Gib2, A Novel Gβ-like/RACK1 Homolog, Functions as a Gβ Subunit in cAMP Signaling and Is Essential in Cryptococcus neoformans*

Canonical G proteins are heterotrimeric, consisting of α, β, and γ subunits. Despite multiple Gα subunits functioning in fungi, only a single Gβ subunit per species has been identified, suggesting that non-conventional G protein signaling exists in this diverse group of eukaryotic organisms. Using the Gα subunit Gpa1 that functions in cAMP signaling as bait in a two-hybrid screen, we have identified a novel Gβ-like/RACK1 protein homolog, Gib2, from the human pathogenic fungus Cryptococcus neoformans. Gib2 contains a seven WD-40 repeat motif and is predicted to form a seven-bladed β propeller structure characteristic of β transducins. Gib2 is also shown to interact, respectively, with two Gγ subunit homologs, Gpg1 and Gpg2, similar to the conventional Gβ subunit Gpb1. In contrast to Gpb1 whose overexpression promotes mating response, overproduction of Gib2 suppresses defects of gpa1 mutation in both melanization and capsule formation, the phenotypes regulated by cAMP signaling and associated with virulence. Furthermore, depletion of Gib2 by antisense suppression results in a severe growth defect, suggesting that Gib2 is essential. Finally, Gib2 is shown to also physically interact with a downstream target of Gpa1-cAMP signaling, Smg1, and the protein kinase C homolog Pkc1, indicating that Gib2 is also a multifunctional RACK1-like protein.

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