Phosphorylation Regulates SLIMB-PERIOD Interactions and Protein Degradation

Post-translational modification (PTM) such as phosphorylation is an integral step in protein regulation. Poly-ubiquitylation is a PTM carried out by E3 Ubiquitin Ligases (E3), which is important for protein degradation and regulation of cellular events. SLIMB is a part of a Drosophila E3 ligase complex with a highly conserved mammalian homologue, β-TrCP. The objective of this investigation was to determine how SLIMB phosphorylation regulates interactions with one of its substrates, PERIOD (PER). PER regulates circadian transcription in animals through conserved mechanisms. Its expression oscillates daily in response to a transcriptional negative feedback loop and SLIMB-dependent degradation. PER PTM regulation is well described, but surprisingly, few studies have characterized how PTM directly regulates SLIMB function, despite observations that SLIMB is phosphorylated. We mapped phosphorylation sites on SLIMB by affinity purification followed by LC-MS/MS and identified phosphosites that regulate SLIMB-PER...