LAUSR

Post-translational modifications play essential roles in finely modulating eukaryotic circadian clock systems. In plants, the effects of O-glycosylation on the circadian clock and the underlying mechanisms remain largely unknown. O-fucosyltransferase SPINDLY (SPY), and O-GlcNAc transferase SECRET AGENT (SEC) are two prominent O-glycosylation enzymes in higher plants, with both overlapped and unique functions in plant growth and development. Unlike the critical role of O-GlcNAc in regulating animal circadian clock, here we demonstrated that nuclear localized SPY but not SEC, specifically modulates the pace of Arabidopsis circadian clock. By identifying the interactome of SPY, we characterized PSEUDO RESPONSE REGULATOR 5 (PRR5), one of core circadian clock components, as a new SPY-interacting protein. PRR5 can be O-fucosylated by SPY in planta, while point mutations in the SPY catalytic domain abolishes the O-fucosylation of PRR5. The protein abundance of PRR5 is strongly increased in spy mutants, while the degradation rate of PRR5 is much reduced, suggesting PRR5 proteolysis is promoted by SPY-mediated O-fucosylation. Moreover, multiple lines of genetic evidence indicate PRR5 is a major downstream target of SPY to specifically mediate its modulation on circadian clock. Collectively, our findings provide a novel insight into the specific role of O-fucosyltransferase activity of SPY in modulating the circadian clock, and implicate O-glycosylation might have an evolutionarily conserved role in modulating circadian clock system, via O-GlcNAcylation in mammals, but via O-fucosylation in higher plants.