Phosphorylation in the intrinsically disordered region of F-BAR protein Imp2 regulates its contractile ring recruitment

The F-BAR protein Imp2 is an important contributor to cytokinesis in the fission yeast Schizosaccharomyces pombe. Because cell cycle-regulated phosphorylation of the central intrinsically disordered region (IDR) of the Imp2 paralog Cdc15 controls Cdc15 oligomerization state, localization and ability to bind protein partners, we investigated whether Imp2 is similarly phosphoregulated. We found that Imp2 is endogenously phosphorylated on 28 sites within its IDR, with the bulk of phosphorylation being constitutive. In vitro, the casein kinase 1 (CK1) isoforms Hhp1 and Hhp2 can phosphorylate 17 sites, and Cdk1 (also known as Cdc2) can phosphorylate the remaining 11 sites. Mutations that prevent Cdk1 phosphorylation result in precocious Imp2 recruitment to the cell division site, and mutations designed to mimic these phosphorylation events delay Imp2 accumulation at the contractile ring (CR). Mutations that eliminate CK1 phosphorylation sites allow CR sliding, and phosphomimetic substitutions at these sites reduce Imp2 protein levels and slow CR constriction. Thus, like Cdc15, the Imp2 IDR is phosphorylated at many sites by multiple kinases. In contrast to Cdc15, for which phosphorylation plays a major cell cycle regulatory role, Imp2 phosphorylation is primarily constitutive, with milder effects on localization and function. This article has an associated First Person interview with the first author of the paper.