Drosophila peptidyl-prolyl cis/trans isomerase-like 4 regulates circadian rhythm by supporting high-amplitude oscillations of PERIOD

Peptidyl-prolyl <i>cis/trans</i> isomerases (PPIases) accelerate proline peptide bond isomerization, affecting substrate protein function. In this study, through RNAi-based behavioral screening of PPIases in <i>Drosophila melanogaster</i>, we identified <i>CG5808</i>, termed <i>Drosophila</i> peptidyl-prolyl <i>cis/trans</i> isomerase-like 4 (dPPIL4), as crucial for circadian rhythm regulation. Knockdown of <i>dppil4</i> in clock cells lengthened the circadian rhythm period and decreased rhythmicity, accompanied by a significant reduction of core clock protein PERIOD (PER). <i>d</i> <i>ppil4</i> knockdown downregulated <i>per</i> 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.