PRENYLATED ISOFORMS OF YEAST CASEIN KINASE-I, INCLUDING THE NOVEL YCK3P, SUPPRESS THE GCS1 BLOCKAGE OF CELL-PROLIFERATION FROM STATIONARY-PHASE

The GCS1 gene of the budding yeast Saccharomyces cerevisiae mediates t he resumption of cell proliferation from the starved, stationary-phase state, Here we identify yeast genes that, in increased dosages, overc ome the growth defect of gcs1 Delta mutant cells, Among these are YCK1 (CK12) and YCK2 (CKT1), encoding membrane-associated casein kinase I, and YCK3, encoding a novel casein kinase I isoform, Some Yck3p gene p roduct was found associated with the plasma membrane, like Yck1p and Y ck2p, but most cofractionated with the nucleus, like another yeast cas ein kinase I isoform, Hrr25p, Genetic studies showed that YCK3 and HRR 25 constitute an essential gene family and that Yck3p can weakly subst itute for Yck1p-Yck2p. For gcs1 Delta suppression, both a protein kina se domain and a C-terminal prenylation motif were shown to be necessar y, An impairment in endocytosis was found for gcs1 Delta mutant cells, which was alleviated by an increased YCK2 gene dosage, The ability of an increased casein kinase I gene dosage to suppress the effects caus ed by the absence of Gcs1 Delta suggests that Gcs1 Delta and Yck1p-Yck 2p affect parallel pathways.