A Protein Kinase Substrate Identified by the Two-Hybrid System

A genetic method, the two-hybrid system, was used to identify four genes encoding proteins that interact with the SNF1 protein kinase from yeast. One of the genes, SIP1, was independently isolated as a multicopy suppressor of defects caused by reduced SNF1 kinase activity, and genetic evidence supports its function in the SNF1 pathway. The SIP1 protein co-immunoprecipitated with SNF1 and was phosphorylated in vitro. Thus, the two-hybrid system, which is applicable to any cloned gene, can be used to detect physical interactions between protein kinases and functionally related substrate proteins.