How to Develop p300 (EP300) PROTACs: Domain Targeting, Linker Design, and Paralog Selectivity

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What Is p300 (EP300), and Which Forms of the Protein Are Worth Degrading with PROTACs?

E1A-associated protein p300, encoded by the EP300 gene, is a large, multidomain transcriptional coactivator and lysine acetyltransferase. It is also known as KAT3B and belongs to the same protein family as CREB-binding protein (CBP), which is encoded by CREBBP and commonly designated KAT3A. Human p300 contains approximately 2,414 amino acids, whereas CBP contains approximately 2,442 amino acids. Although these proteins share approximately 58% overall sequence identity and have highly conserved functional domains, they are not completely interchangeable. They participate in overlapping but distinct transcriptional programs and can exhibit different biological dependencies depending on the research context.

The primary function of p300 is to connect transcription factors, chromatin-associated proteins, and transcriptional machinery. Its histone acetyltransferase (HAT) activity transfers acetyl groups from acetyl-CoA to lysine residues on histones and nonhistone proteins. This modification can alter chromatin accessibility and influence transcriptional activity. In addition, p300 acts as a molecular scaffold through multiple protein-protein interaction domains, allowing it to coordinate transcription factors and regulatory complexes. These combined catalytic and scaffolding functions explain why inhibiting one domain may not completely reproduce the biological effects of removing the entire protein.

From an architectural perspective, p300 contains several independently folded domains connected by flexible or intrinsically disordered regions. The N-terminal region includes the nuclear receptor interaction domain (NRID) and the cysteine-histidine-rich 1 (CH1) region, which incorporates the transcriptional adapter zinc-binding 1 (TAZ1) domain. The KIX domain provides another important interaction surface for transcription factors. The central catalytic region contains the bromodomain, RING domain, plant homeodomain (PHD), and HAT domain, followed by the ZZ-type zinc finger, TAZ2, and interferon-binding domain (IBiD) toward the C-terminus.

These domains provide different opportunities for degrader design. The bromodomain recognizes acetylated lysine residues and offers a defined small-molecule binding pocket. The HAT domain contains a catalytic pocket suitable for inhibitor-derived warheads. In contrast, CH1/TAZ1 and KIX primarily participate in protein-protein interactions, making them attractive targets for peptide-based or peptidomimetic recognition strategies. The NRID and other interaction regions may also provide alternative recognition surfaces when conventional catalytic-domain targeting is unsuitable.

Several p300 protein forms should be considered when defining a PROTAC development strategy:

• Wild-type full-length p300: Retains the complete domain architecture and provides the principal starting point for bromodomain- and HAT-directed degrader development.
• Domain-specific missense variants: Substitutions within the bromodomain, HAT domain, or surrounding structural regions may change ligand recognition, pocket geometry, or protein conformation. Warhead compatibility should therefore be evaluated using the actual variant rather than inferred from wild-type binding.
• Truncated p300 proteins: Nonsense mutations, frameshifts, and deletions can remove specific domains. A degrader directed toward a missing domain cannot engage the truncated protein, even if the corresponding compound efficiently degrades full-length p300.
• Fusion proteins: EP300-related fusion proteins can retain only part of the original p300 architecture. For example, EP300::ZNF384 fusion configurations have been reported to retain the N-terminal EP300 region, including residues approximately 1–510, while lacking downstream KIX, bromodomain, and HAT regions. This creates a different target-selection problem from that of full-length p300.
• CBP as a closely related paralog: Because CBP and p300 share highly conserved ligand-binding domains, researchers should determine whether the intended objective is dual degradation or preferential removal of EP300 while preserving CBP.

An important distinction is that a mutation is not automatically a degrader-resistance hotspot. Its actual effect depends on whether it disrupts the selected warhead-binding site, changes protein folding, removes an accessible ubiquitination surface, or alters the surrounding protein interaction network. Similarly, the p300 K1167 residue identified in recent selective-degrader research is a ubiquitination hotspot rather than a conventional warhead-binding mutation. It should be evaluated as part of degradation geometry and ubiquitin transfer.

Consequently, p300 PROTAC development should begin by defining the target species, retained domains, protein abundance, and desired paralog profile. These factors determine whether a bromodomain ligand, HAT inhibitor, peptide binder, or alternative domain-directed recognition strategy is the most appropriate starting point.

Developed p300/CBP PROTACs and Their Research Progress

Research on p300/CBP degradation has progressed from early bromodomain-directed chemical degraders to more sophisticated HAT-directed, peptide-based, and paralog-selective architectures. Early studies established that the bromodomain could provide an accessible recognition site for bifunctional molecules, while subsequent work demonstrated that catalytic-domain targeting could also induce protein depletion. More recent research has focused on improving linker architecture, reducing molecular size, controlling E3 ligase recruitment, and exploiting differences in ubiquitination between p300 and CBP.

Several representative molecules illustrate how these approaches have evolved. The early dCBP-1 architecture used a GNE-781-derived bromodomain ligand connected to a CRBN recruiter through a PEG-based linker. JET-209 subsequently demonstrated exceptionally potent dual degradation using a GNE-207-derived bromodomain ligand and a lenalidomide-derived CRBN recruiter. In another direction, JQAD1 introduced an A-485-derived HAT-binding warhead and demonstrated preferential p300 degradation in a specific research model.

The development of dCE-2 further highlighted the importance of ternary-complex cooperativity. Its 3-methylcinnoline acetyl-lysine-mimetic warhead showed that high binary binding affinity is not the only determinant of degradation. A favorable protein-protein-ligand arrangement can compensate for relatively modest target-binding affinity. Meanwhile, dCE-1 demonstrated that HAT-directed degradation could depend strongly on both CRBN recruitment and sufficient linker length.

Table 1. Representative Developed p300/CBP PROTACs and Their Research Findings.

Collectively, these studies establish several important principles. First, a ligand that binds both p300 and CBP can still produce selective degradation of one paralog. Second, linker length should be optimized for the specific warhead and E3 recruiter rather than selected according to a universal rule. Third, CRBN is a particularly well-supported recruiter for many p300/CBP degrader architectures, but VHL and MDM2 provide alternative opportunities. Finally, target binding, ternary-complex formation, ubiquitination, and intracellular availability should be evaluated as separate but interconnected stages.

Bromodomain-Directed PROTAC Development for p300/CBP

Target Site: Bromodomain Acetyl-Lysine Reader Pocket and Its Solvent-Exposed Exit Vectors

The p300 bromodomain is a compact acetyl-lysine recognition module located within the central catalytic region, approximately spanning amino acids 1051–1158. Its primary function is to recognize acetylated lysine residues on histones and other proteins, helping recruit p300-associated transcriptional machinery to acetylated chromatin. Structurally, the domain contains a conserved acetyl-lysine recognition pocket surrounded by hydrophobic surfaces and flexible ZA and BC loops. A conserved asparagine residue contributes to acetyl-lysine recognition through hydrogen-bond interactions, while neighboring residues define the geometry and chemical environment of the pocket.

This relatively well-defined recognition cavity makes the bromodomain one of the most practical starting points for small-molecule p300 PROTAC development. However, p300 and CBP bromodomains are highly conserved. Their corresponding ligand-binding pockets exhibit substantial structural similarity, meaning that a bromodomain warhead may bind both proteins with comparable affinity. Therefore, a bromodomain-directed PROTAC should generally be considered a dual-paralog starting point unless degradation selectivity is demonstrated independently.

A second structural consideration is the linker exit vector. The selected attachment position should be accessible to solvent and should allow the linker to extend away from the acetyl-lysine recognition pocket without disrupting the essential hydrogen-bond and hydrophobic interactions that maintain ligand binding. The solvent-exposed tetrahydropyran region of GNE-781 provides a useful precedent. Replacing this region with a conjugatable piperidine enabled attachment of a CRBN-recruiting component while preserving the target-binding architecture. Similarly, the acetamide substituent of the 3-methylcinnoline-based bromodomain ligand provided an alternative attachment vector for dCE-2 development.

Warhead Selection: Bromodomain Inhibitor Chemotypes and Their Conjugation Handles

Bromodomain inhibitor chemotypes provide the most established small-molecule warheads for this strategy. GNE-781 and GNE-207 are useful research precedents because their binding properties and structural features have been explored in the development of p300/CBP degraders. GNE-781 was used to construct dCBP-1 and subsequently dCBP-30, while GNE-207 was incorporated into JET-209. These examples demonstrate that related bromodomain-binding scaffolds can support different degrader architectures depending on the available chemical modification positions and recruited E3 ligase.

Another approach is to use an acetyl-lysine-mimetic scaffold. The 3-methylcinnoline-containing ligand developed for dCE-2 illustrates how fragment-based discovery and structure-guided optimization can generate a new bromodomain-binding chemotype. Although its binary binding affinity was more modest than that of some established inhibitors, the resulting degrader showed favorable ternary-complex cooperativity and effective protein depletion. This is an important reminder that warhead selection should consider the complete degrader rather than maximizing isolated ligand affinity alone.

• Established bromodomain scaffolds: Prioritize GNE-781- or GNE-207-derived structures when a well-characterized recognition mode and a suitable conjugation position are available.
• Acetyl-lysine mimetics: Consider alternative recognition chemotypes when structural optimization identifies a favorable solvent-exposed exit vector.
• Conjugation-site evaluation: Compare several attachment positions and confirm that the modified ligand retains bromodomain binding.
• Dual-target assessment: Evaluate both p300 and CBP binding after ligand functionalization rather than assuming that the parent inhibitor's selectivity profile remains unchanged.

Linker Design: Attachment Point, Length, and Composition for Dual-Paralog Degradation

Bromodomain-directed p300/CBP degraders provide particularly useful evidence that linker design can determine whether a ligand-recruiter combination becomes an effective degrader. In dCBP-1, a flexible PEG4 linker connected the modified GNE-781 scaffold to the C5 position of thalidomide. This architecture supported rapid degradation of both CBP and p300. However, subsequent optimization showed that a long PEG-containing linker was not necessarily required to achieve strong degradation.

The dCE-2 development series provides a more detailed example of linker-length optimization. An initial 11-atom aliphatic linker produced measurable CBP degradation and weaker p300 degradation. Shortening the linker by one atom improved CBP degradation, whereas further shortening to an eight-atom linker caused a substantial loss of activity. Alternative PEG- and piperazine-containing linkers also performed poorly in that particular series. These findings suggest that the relative distance between the bromodomain-bound target and CRBN must be optimized within a narrow structural window.

In contrast, dCBP-30 demonstrated that a directly connected, linker-minimized architecture could outperform longer structures. Its reduced molecular size improved membrane permeability, compensating for a modest reduction in the measured ability to form a ternary complex in a cell-free system. The 2026 Degrader 27 study also showed that linker composition and stereochemistry can influence degradation selectivity, with removal of a linker amide and introduction of a trans-cyclohexyl configuration improving the optimized molecule.

• Attachment position: Begin with a solvent-facing substituent on the bromodomain warhead, such as the modified piperidine position of GNE-781 or the acetamide vector of the cinnoline-based scaffold.
• Linker length: Prepare short, intermediate, and longer matched analogues. Use the dCE-2 series as evidence that a one-atom change can influence degradation substantially.
• Linker composition: Compare PEG, alkyl, heterocyclic, and partially rigidified structures. Do not assume that a more hydrophilic or longer linker will always improve performance.
• Compact architecture: Evaluate direct or near-linkerless attachment when the two binding components can adopt a productive geometry and the reduced molecular size improves cellular availability.

E3 Ligase Selection: CRBN vs. VHL Recruitment for Bromodomain-Directed Degraders

CRBN is the most extensively demonstrated E3 recruiter in published bromodomain-directed p300/CBP PROTAC studies. dCBP-1 uses a pomalidomide-derived recruiter, JET-209 uses a lenalidomide-derived recruiter, and dCE-2 incorporates a thalidomide-derived recruiter. The 2026 p300-selective Degrader 27 also relies on CRBN recruitment. These examples make CRBN a rational first choice when developing a new bromodomain-directed degrader.

VHL remains a useful alternative for determining whether a different E3 surface can improve ternary-complex geometry or target ubiquitination. However, recruiter selection should not be based solely on binary ligand affinity. A CRBN ligand with strong target-cell engagement may still fail if the resulting ternary complex is poorly oriented, while a different recruiter may support a more favorable ubiquitination geometry.

• CRBN-first strategy: Start with a thalidomide- or pomalidomide-derived recruiter when the target ligand offers a suitable attachment vector.
• VHL comparison: Include a VHL-recruiting matched analogue when CRBN-based structures show weak ternary-complex formation or poor degradation.
• Mechanistic confirmation: Evaluate E3 engagement, target ubiquitination, proteasome dependence, and degradation rescue to distinguish genuine PROTAC activity from simple bromodomain inhibition.

Table 2. Recommended Molecular Design Strategies for Bromodomain-Directed p300/CBP PROTACs.

HAT Domain-Directed PROTAC Development for p300/CBP

Target Site: Catalytic Acetyltransferase Pocket and Inhibitor Exit Vectors

The p300 HAT domain is the central catalytic region responsible for transferring acetyl groups from acetyl-CoA to substrate lysine residues. It is located approximately between amino acids 1287 and 1663 and forms part of a larger catalytic core that also contains the bromodomain, RING, and PHD regions. The HAT domain includes an acetyl-CoA-binding region and a substrate-recognition environment that accommodates lysine-containing protein segments.

Unlike a simple isolated binding pocket, the p300 catalytic region is influenced by neighboring structural elements. The RING and TAZ2 regions, together with the regulatory loop associated with the HAT domain, contribute to conformational regulation of catalytic activity. Therefore, a ligand-bound crystal structure should be interpreted in the context of the surrounding domain architecture. A solvent-exposed substituent on a HAT inhibitor may provide a suitable linker exit vector, but its accessibility can change when the ligand is connected to a large E3-recruiting component.

The HAT domain is particularly interesting for PROTAC development because it offers an alternative to bromodomain recognition. It also provides an opportunity to exploit differences in the complete p300- and CBP-containing ternary complexes, even when the inhibitor itself binds both catalytic domains. However, the relatively large and structurally regulated catalytic region requires careful evaluation of ligand attachment position, linker orientation, and the accessibility of surrounding ubiquitination surfaces.

Warhead Selection: Catalytic-Site Inhibitor Scaffolds Amenable to Conjugation

Several HAT inhibitor scaffolds provide useful starting points. A-485 is an established catalytic-domain inhibitor that has been incorporated into the HAT-directed degrader JQAD1. Structural analysis identified a modifiable aryl amine region that could be functionalized through a urea-forming connection to the linker. The resulting architecture retained sufficient target engagement to support p300 degradation in the reported research system.

CPI-1612 provides another important starting point. It has been used in the development of HAT-directed degraders with both CRBN- and VHL-recruiting architectures. The dCE-1 campaign demonstrated that the availability of a suitable ligand modification site does not guarantee successful degradation. Its active CRBN-recruiting design required a sufficiently long linker, while the tested VHL- and IAP-recruiting analogues did not show comparable activity in that particular series.

More recent work has expanded the range of available HAT-binding chemotypes through proline-based inhibitors. In the 2026 PROTAC 25a study, structural analysis of the inhibitor-bound HAT domain identified the phenyl methoxy substituent as a suitable exit vector. The ligand was functionalized with a propargyl-containing intermediate, allowing connection to CRBN-recruiting components through a triazole unit. This strategy provides a useful precedent for developing HAT degraders from scaffolds other than conventional aminopyridine-based inhibitors.

• A-485-derived scaffolds: Use the solvent-accessible aryl amine region as a starting point for linker attachment, while confirming that the modified ligand retains catalytic-domain engagement.
• CPI-1612-derived scaffolds: Explore solvent-exposed positions identified by structural analysis and compare different E3-recruiting systems.
• Proline-based inhibitors: Consider the phenyl methoxy substituent as an attachment vector and evaluate triazole-linked CRBN-recruiting architectures.
• Conjugation validation: Compare the parent inhibitor, linker-bearing intermediate, and complete PROTAC to determine whether reduced degradation results from loss of target recognition or another molecular limitation.

Linker Design: Linker Length Thresholds and Compositional Tuning for HAT Degraders

Linker optimization is particularly important for HAT-directed p300/CBP PROTACs because the catalytic-domain binding site and the recruited E3 surface must be positioned appropriately to support productive ubiquitination. The JQAD1 design provides a relatively long C11 alkyl linker between the A-485-derived warhead and the pomalidomide recruiter. Subsequent research comparing A-485-based conjugates found that longer linkers generally supported more p300 degradation than shorter structures within the tested series.

The dCE-1 study provides another useful example. A linker containing approximately 24 atoms was selected for the active CPI-1612-derived degrader, while a minimum linker length of approximately 21 atoms was required for activity in the tested design series. This finding illustrates that the distance between the HAT-bound ligand and CRBN can be a decisive factor. Shorter structures may fail because the E3 ligase cannot approach the target in a productive orientation, even when both binding components retain their individual recognition properties.

The 2026 proline-based HAT PROTAC study provides a complementary approach using PEG-containing linkers and a triazole connection. Several linker architectures were compared, including PEG, saturated cyclic amine, and alternative heterocyclic structures. The results showed that linker composition influenced both degradation potency and EP300-versus-CBP preference. Importantly, some compounds retained cellular CRBN engagement but did not produce equally effective target degradation, demonstrating that E3 binding alone is insufficient.

• Length optimization: Build a focused linker-length series around the attachment vector. The 21–24-atom range observed in the CPI-1612 dCE-1 series is a useful precedent, not a universal requirement for every HAT warhead.
• PEG-based linkers: Consider PEG segments when a longer spacer and improved flexibility are required, particularly for proline-based HAT inhibitor architectures.
• Alkyl and rigidified linkers: Compare C11-type alkyl structures with heterocyclic or partially rigidified alternatives to assess the influence of conformational restriction.
• Triazole connections: Use click-compatible functional groups when a modular assembly strategy is advantageous, but verify that the connecting unit does not interfere with target or E3 binding.
• Matched comparisons: Hold the warhead and recruiter constant when varying linker composition so that degradation changes can be attributed to the linker architecture.

E3 Ligase Selection: Why CRBN Recruitment Dominates HAT-Directed Designs

CRBN has provided the most consistent foundation for several published HAT-directed p300/CBP degrader series. JQAD1 uses a pomalidomide-derived CRBN recruiter, dCE-1 requires CRBN engagement for its reported activity, and the proline-based 25a series also uses CRBN-recruiting ligands. These examples make CRBN a practical first-choice system when developing a new HAT-directed degrader.

Nevertheless, CRBN should not be regarded as the only possible recruiter. A separate CPI-1612-derived VHL-recruiting degrader demonstrated preferential EP300 degradation, indicating that VHL can also support HAT-directed degradation under an appropriate molecular arrangement. The contrast between these studies suggests that E3 ligase selection must be considered together with the target ligand, linker length, attachment position, and the resulting ternary-complex geometry.

A useful development strategy is to compare CRBN and VHL using matched target-binding scaffolds wherever chemical compatibility permits. The comparison should include E3 engagement, ternary-complex formation, protein ubiquitination, and degradation kinetics. If a recruiter binds efficiently but does not induce protein depletion, changing the E3 system or linker architecture may be more productive than simply increasing the affinity of the HAT inhibitor.

Table 3. Recommended Molecular Design Strategies for HAT-Directed p300/CBP PROTACs.

Paralog-Selective PROTAC Design: Favoring EP300 or CBP

Target Site: Non-Conserved Surfaces and Paralog-Specific Complex Contexts

Achieving selective degradation of EP300 over CBP is challenging because the two proteins share highly conserved domain structures and many of the same ligand-recognition features. A small molecule may bind both bromodomains or both HAT domains with similar affinity, yet the resulting PROTAC may preferentially degrade only one protein. This difference demonstrates that paralog selectivity is not determined exclusively by the target-binding ligand.

The 2026 discovery of Degrader 27 provides an important mechanistic example. Its GNE-781-derived bromodomain ligand showed comparable binding to the p300 and CBP bromodomains. However, the complete degrader formed a more productive ternary complex with p300, resulting in stronger ubiquitination and proteasome recruitment. Further investigation identified lysine 1167 (K1167) as an important p300 ubiquitination site. The corresponding CBP region contains K1203, but this site was not equivalently engaged by Degrader 27.

K1167 should therefore be considered a degradation-relevant ubiquitination hotspot rather than a conventional warhead-binding pocket. Its identification demonstrates that a degrader can exploit differences in the spatial presentation of ubiquitin-accepting residues even when the target-binding domains are highly conserved. This provides a different approach to paralog selectivity: rather than requiring a ligand that binds only p300, the molecule can be designed to position the recruited E3 ligase so that ubiquitin transfer preferentially occurs on p300.

Warhead Selection: Binding Modes and Chemotypes That Bias One Paralog

Warhead selection should begin by distinguishing binding selectivity from degradation selectivity. A p300-binding ligand that has substantially weaker affinity for CBP may provide a straightforward route to selectivity. However, when the two proteins exhibit similar ligand affinity, differences in ternary-complex formation and ubiquitination may still generate a selective degrader.

The GNE-781-derived warhead in Degrader 27 demonstrates the second approach. Its target-binding component recognizes both paralogs, but the complete CRBN-recruiting molecule preferentially degrades p300. The HAT-directed proline-based PROTAC 25a provides another example of preferential EP300 degradation. It achieved approximately 90.8% EP300 reduction compared with 59.6% CBP reduction at the optimal tested concentration, showing that differences in the complete degrader architecture can produce a meaningful preference even when the parent inhibitor recognizes both proteins.

Earlier HAT-directed research also explored p300-preferential degradation through A-485- and CPI-1612-derived architectures. JQAD1 demonstrated a model-dependent preference for p300, while a CPI-1612-derived VHL-recruiting construct also showed EP300-biased degradation. These examples indicate that different target domains and E3 recruitment systems can produce paralog preference through different mechanisms. However, their activity should not be generalized across all cellular systems.

Linker Design: Using Ternary Complex Cooperativity and Geometry to Separate the Paralogs

Linker geometry is a particularly important selectivity variable because it determines the relative orientation of the target and E3 ligase. Even when p300 and CBP bind the same warhead, subtle differences in their surrounding surfaces may produce different ternary-complex arrangements. These differences can affect the proximity of accessible lysine residues to the E3 ligase and ultimately influence ubiquitin transfer.

Degrader 27 illustrates this principle through its optimized linker architecture. Structural modification, including removal of an amide connecting the linker to the E3-recruiting component and adjustment of the cyclohexyl stereochemistry, improved the compound's metabolic stability and degradation activity. The optimized molecule reached a p300 DC₅₀ of approximately 22.5 nM while maintaining its preference over CBP. The study further connected this preference to selective ubiquitination at p300 K1167.

The proline-based HAT PROTAC 25a offers a complementary design precedent. Its PEG-containing linker and triazole connection contributed to a structure that favored EP300 degradation over CBP. This demonstrates that selectivity does not necessarily require a highly rigid linker. A flexible linker can also produce a useful paralog preference when its attachment position, length, and composition support an appropriate protein-protein-ligand arrangement.

• Optimize the exit vector: Compare attachment positions that project the linker toward different regions of the target surface.
• Evaluate linker stereochemistry: Include stereochemical variants when cyclic or conformationally restricted linkers are used.
• Monitor ternary-complex cooperativity: Compare EP300- and CBP-containing complexes rather than relying on target-binding measurements alone.
• Consider ubiquitination geometry: Investigate whether the recruited E3 ligase can approach distinct lysine residues on each paralog.
• Use matched analogues: Change one structural feature at a time to identify which linker modification produces the observed selectivity.

E3 Ligase Selection: Recruiter Identity as an Additional Selectivity Lever

E3 ligase selection can influence paralog preference by changing the protein surface recruited into the ternary complex. CRBN is particularly relevant because both Degrader 27 and HAT PROTAC 25a use CRBN-recruiting architectures. However, VHL has also been used in EP300-preferential HAT-directed degradation, while MDM2 provides an alternative in CH1-directed peptide PROTAC design.

The important consideration is not simply which E3 ligase has the highest expression or strongest ligand affinity. The recruiter must support a productive orientation for ubiquitin transfer to the intended paralog. For this reason, matched CRBN- and VHL-recruiting molecules can be informative when a target ligand is compatible with both systems. The resulting compounds should be compared for EP300 and CBP ternary-complex formation, ubiquitination, degradation kinetics, and maximum degradation.

Table 4. Representative Molecular Strategies for Selective and Dual p300/CBP PROTAC Degradation.

PROTAC Development for p300/CBP Mutations, Truncations, and Fusion Proteins

Target Site: Domains Retained in Mutant, Truncated, and Fusion Forms of p300/CBP

p300 and CBP alterations can change the feasibility of degrader development by affecting the presence, folding, or accessibility of individual domains. Missense mutations may modify the local ligand-binding environment, whereas nonsense mutations, frameshifts, and larger deletions can remove entire functional regions. The first step in a variant-directed program is therefore to determine which portions of the target protein remain expressed and structurally accessible.

For bromodomain-directed PROTACs, substitutions within or around the acetyl-lysine recognition pocket may change the affinity or binding orientation of the selected warhead. A similar problem occurs with HAT-directed degraders when a mutation changes the catalytic pocket or the surrounding structural environment. In either case, the parent inhibitor's activity against wild-type p300 should not be used as the only basis for selecting a mutant-targeting warhead.

Truncated and fusion proteins introduce a more fundamental challenge because the target domain may no longer be present. EP300::ZNF384 provides a useful example of altered domain architecture. Reported fusion configurations retain the N-terminal EP300 region, approximately residues 1–510, which includes the NRID and CH1/TAZ1 region, while downstream KIX, bromodomain, and HAT domains are absent. Consequently, a bromodomain- or HAT-directed PROTAC designed for full-length p300 cannot automatically be expected to recognize this fusion protein. The retained CH1/TAZ1 interaction surface may instead provide a more relevant recognition opportunity.

Warhead Selection: Preserving Engagement Across Altered p300/CBP Species

Warhead selection should be guided by the location of the mutation and the domains retained in the target construct. When a bromodomain substitution weakens the binding of an established bromodomain inhibitor, an alternative HAT-domain ligand may be considered if the catalytic domain remains intact. Conversely, if a mutation affects the HAT pocket, bromodomain- or CH1-directed recognition may provide an alternative route.

For truncated proteins and fusion constructs, domain retention is more important than the original full-length protein annotation. A CH1-directed peptide ligand may be appropriate for a fusion retaining the N-terminal EP300 region, whereas a fusion retaining the catalytic core may remain accessible to bromodomain- or HAT-binding ligands. However, retention of a domain does not guarantee that its recognition surface remains accessible after fusion. The local structural environment and neighboring fusion partner should also be considered.

• Mutation within the bromodomain: Compare alternative bromodomain chemotypes and evaluate whether HAT- or CH1-directed recognition provides a more reliable alternative.
• Mutation within the HAT domain: Assess whether the alteration affects catalytic-pocket recognition and whether a noncatalytic domain can serve as an alternative warhead target.
• Domain deletion: Select a warhead directed toward a region that is demonstrably retained in the expressed protein.
• Fusion proteins: Prioritize binding surfaces retained in the fusion and evaluate whether the fusion junction changes ligand accessibility.

Linker Design: Accommodating Altered Domain Architecture and Fusion Partner Context

A linker that works with full-length p300 may not be optimal for a truncated or fusion protein. Changes in domain composition can alter the position of the ligand-binding surface, the accessibility of neighboring lysine residues, and the orientation of the recruited E3 ligase. Therefore, linker optimization should use the actual target construct rather than assuming that the geometry of wild-type p300 remains unchanged.

For mutation-directed designs, the first priority is to determine whether linker attachment changes the warhead's ability to recognize the altered pocket. If the mutation changes the orientation of a bound ligand, an otherwise effective linker may project toward the protein surface and create steric interference. In this situation, alternative attachment positions or different linker compositions may improve the presentation of the E3 recruiter.

Fusion-directed designs require additional attention to the junction between the retained p300 region and the fusion partner. A peptide or small-molecule warhead may still recognize its original domain, but the fusion partner can change the local conformational environment or block the expected linker trajectory. A practical strategy is to compare the isolated retained domain, the complete fusion construct, and the full-length protein when evaluating target engagement and degradation.

E3 Ligase Selection: Matching Recruitment to the Disease Context and E3 Expression Profile

E3 ligase selection should be based on the biological context in which the mutant or fusion protein is expressed. CRBN and VHL provide established starting points for small-molecule PROTAC architectures, while MDM2 may be relevant for peptide-based designs. Their relative suitability depends on expression, activity, subcellular availability, and the geometry of the recruited complex.

A mutation or fusion may also change the accessibility of potential ubiquitination sites without directly affecting the warhead-binding domain. In such cases, changing the E3 recruiter may be more effective than repeatedly modifying the target-binding ligand. The complete system should be assessed for target engagement, ternary-complex formation, ubiquitination, and protein depletion.

Table 5. Recommended PROTAC Strategies for Altered p300/CBP Protein Forms.

Peptide-Based PROTAC Development for p300/CBP Scaffolding Domains

Target Site: CH1/TAZ1, KIX, and Other Protein-Protein Interaction Surfaces

The scaffolding domains of p300 and CBP provide alternative recognition opportunities when conventional small-molecule warheads are difficult to develop. Unlike the relatively enclosed bromodomain and HAT pockets, these regions often present broad protein-protein interaction surfaces. Their biological functions depend on recognizing transcription factors, regulatory proteins, and other coactivators through multiple interaction hotspots.

The CH1/TAZ1 domain is particularly interesting because it contains a cysteine- and histidine-rich structural region that supports interactions with transcriptional regulators. One well-characterized interaction involves the p300 CH1 domain and CITED2. The corresponding complex provides a structural template for developing peptide binders against the CH1 surface. The KIX domain is another potential target because it recognizes multiple transcription-factor interaction motifs, while TAZ2 and IBiD offer additional protein-interaction surfaces for future ligand discovery.

Among these possibilities, CH1-directed peptide PROTAC development has the clearest direct research precedent. A reported p300 peptide degrader used a CITED2-derived recognition sequence to bind the CH1 domain and recruit MDM2. This approach demonstrates how a relatively shallow protein-protein interaction surface can be converted into a degradation strategy without requiring a conventional small-molecule binding cavity.

Warhead Selection: Peptide and Peptidomimetic Binders for p300/CBP Interfaces

The reported CH1-directed p300 peptide PROTAC was developed from a 15-amino-acid segment derived from the p300/CITED2 complex structure. Structure-guided sequence optimization produced the p300-binding sequence PWIWDGDNKDDNSTD. This sequence was designed to improve recognition of the CH1 interaction surface while retaining the essential contact pattern of the original protein interface.

A separate peptide segment was used to recruit MDM2. The MDM2-binding sequence TSFEQFWAWLWP was connected to the p300-binding peptide to create a bifunctional recognition architecture. The reported peptide construct showed binding affinities of approximately 78.3 nM for p300 and 101.4 nM for MDM2 in the corresponding binding assays. These results provide a practical precedent for converting a protein-protein interaction surface into a peptide-based degrader.

For new programs, peptide selection should focus on the interaction residues that contribute most strongly to domain recognition. Structure-guided sequence optimization can be used to improve binding complementarity and peptide stability. Where a linear peptide exhibits insufficient conformational stability, constrained peptides, stapled structures, or peptidomimetic modifications may be explored. However, modifications should be introduced carefully because changes to the peptide backbone or side-chain orientation can disrupt the original recognition interface.

• CH1-directed peptide: Use the CITED2-derived PWIWDGDNKDDNSTD sequence as a published starting point for p300 CH1 recognition.
• KIX-directed peptides: Explore peptide motifs derived from known KIX-binding transcription-factor interfaces, while confirming their affinity and specificity independently.
• Peptidomimetic binders: Consider backbone modification or conformational constraints when improved stability and binding persistence are required.
• Interface specificity: Compare binding to p300 and CBP domains to determine whether the selected peptide offers paralog preference or recognizes both proteins.

Linker Design: Conjugation Site, Spacer Length, and Cell-Penetration Considerations

Peptide-based PROTACs require a different linker strategy from conventional small-molecule degraders. The linker must preserve the recognition surfaces of both peptide segments while allowing the target and E3 ligase to form a productive complex. In the reported p300 peptide degrader, a flexible GGGSGGG spacer was placed between the p300-binding and MDM2-binding sequences. This glycine- and serine-rich architecture provides conformational flexibility and helps reduce steric interference between the two recognition elements.

A cysteine residue was added to the N-terminus of the complete peptide construct to provide a conjugation handle for a nano-selenium delivery system. The carrier was used to address two major limitations of peptide-based degraders: poor stability and limited cellular penetration. This is an important consideration because a peptide may show good target and E3 binding in biochemical assays but still produce insufficient intracellular degradation if it cannot reach the relevant proteins.

For new designs, linker optimization should consider the distance between the two peptide-binding surfaces, the flexibility of the spacer, and the chemical compatibility of the conjugation site. Excessive flexibility may increase conformational entropy and reduce productive complex formation, while an overly short spacer can cause steric interference. Peptide degradation, aggregation, and cellular uptake should also be evaluated alongside ternary-complex formation.

• Flexible spacer: Use glycine/serine-rich sequences such as GGGSGGG as an initial design and compare shorter or longer variants.
• Conjugation position: Select terminal or solvent-exposed peptide positions that do not disrupt the target-binding interface.
• Peptide stabilization: Evaluate terminal modifications, conformational constraints, and peptidomimetic substitutions when proteolytic instability limits performance.
• Delivery considerations: Consider an appropriate carrier strategy when poor membrane penetration prevents sufficient intracellular exposure.

E3 Ligase Selection: MDM2 Recruitment and Alternatives for Larger, More Polar Warheads

MDM2 is a demonstrated E3 ligase recruiter for CH1-directed p300 peptide degradation. In the reported architecture, the MDM2-binding peptide promotes proximity between MDM2 and p300, facilitating target ubiquitination and subsequent protein depletion. An additional feature of this design is that the MDM2-binding segment interacts with the region involved in p53 recognition. Consequently, the construct can influence the MDM2-p53 interaction as well as p300 abundance. These effects should be measured separately when interpreting the biological response.

CRBN and VHL offer alternative E3 recruitment systems for future peptide-based p300 degraders. Their use may be attractive when a small-molecule recruiter can be connected to the peptide without compromising its recognition surface. However, the reported CH1 peptide architecture provides direct experimental support for MDM2 rather than establishing CRBN- or VHL-based peptide conjugates as equally validated alternatives.

E3 selection should therefore consider the size and polarity of the peptide warhead, the accessibility of the target interaction surface, and the availability of the recruited ligase. Mechanistic experiments should confirm that p300 degradation depends on the intended E3 system and distinguish degradation from changes in p53-associated signaling or other independent biological effects.

Table 6. Recommended Molecular Design Strategies for Peptide-Based p300 PROTACs.

BOC Sciences Support for p300 (EP300) PROTAC Development

p300/CBP Target and Domain Feasibility Assessment

• Define the protein species: wild-type dual paralogs, EP300-dependent form, mutant, truncation, or fusion.
• Map retained domains using our practical guide, How to Select and Validate Targets for Protein Degrader Development.
• Evaluate the nuclear context based on Protein Degrader Design Strategies According to Target Protein Subcellular Localization.
• Weigh protein turnover, accessible lysines, and ligase availability before choosing a warhead class.

Warhead and Ligand Design for Bromodomain, HAT, and Scaffolding Surfaces

• Evaluate bromodomain-binding, HAT-directed, and peptide warheads against the defined protein form and desired paralog selectivity.
• Identify solvent-exposed exit vectors and prepare multiple attachment-site analogues.
• Supply warheads, reference degraders, and screening library sets for comparative evaluation.
• Reassess target binding using both linker-bearing intermediates and complete degrader molecules.

Linker and E3 Ligase System Design with Custom PROTAC Synthesis

• Build design matrices varying attachment site, length, composition, and rigidity using diverse linker structures and click chemistry-based linker design strategies.
• Compare CRBN, VHL, IAP, and MDM2 recruiters early in development, supported by E3 ligase ligand design.
• Prepare parent warhead, non-degrading controls, and matched-linker controls.
• Provide purification and analytical characterization of custom p300 degrader candidates.

Ternary Complex, Degradation, and Paralog Selectivity Evaluation

• Measure binary binding and cellular target engagement after warhead derivatization.
• Resolve EP300 and CBP depletion separately through DC₅₀, D_max, degradation kinetics, and hook-effect analysis.
• Confirm the degradation mechanism using PROTAC diastereomer-based negative controls.
• Extend promising candidates through PROTAC in vitro metabolism and PROTAC in vivo evaluation in animal models.

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