Drosophila ubiquitin-specific peptidase 14 stabilizes the PERIOD protein by regulating a ubiquitin ligase SLIMB

The circadian clock orchestrates behavior and physiology through the oscillation of key clock proteins like PERIOD (PER). Here, we investigate the role of ubiquitin-specific peptidase 14 (USP14) in modulating PER stability and circadian rhythms in Drosophila. We find that overexpression of USP14 in clock cells reduces PER protein levels without altering its mRNA levels whereas USP14 knockdown increases PER protein levels, suggesting that USP14 regulates PER post-translationally. Interestingly, despite these alterations in PER levels, neither USP14 overexpression nor knockdown significantly impacts circadian behavioral rhythms, likely because of slight effects on PER levels in small ventral lateral neurons (sLN_vs). Further analysis shows that USP14 physically interacts with Supernumerary Limbs (SLIMB), a protein involved in PER degradation. Moreover, reducing slimb expression mitigates the effects of USP14 on PER protein stability. Mass spectrometry identifies two ubiquitination sites on PER (Lys1117 and Lys1118) critical for its degradation. Expression of PER^{1117A, 1118A} mutant in per⁰¹ background impairs circadian rhythm strength. In conclusion, this study demonstrates that Drosophila USP14 indirectly modulates PER protein stability by affecting SLIMB and highlights the critical role of specific ubiquitination sites on PER in maintaining circadian rhythms.