A ubiquitin-proteasome pathway degrades the inner nuclear membrane protein Bqt4 to maintain nuclear membrane homeostasis

Aberrant accumulation of inner nuclear membrane (INM) proteins is associated with deformed nuclear morphology and mammalian diseases. However, the mechanisms underlying the maintenance of INM homeostasis remain poorly understood. In this study, we explored the degradation mechanisms of the INM protein Bqt4 in the fission yeast Schizosaccharomyces pombe. We have previously shown that Bqt4 interacts with the transmembrane protein Bqt3 at the INM and is degraded in the absence of Bqt3. Here, we reveal that excess Bqt4, unassociated with Bqt3, is targeted for degradation by the ubiquitin-proteasome system localized in the nucleus and Bqt3 antagonizes this process. The degradation process involves the Doa10 E3 ligase complex at the INM. Bqt4 is a tail-anchored protein and the Cdc48 complex is required for its degradation. The C-terminal transmembrane domain of Bqt4 was necessary and sufficient for proteasome-dependent protein degradation. Accumulation of Bqt4 at the INM impaired cell viability with nuclear envelope deformation, suggesting that quantity control of Bqt4 plays an important role in nuclear membrane homeostasis.