bSut1 is a transcriptional regulator of the Zn(II) 2 Cys 6 family in the budding yeast Saccharomyces cerevisiae. The only function that has been attributed to Sut1 is sterol uptake under anaerobic conditions. Here, we show that Sut1 is also expressed in the presence of oxygen, and we identify a novel function for Sut1. SUT1 overexpression blocks filamentous growth, a response to nutrient limitation, in both haploid and diploid cells. This inhibition by Sut1 is independent of its function in sterol uptake. Sut1 downregulates the expression of GAT2, HAP4, MGA1, MSN4, NCE102, PRR2, RHO3, and RHO5. Several of these Sut1 targets (GAT2, HAP4, MGA1, RHO3, and RHO5) are essential for filamentation in haploids and/or diploids. Furthermore, the expression of the Sut1 target genes, with the exception of MGA1, is induced during filamentous growth. We also show that SUT1 expression is autoregulated and inhibited by Ste12, a key transcriptional regulator of filamentation. We propose that Sut1 partially represses the expression of GAT2, HAP4, MGA1, MSN4, NCE102, PRR2, RHO3, and RHO5 when nutrients are plentiful. Filamentation-inducing conditions relieve this repression by Sut1, and the increased expression of Sut1 targets triggers filamentous growth.

In Saccharomyces cerevisiae, the Rho-type GTPase Cdc42 regulates polarized growth through its effectors, including the p21-activated kinases (PAKs) Ste20, Cla4, and Skm1. Previously, we demonstrated that Ste20 interacts with several proteins involved in sterol synthesis that are crucial for cell polarization. Under anaerobic conditions, sterols cannot be synthesized and need to be imported into cells. Here, we show that Ste20, Cla4, and Skm1 form a complex with Sut1, a transcriptional regulator that promotes sterol uptake. All three PAKs can translocate into the nucleus and down-regulate the expression of genes involved in sterol uptake, including the Sut1 targets AUS1 and DAN1 by a novel mechanism. Consistently, deletion of either STE20, CLA4, or SKM1 results in an increased sterol influx and PAK overexpression inhibits sterol uptake. For Ste20, we demonstrate that the down-regulation of gene expression requires nuclear localization and kinase activity of Ste20. Furthermore, the Ste20-mediated control of expression of sterol uptake genes depends on SUT1 but is independent of a mitogen-activated protein kinase signaling cascade. Together, these observations suggest that PAKs translocate into the nucleus, where they modulate expression of sterol uptake genes via Sut1, thereby controlling sterol homeostasis. INTRODUCTIONThe small Rho GTPase Cdc42 plays a central role in the regulation of cellular polarity in eukaryotic cells (EtienneManneville, 2004;Jaffe and Hall, 2005). In the budding yeast Saccharomyces cerevisiae, Cdc42 regulates different types of polarized growth during various phases of its life cycle, including budding during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon nutrient limitation (Park and Bi, 2007). Cdc42 promotes polarized growth through multiple pathways, including polarization of the actin cytoskeleton and directed vesicle trafficking. Among the Cdc42 effectors that trigger these pathways are Ste20, Cla4 and Skm1, members of the p21-activated kinase (PAK) family of serine/threonine protein kinases (Hofmann et al., 2004;Park and Bi, 2007).Cdc42 recruits Ste20 and Cla4 from the cytoplasm and activates them at the plasma membrane at sites of polarized growth. Therefore, Ste20 and Cla4 are enriched at tips of buds and mating projections (Peter et al., 1996;Leberer et al., 1997;Holly and Blumer, 1999). The recruitment of Ste20 and Cla4 to these sites is governed by protein-protein as well as protein-lipid interactions. PAKs carry a conserved Cdc42/ Rac-interactive binding (CRIB) domain that mediates binding to Cdc42 and regulates their activity (Cvrckova et al., 1995;Peter et al., 1996;Leberer et al., 1997;Lamson et al., 2002;Ash et al., 2003). Ste20 and Cla4 also bind to Bem1, a scaffold protein that brings Cdc42, its activator Cdc24 and either Ste20 or Cla4, into proximity (Leeuw et al., 1998;Gulli et al., 2000;Bose et al., 2001;Yamaguchi et al., 2007). In addition, phosphoinositide-binding domains promote the association with membrane lip...

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