The Snf1 protein kinase is an essential component of the glucose starvation signalling pathway in Saccharomyces cerevisiae. We have used the two-hybrid system to identify a new protein, Sip5, that interacts with the Snf1 kinase complex in response to glucose limitation. Coimmunoprecipitation studies confirmed the association of Sip5 and Snf1 in cell extracts. We found that Sip5 also interacts strongly with Reg1, the regulatory subunit of the Reg1/Glc7 protein phosphatase 1 complex, in both two-hybrid and coimmunoprecipitation assays. Previous work showed that Reg1/Glc7 interacts with the Snf1 kinase under glucose-limiting conditions and negatively regulates its activity. Sip5 is the first protein that has been shown to interact with both Snf1 and Reg1/Glc7. Genetic analysis showed that the two-hybrid interaction between Reg1 and Snf1 is reduced threefold in a sip5Delta mutant. These findings suggest that Sip5 facilitates the interaction between the Reg1/Glc7 phosphatase and the Snf1 kinase.