Expression cloning of the Candida albicans CSA1 gene encoding a mycelial surface antigen by sorting of Saccharomyces cerevisiae transformants with monoclonal antibody-coated magnetic beads

The mycelial surface antigen recognized by monoclonal antibody (mAb) 4E1 ha s previously been shown to be present predominantly in the terminal third o f the hyphal structures in Candida albicans. We report here the expression cloning of the corresponding gene (CSA1) by mAb 4E1-coated magnetic beads s orting of Saccharomyces cerevisiae transformants expressing a C. albicans g enomic library. The strategy is both highly selective and highly sensitive and provides an additional genetic tool for the cloning and characterizatio n of C. albicans genes encoding surface proteins. CSA1 is an intronless gen e encoding a 1203-residue protein composed of repetitive motifs and domains . Northern analysis indicates that CSA1 is preferentially expressed during the mycelial growth phase, although a low level of CSA1 mRNA can be detecte d in the yeast form. As evidenced by indirect immunofluorescence microscopy with mAb 4E1, Csa1p is not randomly distributed over the surface of yeast cells, but localizes predominantly in the growing buds. This suggests that the distribution of Csa1p may be restricted to sites of cell surface elonga tion. Both heterozygous and homozygous C. albicans csa1 Delta mutants are v iable. Upon induction of mycelial growth, the number and size of hyphal str uctures derived from the mutants are similar to those observed in the paren tal wildtype strain. The physiological role of Csa1p has yet to be determin ed. However, the presence in Csa1p of repeated cysteine-rich hydrophobic do mains with significant sequence similarity to motifs found in surface prote ins (Ag2 and Pth11) from two distantly related fungal pathogens (Coccidioid es immitis and Magnaporthe grisea respectively) suggests a common function in host interaction.