Acute glucose deprivation rapidly but transiently depolarizes the actin cytoskeleton and inhibits translation initiation in Saccharomyces cerevisiae. Neither rapid actin depolarization nor translation inhibition upon glucose removal occurs in a reg1 disruptant, which is defective in glucose repression, or in the tpk1 w mutant, which has weak cAPK activity. In the absence of additional glucose, recovery of either actin polarization or translation initiation relies upon respiration, the Snf1p protein kinase, and the transcription factors Msn2p and Msn4p. The readdition of glucose to glucose-starved cells causes a rapid recovery of actin polarization as well as translation initiation without respiration. These results indicate that the simultaneous regulation of actin polarization and translation initiation is divided into three reactions: 1) rapid shutdown depending on Reg1p and cAPK after glucose removal, 2) slow adaptation depending on Snf1p and Msn2p/4p in the absence of glucose, and 3) rapid recovery upon readdition of glucose. On glucose removal, translation initiation is rapidly inhibited in a rom2 disruptant, which is defective in rapid actin depolarization, whereas rapid actin depolarization occurs in a pop2/caf1 disruptant, which is defective in rapid inhibition of translation initiation. Thus, translation initiation and actin polarization seem to be simultaneously but independently regulated by glucose deprivation.
INTRODUCTIONThe actin cytoskeleton provides the structural basis for cell polarity in the yeast Saccharomyces cerevisiae and is organized primarily into two morphologically distinct structures: cortical patches and cables (Botstein et al., 1997). Cortical patches are discrete cytoskeletal bodies at the plasma membrane, whereas actin cables are long bundles of actin filaments that are oriented along the mother-bud axis. Both structures are polarized during the budding phase of the cell cycle. During early G1 phase, cortical patches and cables are randomly distributed. As a bud emerges, cortical patches cluster at the growing tip and cables extend from the mother cell into the bud. Near the end of bud growth, the patches and cables redistribute randomly, disrupting the asymmetric distribution of the actin cytoskeleton. At the end of mitosis, patches accumulate in and cables reorient toward the mother-bud neck in connection with cytokinesis and septum formation (Botstein et al., 1997).The actin cytoskeleton is also regulated in response to environmental stimuli. Mating pheromone causes polarization of the actin cytoskeleton toward the tip of the mating projection via the Cdc42p GTPase (Read et al., 1992;Leberer et al., 1997). In contrast, stresses such as osmotic shock, heat shock, and glucose starvation depolarize the actin cytoskeleton in budded cells as shown in Figure 1A (Novick et al., 1989;Chowdhury et al., 1992;Delley and Hall, 1999). In particular, the depolarization by heat or osmotic stress is known to be a rapid and transient reaction. A recent report has indicated that the rapid depol...