Synthetic-evolution reveals that phosphoregulation of the mitotic kinesin-5 Cin8 is constrained

Abstract

Cdk1 has been found to phosphorylate the majority of its substrates in disordered regions. These phosphorylation sites do not appear to require precise positioning for their function. The mitotic kinesin-5 Cin8 was shown to be phosphoregulated at three Cdk1 sites in disordered loops within its catalytic motor domain. Here, we examined the flexibility of Cin8 phosphoregulation by analyzing the phenotypes of synthetic Cdk1-sites that were systematically generated by single amino-acid substitutions, starting from a phosphodeficient variant of Cin8. Out of 29 synthetic Cdk1 sites that we created, eight were non-functional; 19 were neutral, similar to the phosphodeficient variant; and two gave rise to phosphorylation-dependent spindle phenotypes. Of these two, one site resulted in novel phosphoregulation, and only one site, immediately adjacent to a native Cdk1 site, produced phosphoregulation similar to wild-type. This study shows that, while the gain of a single phosphorylation site can confer regulation and modulate the dynamics of the spindle, to achieve optimal regulation of a mitotic kinesin-5 motor protein, phosphoregulation has to be site-specific and precise.