Candida albicans and Saccharomyces cerevisiae expressing ALA1/ALS5 adhere to accessible threonine,serine, or alanine patches

Saccharomyces cerevisiae transformed with Candida albicans ALA1/ALS5 exhibits adherence properties similar to C. albicans. Adherence of the fungi to immobilized proteins involves hydrogen bonds, is stable to shear forces, and is resistant to competition from various biological molecules. The specificity determinants of target recognition in Ala1/Als5p-mediated adherence are not known. To determine features of target recognition, proteins and small peptides were covalently coupled at the N-terminus to the surface of carboxylate-modified magnetic beads. C. albicans yeast cells, germ tubes and pseudohyphae and S. cerevisiae expressing the adhesin, Ala1/Als5p, adhered to beads coated with fibronectin, laminin, type IV collagen, bovine serum albumin, and casein. No adherence to beads was observed if a single amino acid was coupled to the beads. However, 10-mer homopolymers of threonine, serine, and alanine served as ligands for adherence. The presence of a minimum of four contiguous threonine residues in a peptide was required for maximal adherence. Coupling of 10-mer peptides from fibronectin and Ala1/Als5p each possessing 5-7 threonine or serine residues also initiated adherence. On the other hand, a collagen and a fibronectin 10-mer peptide with few threonine and serine residues and lysine at the C-terminus did not serve as adherence ligands. Both of them are converted to adherence ligands by adding threonine or serine residues at the C-terminus or removing the lysine residue and adding threonine residues anywhere in the peptide. The presence of lysine at the C-terminus may have resulted in coupling of the peptides at both the N- and C-termini, thus making the threonine residues inaccessible for adherence. Thus, Ala1/Als5p recognizes patches of certain amino acids, which must be accessible before adherence will occur.