Fungal Gib2, an Atypical Gβ Subunit

Abstract

The genome of Cryptococcus neoformans , which causes meningoencephalitis, has been sequenced, revealing three Gα subunits (Gpa1, Gpa2, and Gpa3) and a single Gβ subunit (Gbp1). Palmer et al . used a yeast two-hybrid screen to identify Gib2 (Gpa1-interacting protein-β) as a partner for Gpa1, and this interaction was confirmed by protein pull-down assay. Although sequence comparisons indicated that Gib2 was more similar to proteins of the RACK1 (receptor for activated protein kinase C1) family than to Gβ proteins, modeling of the protein structure revealed a seven-bladed β propeller structure like that of Gβ proteins. Yeast two-hybrid assays and pull-down assays confirmed that Gib2 interacted with C. neoformans Gγ proteins (Gpg1 and Gpg2). Overexpression or suppression of Gib2 expression did not alter responsiveness to mating pheromone. However, the Gib2 gene was essential, and analysis of gpa1 -deficient strains overexpressing Gib2 showed that Gib2 restored melanin production and capsule formation, two virulence factors. Gib2 also restored cyclic adenosine monophosphate (cAMP) production in gpa1 -deficient strains. Thus, Gib2 appears to serve as an atypical Gβ in a cAMP pathway that contributes to virulence in C. neoformans . D. A. Palmer, J. K. Thompson, L. Li, A. Prat, P. Wang, Gib2, a novel Gβ-like/RACK1 homolog, functions as a Gβ subunit in cAMP signaling and is essential in Cryptococcus neoformans . J. Biol. Chem. 281 , 32596-32605 (2006). [Abstract] [Full Text]