Post-translational Regulation of the Arabidopsis Circadian Clock through Selective Proteolysis and Phosphorylation of Pseudo-response Regulator Proteins

Abstract

One core group of circadian clock components in Arabidopsis that controls the pace of the central oscillator is comprised of five PRR (Pseudo-Response Regulator) proteins whose biochemical function in the clock remains unclear. PRR1/TOC1 and PRR5 are the only likely proteolytic substrates of the E3 ubiquitin ligase SCF^ZTL within this PRR family. We demonstrate a functional significance for the phosphorylated forms of PRR5, TOC1, and PRR3. Each PRR protein is differentially phosphorylated over the circadian cycle. The more highly phosphorylated forms of PRR5 and TOC1 interact best with the F-box protein ZTL (ZEITLUPE). TOC1 and PRR3 interact in vivo and phosphorylation of both is necessary for their optimal binding in vitro. TOC1/PRR3 phosphorylation-dependent interaction may protect TOC1 from ZTL-mediated degradation, resulting in an enhanced amplitude of TOC1 cycling.