Genetic and carbon source regulation of phosphorylation of Sip1p, a Snf1p-associated protein involved in carbon response in Saccharomyces cerevisiae.

The SIP1 gene of Saccharomyces cerevisiae is a carbon-catabolite-specific negative regulator of GAL gene transcription and acts as a multicopy suppressor of growth defects associated with impaired Snf1p protein kinase activity. The Sip1 protein is known to undergo phosphorylation when associated in vitro with the Snf1 protein kinase. We have carried out in vivo studies of the genetic and carbon control of Sip1p phosphorylation. Metabolic labeling reveals phosphorylation of Sip1p under both carbon catabolite-repressing and non-repressing conditions and in both SNF1 wild-type and snf1-deletion cells. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblot assay, we detect apparent changes in Sip1p phosphorylation states in response to changes in carbon source. At least one dephosphorylation of Sip1p occurs with a shift from non-repressing carbon source to repressing carbon source. The MIG1 gene, acting through SNF1-dependent and SNF1-independent pathways, is required for some Sip1p phosphorylations. REG1 appears to be required for at least one dephosphorylation of Sip1p, whereas SSN6 appears to be required for at least one phosphorylation of Sip1p. These results reveal new complexities in carbon response signaling, and may reflect the involvement of the Sip1 protein in the same complex as the Mig1 and Ssn6 proteins.