Snf1 kinase complexes with different beta subunits display stress-dependent preferences for the three Snf1-activating kinases.

Three upstream kinases, Pak1, Tos3 and Elm1, are able to activate the Snf1 kinase. Since the Snf1 kinase itself assembles into three complexes that differ in their beta subunit identity, the possibility exists that each upstream kinase might be dedicated to a single isoform of the Snf1 kinase. To test this dedicated activator hypothesis, we generated a series of yeast strains that lacked different combinations of upstream kinases and beta subunits. Cells expressing only one of the three upstream kinases exhibited distinct abilities to activate Snf1, depending on the beta subunit present in the Snf1 kinase complex and the stress imposed on the cells. Pak1 and Gal83 were the most promiscuous. Pak1 was able to activate all three isoforms of the Snf1 kinase under all stress conditions tested. The Gal83 isoform of Snf1 was able to be activated by any of the three upstream kinases under aerobic growth conditions but showed a preference for Pak1 during growth on raffinose. Our results indicate that the three Snf1-activating kinases are not dedicated to specific isoforms of the Snf1 kinase. Instead, the different isoforms of the Snf1 kinase display stress-dependent preferences for the Pak1, Tos3 and Elm1 kinases.