The Yck2 yeast casein kinase 1 isoform shows cell cycle-specific localization to sites of polarized growth and is required for proper septin organization

Casein kinase 1 protein kinases are ubiquitous and abundant Ser/Thr-specifi c protein kinases with activity on acidic substrates. In yeast, the product s of the redundant YCK1 and YCK2 genes are together essential for cell viab ility. Mutants deficient for these proteins display defects in cellular mor phogenesis, cytokinesis, and endocytosis. Yck1p and Yck2p are peripheral pl asma membrane proteins, and we report here that the localization of Yck2p w ithin the membrane is dynamic through the cell cycle. Using a functional gr een fluorescent protein (GFP) fusion, we have observed that Yck2p is concen trated at sites of polarized growth during bud morphogenesis. At cytokinesi s, GFP-Yck2p becomes associated with a ring at the bud neck and then appear s as a patch of fluorescence, apparently coincident with the dividing membr anes. The bud neck association of Yck2p at cytokinesis does not require an intact septin ring, and septin assembly is altered in a Yck-deficient mutan t. The sites of GFP-Yck2p concentration and the defects observed for Yck-de ficient cells together suggest that Yck plays distinct roles in morphogenes is and cytokinesis that are effected by differential localization.