The Snf1/AMPK protein kinase family is widely conserved in eukaryotes. In Saccharomyces cerevisiae, the Snf1 kinase is an essential element of the glucose response pathway and has diverse regulatory roles. The Snf1 complex contains one of the related proteins Sip1, Sip2 and Gal83, which are also conserved in higher eukaryotes. Previous studies showed that the Sip1/Sip2/Gal83 component plays a structural role in the complex. We present evidence that this component also mediates the interaction of the Snf1 kinase complex with specific targets. We show that Gal83 mediates the association of the kinase with Sip4, a Snf1-regulated transcription activator of gluconeogenic genes. Gal83 interacts with Sip4 in two-hybrid assays in vivo, and bacterially expressed proteins bind in vitro. Moreover, Gal83 is required for the two-hybrid interaction of Sip4 with the Snf1 kinase. Gal83 also facilitates the rapid Snf1-dependent phosphorylation and activation of Sip4 in response to glucose limitation, indicating that Gal83 mediates the functional interaction of Snf1 with Sip4. Evidence indicates that Sip1 and Sip2 do not interact with Sip4. We propose that members of the Sip1/Sip2/Gal83 family confer specificity to the kinase complex in its interactions with target proteins.