The signaling mucin Msb2 interacts with the HOG pathway osmosensor Sho1 and polarity establishment GTPase Cdc42 to promote MAPK-dependent activation of the filamentous growth pathway.
Paul J. Cullen (1), Ellie Graham (2), Walid Sabbagh (3), Lee Bardwell (3), George F. Sprague, Jr. (2)
(1) Institute of Molecular Biology, University of Oregon, Eugene OR 97403, Address starting September 1st: Department of Biological Sciences, SUNY at Buffalo, Buffalo, NY 14260;
(2) Institute of Molecular Biology, University of Oregon, Eugene OR 97403;
(3) Department of Developmental and Cell Biology, University of California, Irvine, Irvine, CA 92697
Budding yeast undergo filamentous growth (FG), a developmental response to nutrient limitation thought to facilitate foraging. One of the signaling pathways required for FG is a canonical Cdc42/PAK and MAPK-dependent pathway. At the head of this FG pathway are two integral-membrane cell-surface factors: Sho1, which is also the presumptive osmosensor for the HOG pathway, and Msb2, which appears to be a FG-pathway specific factor and is a glycosylated, signaling mucin. What is the relationship between Msb2 and Sho1, and how do they connect to downstream components of the FG pathway? We show here by coimmunoprecipitation (coIP) analysis that Msb2 and Sho1interact. Moreover, the interaction is dynamic: induction of the FG pathway stimulates interaction between the two proteins. We also demonstrate that Msb2 interacts with Cdc42 by 2-hybrid, coIP, and in vitro binding assays. In contrast to the Msb2-Sho1 interaction, the Msb2-Cdc42 interaction is not influenced by activation of the FG pathway. Msb2 and Sho1 induce phosphorylation and activation of the MAPK for the FG pathway, Kss1, but not the MAPK for the mating pathway, Fus3. Moreover, deletion of two distinct N-terminal regions of the Msb2 protein, one of which is the mucin domain, creates hyperactive variants of Msb2. Taken together, our data suggest that Msb2, Sho1, and Cdc42 comprise a complex and that Msb2 directly promotes the function of these general factors towards the FG pathway.
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