Function of mammalian LKB1 and Ca2+/calmodulin-dependent protein kinase kinase a as Snf1-activating kinases in yeast

The Snf1/AMP-activated protein kinase (AMPK) family is important for metabolic regulation in response to stress. In the yeast Saccharomyces cerevisiae, the Snf1 kinase cascade comprises three Snf1-activating kinases, Pak1, Tos3, and Elm1. The only established mammalian AMPK kinase is LKB1. We show that LKB1 functions heterologously in yeast. In pak1D tos3D elm1D cells, LKB1 activated Snf1 catalytic activity and conferred a Snf+ growth phenotype. Coexpression of STRADa and MO25a, which form a complex with LKB1, enhanced LKB1 function. Thus, the Snf1/AMPK kinase cascade is functionally conserved between yeast and mammals. Ca2+/calmodulin-dependent kinase kinase (CaMKK) shows more sequence similarity to Pak1, Tos3, and Elm1 than does LKB1. When expressed in pak1D tos3D elm1D cells, CaMKKa activated Snf1 catalytic activity, restored the Snf+ phenotype, and also phosphorylated the activation loop threonine of Snf1 in vitro. These findings indicate that CaMKKa is a functional member of the Snf1/AMPK kinase family and support CaMKKa as a likely candidate for an AMPK kinase in mammalian cells. Analysis of the function of these heterologous kinases in yeast provided insight into the regulation of Snf1. When activated by LKB1 or CaMKKa, Snf1 activity was significantly inhibited by glucose, suggesting that a mechanism independent of the activating kinases can mediate glucose signaling in yeast. Finally, this analysis provided evidence that Pak1 functions in another capacity, besides activating Snf1, to regulate the nuclear enrichment of Snf1 protein kinase in response to carbon stress.