CLONING AND CHARACTERIZATION OF PRA1, A GENE ENCODING A NOVEL PH-REGULATED ANTIGEN OF CANDIDA-ALBICANS

Candida albicans is an opportunistic fungal pathogen of humans. The ce ll wall of the organism defines the interface between the pathogen and host tissues and is likely to play an essential and pivotal sole in t he host-pathogen interaction. The components of the cell wall critical to this interaction are undefined. Immunoscreening of a lambda expres sion library with sera raised against mycelial cell walls of C. albica ns was used to identify genes encoding cell surface proteins. One of t he positive clones represented a candidal gene that was differentially expressed in response to changes in the pH of the culture medium. Max imal expression occurred at neutral pH, with no expression detected be low pH 6.0. On the basis of the expression pattern, the corresponding gene was designated PRA1, for pH-regulated antigen. The protein predic ted from the nucleotide sequence was 299 amino acids long with motifs characteristic of secreted glycoproteins. The predicted surface locali zation and N glycosylation of the protein were directly demonstrated b y cell fractionation and immunoblot analysis. Deletion of the gene imp arted a temperature-dependent defect in hypha formation, indicating a role in morphogenesis. The PRA1 protein was homologous to surface anti gens of Aspergillus spp. which react with serum from aspergillosis pat ients, suggesting that the PRA1 protein may have a role in the host pa rasite interaction during candidal infection.