Molecular mechanisms of the Asx C-terminal region for genomic targeting of PR-DUB in Drosophila

Polycomb-repressive deubiquitinase (PR-DUB) removes monoubiquitin from histone H2A at lysine 119 in mammals and lysine 118 in Drosophila (H2Aub1). This reaction requires interaction of the PR-DUB catalytic subunit Bap1 with the N terminus of its partner Asx. In Drosophila , Asx truncation mutants lacking the C-terminal low-complexity region and PHD finger display severe PR-DUB loss-of-function phenotypes. PR-DUB containing C-terminally truncated Asx retains deubiquitinase activity but fails to localize to Polycomb target genes, resulting in excessive H2Aub1 accumulation that disrupts repression. Unexpectedly, the conserved interaction between the Asx PHD finger and Mbd5 is dispensable for viability but fine-tunes H2Aub1 deubiquitination at a subset of Polycomb target genes. In contrast, a short domain in the Asx C terminus that associates with the DNA-binding domain of the Polycomb protein Pho is essential for viability. The Asx–Pho interaction is required for recruitment of PR-DUB to most Polycomb target genes, where it promotes H2Aub1 deubiquitination and preserves Polycomb repression. Unlike the widely conserved PHD finger, the Pho-interacting motif in Asx is conserved only across arthropods. The Asx C terminus therefore constitutes a modular interaction platform in which conserved protein interactions modulate PR-DUB activity, whereas evolutionarily restricted interactions direct its genomic targeting.