TOS3 encodes a protein kinase that phosphorylates and activates the Snf1p kinase, which is a member of the family of eukaryotic AMP-activated protein kinases (AMPK) that play key roles in cellular response to nutrient stress (3). Snf1p activation is required for utilization of nonfermentable and non-preferred carbon sources, such as raffinose and glycerol-ethanol (reviewed in 7).

Three kinases function as upstream activators of Snf1p: Tos3p, Sak1p (formerly called Pak1p), and Elm1p. These kinases exhibit sequence similarity in the 300-residue kinase domain, while the N- and C-terminal regions are divergent. Each kinase activates Snf1p through phosphorylation of threonine 210, a conserved residue in the activation loop of the Snf1p kinase domain (3). Tos3p, Sak1p, and Elm1p exhibit a considerable degree of functional redundancy, as only the triple null mutant confers a snf1- phenotype (3, 2). Studies attempting to elucidate distinct roles for the three kinases have shown that, while each kinase does not specifically activate one of the three known isoforms of Snf1p, these isoforms do exhibit preferences for a particular kinase, depending in part on the nature of the available carbon source (8, 4).

TOS3 (target of SBF) was first identified by a genomic screen for promoters that are bound by the SBF (Swi4p-Swi6p) complex, which regulates transcription during the cell cycle (9). TOS3 RNA does not appear to be cell-cycle regulated (10) nor is it regulated in response to carbon source (7). Tos3p localizes to the cytoplasm regardless of the carbon source present during cell growth (7). It appears that Tos3p function is most important during growth on nonfermentable carbon sources. When tos3 null mutants are grown on glycerol-ethanol, they exhibit slow growth, reduced Snf1p catalytic activity, and reduced activation of the Snf1p-dependent CSREs (carbon source responsive elements) of gluconeogenic genes, relative to wild type cells (7).

The catalytic domain of Tos3p is similar to mammalian LKB1 tumor suppressor kinase, which is essential for mammalian embryonic development and is mutated in Peutz-Jeghers syndrome (11, 12, 13). Both LKB1 and Tos3p phosphorylate mammalian AMPK in vitro, providing evidence for their similar function as upstream kinases for the AMPK protein family members (3).

Last updated: 2006-10-24