Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 281, Issue 43, Pages 32596-32605Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M602768200
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- NIAID NIH HHS [AI 054958] Funding Source: Medline
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Canonical G proteins are heterotrimeric, consisting of alpha, beta, and gamma subunits. Despite multiple G alpha subunits functioning in fungi, only a single G beta subunit per species has been identified, suggesting that non-conventional G protein signaling exists in this diverse group of eukaryotic organisms. Using the G alpha subunit Gpa1 that functions in cAMP signaling as bait in a two-hybrid screen, we have identified a novel G beta-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 beta propeller structure characteristic of beta transducins. Gib2 is also shown to interact, respectively, with two G gamma subunit homologs, Gpg1 and Gpg2, similar to the conventional G beta 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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