BACKGROUND: The pathogenic fungus Candida albicans is capable of a morphological transition from a unicellular budding yeast to a filamentous form. Extensive filamentous growth leads to the formation of mycelia displaying hyphae with branches and lateral buds. Hyphae have been observed to adhere to and invade host tissues more readily than the yeast form, suggesting that filamentous growth may contribute to the virulence of this major human pathogen. A molecular and genetic understanding of the potential role of morphological switching in the pathogenicity of C. albicans would be of significant benefit in view of the increasing incidence of candidiasis. RESULTS: The CaCLA4 gene of C. albicans was cloned by functional complementation of the growth defect of cells of the budding yeast Saccharomyces cerevisiae deleted for the STE20 gene and the CLA4 gene. CaCLA4 encodes a member of the Ste20p family of serine/threonine protein kinases and is characterized by a pleckstrin homology domain and a Cdc42p-binding domain in its amino-terminal non-catalytic region. Deletion of both alleles of CaCLA4 in C. albicans caused defects in hyphal formation in vitro, in both synthetic liquid and solid media, and in vivo in a mouse model for systemic candidiasis. The gene deletions reduced colonization of the kidneys in infected mice and suppressed C. albicans virulence in the mouse model. CONCLUSIONS: Our results demonstrate that the function of the CaCla4p protein kinase is essential for virulence and morphological switching of C. albicans in a mouse model. Thus, hyphal formation of C. albicans mediated by CaCla4p may contribute to the pathogenicity of this dimorphic fungus, suggesting that regulators of morphological switching may be useful targets for antifungal drugs.
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