Photocrosslinking Probes Uncover New Nonhistone Partners of the TRIM24 Bromodomain
- TRIM24 is a dual-module epigenetic "reader" whose bromodomain binds acetylated histones and is an established cancer drug-discovery target, though its nonhistone binding partners have been poorly defined.
- Researchers installed genetically encoded photocrosslinkers, AzF and BzF, at residue L922 within the bromodomain's acetyllysine pocket, preserving histone binding while enabling UV-triggered covalent capture of interacting proteins.
- Quantitative proteomics identified the nonhistone proteins NPM1 (AzF-enriched) and CDK9 (BzF-enriched), plus candidates MAP4, RAN, and SND1.
- ChIP-seq showed TRIM24, CDK9, and NPM1 co-localize near transcription start sites of cancer-associated genes; KEGG analysis tied the hits to several signaling pathways.
- The approach shows photocrosslinking can extend reader-domain interactome mapping beyond histones, yielding new candidate leads for chemical-probe and target-discovery work on TRIM24.
"Selective bromodomain inhibitor" just got a harder definition. NPM1 and CDK9 turned up riding along at L922 — the acetyllysine pocket where any TRIM24 bromodomain probe or degrader warhead would sit — so selectivity now has to mean selective against displacing these too, not just against other BRD family members. Most bromodomain campaigns are still built structure-first off that pocket alone, chasing histone-mimetic potency; this says the pocket's biology extends past histones, so cellular phenotype from a TRIM24 chemical probe could be confounded by NPM1/CDK9 disruption rather than histone reading per se. No compound here, just a crosslinker — useful, but MAP4, RAN and SND1 are "candidates" in a proteomics list, not validated partners; I'd want co-IP/FP confirmation and dose-response competition before building any SAR narrative around them.