Yeast glycogen synthase kinase 3 is involved in protein degradation in cooperation with Bul1, Bul2, and Rsp5

The yeast Saccharomyces cerevisiae has four genes, MCK1, MDS1 (RIM11), MRK1 , and YOL128c, that encode glycogen synthase kinase 3 (GSK-3) homologs. The gsk-3 null mutant, in which these four genes are disrupted, shows temperat ure sensitivity, which is suppressed by the expression of mammalian GSK-3 b eta and by an osmotic stabilizer. Suppression of temperature sensitivity by an osmotic stabilizer is also observed in the bul1 bul2 double null mutant , and the temperature sensitivity of the bul1 bul2 double null mutant is su ppressed by multiple copies of MCK1. We have screened rog mutants (revertan ts of gsk-3) which suppress the temperature sensitivity of the mck1 mds1 do uble null mutant and found that two of them, rog1 and rog2, also suppress t he temperature sensitivity of the bul1 bul2 double null mutant. Bul1 and Bu l2 have been reported to bind to Rsp5, a hect (for homologous to E6-associa ted-protein carboxyl terminus)-type ubiquitin ligase, but involvement of Bu l1 and Bul2 in protein degradation has not been demonstrated. We find that Rog1, but not Rog2, is stabilized in the gsk-3 null and the bul1 bul2 doubl e nub mutants. Rog1 binds directly to Rsp5, and their interaction is depend ent on GSK-3. Furthermore, Rog1 is stabilized in the npi1 mutant, in which RSP5 expression levels are reduced. These results suggest that yeast GSK-3 regulates the stability of Rog1 in cooperation with Bul1, Bul2, and Rsp5.