The git5 Gß and git11 G Form an Atypical Gß Dimer Acting in the Fission Yeast Glucose/cAMP Pathway

Corresponding author: Charles S. Hoffman, Boston College, Biology Department, Higgins Hall 401B, Chestnut Hill, MA 02467., hoffmacs{at}bc.edu (E-mail)

Communicating editor: J. RINE

Fission yeast adenylate cyclase, like mammalian adenylate cyclases, is regulated by a heterotrimeric G protein. The gpa2 G and git5 Gß are both required for glucose-triggered cAMP signaling. The git5 Gß is a unique member of the Gß family in that it lacks an amino-terminal coiled-coil domain shown to be essential for mammalian Gß folding and interaction with G subunits. Using a git5 bait in a two-hybrid screen, we identified the git11 G gene. Co-immunoprecipitation studies confirm the composition of this Gß dimer. Cells deleted for git11 are defective in glucose repression of both fbp1 transcription and sexual development, resembling cells lacking either the gpa2 G or the git5 Gß. Overexpression of the gpa2 G partially suppresses loss of either the git5 Gß or the git11 G, while mutational activation of the G fully suppresses loss of either Gß or G. Deletion of gpa2 (G), git5 (Gß), or git11 (G) confer quantitatively distinct effects on fbp1 repression, indicating that the gpa2 G subunit remains partially active in the absence of the Gß dimer and that the git5 Gß subunit remains partially active in the absence of the git11 G subunit. The addition of the CAAX box from the git11 G to the carboxy-terminus of the git5 Gß partially suppresses the loss of the G. Thus the G in this system is presumably required for localization of the Gß dimer but not for folding of the Gß subunit. In mammalian cells, the essential roles of the Gß amino-terminal coiled-coil domains and G partners in Gß folding may therefore reflect a mechanism used by cells that express multiple forms of both Gß and G subunits to regulate the composition and activity of its G proteins.

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