A Quality Control Pathway Clears Stalled PIWI Target Complexes via the Tudor Protein Chum
The PIWI-interacting RNA (piRNA) pathway safeguards animal germ cells by silencing transposons. In the cytoplasm, piRNAs guide PIWI proteins to cleave target RNAs, and the fragments are recycled as precursors for new piRNAs in the ping-pong cycle. In silkworms, ping-pong proceeds through reciprocal cleavage between Siwi and BmAgo3. Upon Siwi depletion, BmAgo3 and its cleavage products aberrantly accumulate, but how this unproductive state is recognized and resolved remains unclear. Here, we identify Chum, an uncharacterized about 40 kDa Tudor domain protein that is selectively recruited to the stalled BmAgo3 target complex under Siwi depletion, revealing a previously unrecognized surveillance mechanism in the piRNA system. Chum preferentially associates with the Gtsf1L bound form of BmAgo3, where Gtsf1L potentiates BmAgo3 slicer activity, and negatively regulates BmAgo3 at the protein level. Mechanistically, Chum promotes K48 linked polyubiquitination of BmAgo3, at least in part via the SCF TrCP E3 ligase complex, thereby driving BmAgo3 degradation. We propose that Chum acts as a quality control adaptor that recognizes Gtsf1L engaged BmAgo3 stalled on target RNAs and presents it as bait for the ubiquitination machinery, thereby resolving trapped ping-pong intermediates and supporting efficient piRNA production.