The Role of Cdc42p Adaptors in Regulating the Filamentous Growth MAP Kinase Pathway.

Rho GTPases regulate cell polarity and signal transduction pathways to control morphogenetic responses in different settings 1. In yeast, the Rho GTPase Cdc42p regulates cell polarity and mitogen-activated protein kinase (MAPK) pathways [mating, filamentous growth or fMAPK, and HOG 2]. Although much is known about how Cdc42p regulates cell polarity and mating, how Cdc42p is activated in the fMAPK pathway is not clear. This question is relevant because filamentous growth is a developmental fungal response to nutirent limitation that occurs in mnay fungal species, including pathogens 3-5. Moreover, although the signaling glycoprotein (Msb2) that regulates the fMAPK pathway has been characterized 6-7, how Msb2p connects to Cdc42p is not clear. To begin to address this question, Cdc42p-dependent MAPK pathways were compared in the filamentous (∑1278b) strain background. Each MAPK pathway showed a unique activation profile, with the fMAPK pathway exhibiting slow activation kinetics compared to the mating and HOG pathways. A previously characterized version of Cdc42p, Cdc42pE100A 8, that is specifically defective for fMAPK pathway signaling, was also defective for interaction with Bem4p, the pathway-specific adaptor for the fMAPK pathway 9. Corresponding residues in Bem4p were identified that were required for interaction with Cdc42p and fMAPK pathway signaling. The polarity adaptor Bem1p also regulated the fMAPK pathway. In the fMAPK pathway, Bem1p recruited the p21-activated kinase (PAK) Ste20p to the plasma membrane, cycled between an open and closed conformation, and interacted with the GEF Cdc24p. Bem1p also regulated effector pathways in different ways, behaving as a multi-functional adaptor in some pathways and like a passive scaffold in others. Genetic suppression tests showed that Bem4p and Bem1p, together with Rsr1p – the bud site GTPase that also regulates the fMAPK pathway 10 - regulated the fMAPK pathway in an ordered sequence. Collectively, the study demonstrates the unique and sequential roles that Rho GTPase adaptors have in regulating MAPK pathways.