IDENTIFICATION OF A NOVEL CASEIN KINASE-1 HOMOLOG IN FISSION YEAST SCHIZOSACCHAROMYCES-POMBE

Fission yeast cells lacking either the ste9(+) - or rum1(+) function c annot enter the cell differentiation pathway upon nutritional starvati on. Sterility in both mutants is suppressed by the srs1-S41 mutation. A gene encoding a novel casein kinase-1 (CK1) isoform, cki3(+), was is olated as a high-copy-number suppressor gene of the srs1 mutation. Cki 3 protein is structurally more related to the Cki/Yck subfamily protei ns than those of the Hhp/Hrr25 subfamily. A mutant cki3 gene in which a highly conserved lysine residue in the kinase subdomain II was subst ituted to arginine lost the ability to recover the growth defect in th e srs1 mutant, indicating that catalytic activity was necessary for su ppression. Gene disruption revealed that cki3(+) was dispensable for c ell viability, and cells lacking functional cki3(+) exhibited no chara cteristic phenotype. Thus, S. pombe has three highly related CK1 isofo rms (Cki1, Cki2 and Cki3), but none of them has an essential function. (C) 1998 Elsevier Science B.V. All rights reserved.