The Cdc42 effectors Ste20, Cla4 and Skm1 down-regulate the expression of genes involved in sterol uptake by a MAPK-independent pathway

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 (MAPK) signalling cascade. Taken together, these observations suggest that PAKs translocate into the nucleus, where they modulates expression of sterol uptake genes via Sut1, thereby controlling sterol homeostasis. 2 ht tp :// do c. re ro .c h