PPIase dPPIL4 regulates high-amplitude PERIOD oscillations in Drosophila
Peptidyl-prolyl cis/trans isomerases (PPIases) accelerate proline peptide bond isomerization, affecting substrate protein function. In this study, through RNAi-based behavioral screening of PPIases in Drosophila melanogaster, we identified CG5808, termed Drosophila Peptidyl-Prolyl cis/trans Isomerase-like 4 (dPPIL4), as crucial for circadian rhythm regulation. Knockdown of dppil4 in clock cells lengthened the circadian rhythm period and decreased rhythmicity, accompanied by a significant reduction of core clock protein PERIOD (PER). Dppil4 knockdown downregulated per transcription and reduced phosphorylation at Ser5 in the RNA polymerase II C-terminal domain, critical for transcription elongation. In addition, dPPIL4 stabilized Cullin1 of the Skp1-Cullin1-F-box protein complex, a key regulator of PER degradation. Our findings suggest that dPPIL4 supports high-amplitude PER oscillation by enhancing both synthesis and degradation processes in a timely manner.  In conclusion, our study underscores the importance of high-amplitude PER oscillations in PER for robust circadian rhythms and highlights the critical role of dPPIL4 in this process. Overall design: Control flies and dppil4 knock-down flies (with knockdown in clock cells) were entrained under a 12:12 light-dark cycle. On the fourth day of entrainment, flies were collected at ZT8 and ZT16. Fly heads were isolated for RNA extraction, followed by RNA-sequencing.