Abstract: Members of the AMP-activated protein kinase family, including the Snf1 kinase of Saccharomyces cerevisiae, are activated under conditions of nutrient stress. AMP-activated protein kinases are heterotrimeric complexes composed of a catalytic alpha subunit and regulatory beta and gamma subunits. In this study, the role of the beta subunits in the regulation of Snf1 activity was examined. Yeast express three isoforms of the AMP-activated protein kinase consisting of the Snf1(alpha), Snf4 (gamma) and one of three alternative beta subunits, either Sip1, Sip2 or Gal83. The Gal83 isoform of the Snf1 complex is the most abundant and was analyzed in the greatest detail. All three beta subunits contain a conserved domain referred to as the glycogen binding domain. Deletion of this domain from Gal83 results in a deregulation of the Snf1 kinase as judged by a constitutive activity independent of glucose availability. In contrast, deletion of this homologous domain from the Sip1 and Sip2 subunits had little effect on Snf1 kinase regulation. Therefore the different Snf1 kinase isoforms are regulated through distinct mechanisms which may contribute to their specialized roles in different stress response pathways. In addition, the beta subunits are subjected to phosphorylation. The responsible kinases were identified as Snf1 and casein kinase II. The significance of the phosphorylation is unclear since deletion of the region containing the phosphorylation sites in Gal83 had little effect on the regulation of Snf1 in response to glucose limitation.