Overexpression of the Candida albicans ALA1 gene in Saccharomyces cerevisiae results in aggregation following attachment of yeast cells to extracellular matrix proteins, adherence properties similar to those of Candida albicans.

Abstract

Candida albicans maintains a commensal relationship with human hosts, probably by adhering to mucosal tissue in a variety of physiological conditions. We show that adherence due to the C. albicans gene ALA1 when transformed into Saccharomyces cerevisiae, is comprised of two sequential steps. Initially, C. albicans rapidly attaches to extracellular matrix (ECM) protein-coated magnetic beads in small numbers (the attachment phase). This is followed by a relatively slower step in which cell-to-cell interactions predominate (the aggregation phase). Neither of these phases is observed in S. cerevisiae. However, expression of the C. albicans ALA1 gene from a low-copy vector causes S. cerevisiae transformants to attach to ECM-coated magnetic beads without appreciable aggregation. Expression of ALA1 from a high-copy vector results in both attachment and aggregation. Moreover, transcriptional fusion of ALA1 with the galactose-inducible promoters GALS, GALL, and GAL1, allowing for low, moderate, and high levels of inducible transcription, respectively, causes attachment and aggregation that correlates with the strength of the GAL promoter. The adherence of C. albicans and S. cerevisiae overexpressing ALA1 to a number of protein ligands occurs over a broad pH range, is resistant to shear forces generated by vortexing, and is unaffected by the presence of sugars, high salt levels, free ligands, or detergents. Adherence is, however, inhibited by agents that disrupt hydrogen bonds. The similarities in the adherence and aggregation properties of C. albicans and S. cerevisiae overexpressing ALA1 suggest a role in adherence and aggregation for ALA1 and ALA1-like genes in C. albicans.