Abstract
Cryptococcus neoformans causes often-fatal fungal meningoencephalitis in immunocompromised individuals. While the exact disease mechanisms remain elusive, signal transduction pathways mediated by key elements such as G-protein α subunit Gpa1, small GTPase Ras1, and atypical Gβ-like/RACK1 protein Gib2 are known to play important roles in C. neoformans virulence. Gib2 is important for normal growth, differentiation, and pathogenicity, and it also positively regulates cAMP levels in conjunction with Gpa1. Interestingly, Gib2 displays a scaffold protein property by interacting with a wide variety of cellular proteins. To explore Gib2 global regulatory functions, we performed two-dimensional differential gel electrophoresis (DIGE) analysis and found that GIB2 disruption results in an increased expression of 304 protein spots (43.4%) and a decreased expression of 396 protein spots (56.6%). Analysis of 96 proteins whose expression changes were deemed significant (≥ +/- 1.5- fold) revealed that 75 proteins belong to at least 12 functional protein groups. Among them, eight groups have the statistical stringency of p ≤ 0.05, and four groups, including Hsp70/71 heat shock protein homologs and ribosomal proteins, survived the Bonferroni correction. This finding is consistent with earlier established roles for the human Gβ-like/RACK1 and the budding yeast Saccharomyces cerevisiae Asc1. It suggests that Gib2 could also be part of the complex affecting ribosomal biogenesis and protein translation in C. neoformans. Since eukaryotic Hsp70/71 proteins are involved in the facilitation of nascent protein folding, processing, and protection of cells against stress, we also propose that Gib2-regulated stress responses are linked to fungal virulence. Collectively, our study supports a conserved role of Gβ-like/RACK/Gib2 proteins in the essential cellular process of ribosomal biogenesis and protein translation. Our study also highlights a multifaceted regulatory role of Gib2 in the growth and pathogenicity of C. neoformans.
Highlights
Cryptococcus neoformans is a basidiomycetous fungal pathogen that has a predilection for the human central nervous system causing meningoencephalitis in individuals with a compromised immune status
To explore the global regulatory role of Gib2 in C. neoformans, we examined genome-wide targets of Gib2 by comparative proteomic analysis using two-dimensional difference gel electrophoresis (DIGE) coupled with mass spectrometry that yields novel findings
The fungus exhibits a predilection for the human central nervous system where it can cause potentially fatal meningoencephalitis if untreated
Summary
Cryptococcus neoformans is a basidiomycetous fungal pathogen that has a predilection for the human central nervous system causing meningoencephalitis in individuals with a compromised immune status. Guanine nucleotide binding protein (G-protein)-mediated signal transduction pathways are one of the most important mechanisms by which eukaryotic cells sense extracellular signals and integrate them via intrinsic signals or pathways, such as cAMP or the MAP kinase pathways (reviewed in [2]). In C. neoformans, three Gα protein subunits Gpa, Gpa, and Gpa function in two distinct signaling pathways [3]. Adenylyl cyclase Cac, protein kinase A regulatory subunit Pkr and catalytic subunit Pka, and Cac associated protein Aca govern a cAMP-dependent signaling pathway to directly impact fungal pathogenicity [4,5,6]. Gpa and Gpa function in a distinct pathway(s) to regulate pheromone responsive mating and differentiation that is to certain degree linked to virulence [3,7,8]
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