Cunle Wu

Interaction of a G-protein |[beta]|-subunit with a conserved sequence in Ste20/PAK family protein kinases

Serine/threonine protein kinases of the Ste20/PAK family have been implicated in the signalling from heterotrimeric G proteins to mitogen-activated protein (MAP) kinase cascades,. In the yeast Saccharomyces cerevisiae, Ste20 is involved in transmitting the mating-pheromone signal from the βγ-subunits (encoded by the STE4 and STE18 genes, respectively) of a heterotrimeric G protein to a downstream MAP kinase cascade. We have identified a binding site for the G-protein β-subunit (Gβ) in the non-catalytic carboxy-terminal regions of Ste20 and its mammalian homologues, the p21-activated protein kinases (PAKs). Association of Gβ with this site in Ste20 was regulated by binding of pheromone to the receptor. Mutations in Gβ and Ste20 that prevented this association blocked activation of the MAP kinase cascade. Considering the high degree of structural and functional conservation of Ste20/PAK family members and G-protein subunits, our results provide a possible model for a role of these kinases in Gβγ-mediated signal transduction in organisms ranging from yeast to mammals.

Functional characterization of the Cdc42p binding domain of yeast Ste20p protein kinase.

Ste20p from Saccharomyces cerevisiae belongs to the Ste20p/p65PAK family of protein kinases which are highly conserved from yeast to man and regulate conserved mitogen‐activated protein kinase pathways. Ste20p fulfills multiple roles in pheromone signaling, morphological switching and vegetative growth and binds Cdc42p, a Rho‐like small GTP binding protein required for polarized morphogenesis. We have analyzed the functional consequences of mutations that prevent binding of Cdc42p to Ste20p. The complete amino‐terminal, non‐catalytic half of Ste20p, including the conserved Cdc42p binding domain, was dispensable for heterotrimeric G‐protein‐mediated pheromone signaling. However, the Cdc42p binding domain was necessary for filamentous growth in response to nitrogen starvation and for an essential function that Ste20p shares with its isoform Cla4p during vegetative growth. Moreover, the Cdc42p binding domain was required for cell–cell adhesion during conjugation. Subcellular localization of wild‐type and mutant Ste20p fused to green fluorescent protein showed that the Cdc42p binding domain is needed to direct localization of Ste20p to regions of polarized growth. These results suggest that Ste20p is regulated in different developmental pathways by different mechanisms which involve heterotrimeric and small GTP binding proteins.

Pheromone Response in Yeast: Association of Bem1p with Proteins of the MAP Kinase Cascade and Actin

Haploid cells of the yeast Saccharomyces cerevisiae respond to mating pheromones with polarized growth toward the mating partner. This morphological response requires the function of the cell polarity establishment protein Bem1p. Immunochemical and two-hybrid protein interaction assays revealed that Bem1p interacts with two components of the pheromone-responsive mitogen-activated protein (MAP) kinase cascade, Ste20p and Ste5p, as well as with actin. Mutants of Bem1p that are associated with defective pheromone-induced polarized morphogenesis interacted with Ste5p and actin but not with Ste20p. Thus, the association of Bem1p with Ste20p and Ste5p may contribute to the conveyance of spatial information that regulates polarized rearrangement of the actin cytoskeleton during yeast mating.

Activation of Myosin-I by Members of the Ste20p Protein Kinase Family

The heavy chain of myosin-ID isolated from Dictyostelium was identified as an in vitro substrate for members of the Ste20p family of serine/threonine protein kinases which are thought to regulate conserved mitogen-activated protein kinase pathways. Yeast Ste20p and Cla4p and mammalian p21-activated protein kinase (PAK) phosphorylated the heavy chain to 0.5-0.6 mol of Pi/mol and stimulated the actin-dependent Mg2+-ATPase activity to an extent equivalent to that of the Ste20p-like myosin-I heavy chain kinase isolated from Dictyostelium. PAK purified from rat brain required GTPγS-Cdc42 to express full activity, whereas recombinant mouse mPAK3 fused to glutathione S-transferase and purified from bacteria, and Ste20p and Cla4p purified from yeast extracts were fully active without GTPγS-Cdc42. These results suggest, together with the high degree of structural and functional conservation of Ste20p family members and myosin-I isoforms, that myosin-I activation by Ste20p family protein kinases may contribute to the regulation of morphogenetic processes in organisms ranging from yeast to mammalian cells.

The phosphorylation site for Ste20p-like protein kinases is essential for the function of myosin-I in yeast.

The budding yeast Saccharomyces cerevisiae has two functionally redundant myosin-I isoforms encoded by the MYO3 and MYO5 genes. The function shared by these myosin proteins is required for proper yeast budding. Serine residue 357 in the head domain of Myo3p, conserved among myosin-I proteins including yeast Myo5p, was identified as a unique phosphorylation site for the serine/threonine protein kinase Ste20p and its closely related isoform Cla4p. These protein kinases share a function that is also essential for budding. Replacement of serine 357 with alanine disrupted the in vivo function of Myo3p, whereas this function was maintained by changing the serine residue to aspartate. This mutant version failed to compensate the growth defect of cells which lack both Ste20p and Cla4p, suggesting that myosin-I is not the only essential target of these protein kinases. Our results suggest that phosphorylation of the head domain by Ste20p-like protein kinases plays an essential role in the function of myosin-I in yeast cells.

Rho5p is involved in mediating the osmotic stress response in Saccharomyces cerevisiae, and its activity is regulated via Msi1p and Npr1p by phosphorylation and ubiquitination.

ABSTRACT Small GTPases of the Rho family act as molecular switches, and modulation of the GTP-bound state of Rho proteins is a well-characterized means of regulating their signaling activity in vivo. In contrast, the regulation of Rho-type GTPases by posttranslational modifications is poorly understood. Here, we present evidence of the control of the Saccharomyces cerevisiae Rho-type GTPase Rho5p by phosphorylation and ubiquitination. Rho5p binds to Ste50p, and the expression of the activated RHO5(Q91H) allele in an Δste50 strain is lethal under conditions of osmotic stress. An overexpression screen identified RGD2 and MSI1 as being high-copy suppressors of the osmotic sensitivity of this lethality. Rgd2p had been identified as being a possible Rho5p GTPase-activating protein based on an in vitro assay; this result supports its function as a regulator of Rho5p activity in vivo. MSI1 was previously identified as being a suppressor of hyperactive Ras/cyclic AMP signaling, where it antagonizes Npr1p kinase activity and promotes ubiquitination. Here, we show that Msi1p also acts via Npr1p to suppress activated Rho5p signaling. Rho5p is ubiquitinated, and its expression is lethal in a strain that is compromised for proteasome activity. These data identify Rho5p as being a target of Msi1p/Npr1p regulation and describe a regulatory circuit involving phosphorylation and ubiquitination.

Phosphorylation of the MAPKKK Regulator Ste50p in Saccharomyces cerevisiae: a Casein Kinase I Phosphorylation Site Is Required for Proper Mating Function

ABSTRACT The Ste50 protein of Saccharomyces cerevisiae is a regulator of the Ste11p protein kinase. Ste11p is a member of the MAP3K (or MEKK) family, which is conserved from yeast to mammals. Ste50p is involved in all the signaling pathways that require Ste11p function, yet little is known about the regulation of Ste50p itself. Here, we show that Ste50p is phosphorylated on multiple serine/threonine residues in vivo. Threonine 42 (T42) is phosphorylated both in vivo and in vitro, and the protein kinase responsible has been identified as casein kinase I. Replacement of T42 with alanine (T42A) compromises Ste50p function. This mutation abolishes the ability of overexpressed Ste50p to suppress either the mating defect of a ste20 ste50 deletion mutant or the mating defect of a strain with a Ste11p deleted from its sterile-alpha motif domain. Replacement of T42 with a phosphorylation-mimetic aspartic acid residue (T42D) permits wild-type function in all assays of Ste50p function. These results suggest that phosphorylation of T42 of Ste50p is required for proper signaling in the mating response. However, this phosphorylation does not seem to have a detectable role in modulating the high-osmolarity glycerol synthesis pathway.