TRIMming the nuclear pore

Molecular glues are small-molecule compounds that promote interactions between a protein of interest and an E3 ubiquitin ligase, leading to selective degradation of the protein of interest. However, these glues are often identified serendipitously through phenotypic screening. Lu et al. have now identified a glue called acepromazine (ACE) that promotes formation of a TRIM21–NUP98 ternary complex, resulting in degradation of nuclear pore proteins. ACE was identified in a chemical screen for compounds that enhance cytotoxicity against the inflammatory cytokine IFNγ. ACE was metabolically converted by aldo-keto reductases into a reduced product called S-ACE-OH. The team performed a genome-wide CRISPR screen and identified the E3 ubiquitin ligase TRIM21 as a key regulator of S-ACE-OH activity. Quantitative proteomic analysis of ACE-sensitive cell lines revealed specific proteosome-mediated depletion of nuclear pore proteins and subsequent fragmentation of the nuclear pore complex structure. CRISPR suppressor scanning screens identified mutations in the nucleoporin NUP98 that increased resistance to ACE-mediated degradation, while isothermal titration calorimetry analysis showed that S-ACE-OH promoted formation of a TRIM21–NUP98 ternary complex. A co-crystal structure of S-ACE-OH with TRIM21 identified a shallow hydrophobic pocket that was potentially ligandable and could enable design of a TRIM21-mediated degrader against the bromodomain of BRD4, which they called TrimTAC. The team observed that a fusion protein consisting of NUP98 with the BRD4 bromodomain could form nuclear condensates in cells, which could then be degraded by the TrimTAC. Fusion of the BRD4 bromodomain to other condensate-forming proteins also resulted in TrimTAC-mediated degradation, with TrimTAC sparing monomeric proteins. Although the biochemical basis of TrimTAC-mediated degradation of multimeric complexes remains unclear, TrimTAC offers intriguing new opportunities to alter condensates.

Original reference: Cell 187, 7126–7142 (2024)