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Vol. 19, Issue 1, 181-197, January 2008

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Distinct Roles for Two G–Gβ Interfaces in Cell Polarity Control by a Yeast Heterotrimeric G Protein

Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, MA 01605

Submitted April 27, 2007; Revised October 9, 2007; Accepted October 22, 2007
Monitoring Editor: Daniel Lew

Saccharomyces cerevisiae mating pheromones trigger dissociation of a heterotrimeric G protein (Gβ) into G-guanosine triphosphate (GTP) and Gβ. The Gβ dimer regulates both mitogen-activated protein (MAP) kinase cascade signaling and cell polarization. Here, by independently activating the MAP kinase pathway, we studied the polarity role of Gβ in isolation from its signaling role. MAP kinase signaling alone could induce cell asymmetry but not directional growth. Surprisingly, active Gβ, either alone or with G-GTP, could not organize a persistent polarization axis. Instead, following pheromone gradients (chemotropism) or directional growth without pheromone gradients (de novo polarization) required an intact receptor–Gβ module and GTP hydrolysis by G. Our results indicate that chemoattractant-induced cell polarization requires continuous receptor–Gβ communication but not modulation of MAP kinase signaling. To explore regulation of Gβ by G, we mutated Gβ residues in two structurally distinct G–Gβ binding interfaces. Polarity control was disrupted only by mutations in the N-terminal interface, and not the Switch interface. Incorporation of these mutations into a Gβ–G fusion protein, which enforces subunit proximity, revealed that Switch interface dissociation regulates signaling, whereas the N-terminal interface may govern receptor–Gβ coupling. These findings raise the possibility that the Gβ heterotrimer can function in a partially dissociated state, tethered by the N-terminal interface.

Address correspondence to: Peter M. Pryciak (peter.pryciak{at}umassmed.edu)

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