Ubiquitin-dependent Degradation of Sic1
In order to be recognized by Cdc4 of the SCF complex, Sic1 has to be phosphorylated, often by Cyclin-Cdk complexes, at least at 6 of the 9 cdk sites (Fig. 2). Sic1 can also be phosphorylated by other kinases, such as Pho85-Pc11, a kinase which becomes essential when Cln1 and Cln2 are absent. Sic1 has also a role in the response to osmostress. The stress-activated protein kinase (SAPK) Hog1 phosphorylates Sic1 at a single residue at the carboxyl terminus. This leads to downregulation of cyclin expression and Sic1 stabilization which arrests the cell cycle.
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