Identification of <i>Candida albicans ALS2</i> and <i>ALS4</i> and Localization of Als Proteins to the Fungal Cell Surface

<jats:title>ABSTRACT</jats:title> <jats:p> Additional genes in the growing <jats:italic>ALS</jats:italic> family of <jats:italic>Candida albicans</jats:italic> were isolated by PCR screening of a genomic fosmid library with primers designed from the consensus tandem-repeat sequence of <jats:italic>ALS1</jats:italic> . This procedure yielded fosmids encoding <jats:italic>ALS2</jats:italic> and <jats:italic>ALS4. ALS2</jats:italic> and <jats:italic>ALS4</jats:italic> conformed to the three-domain structure of <jats:italic>ALS</jats:italic> genes, which consists of a central domain of tandemly repeated copies of a 108-bp motif, an upstream domain of highly conserved sequences, and a domain of divergent sequences 3′ of the tandem repeats. Alignment of five predicted Als protein sequences indicated conservation of N- and C-terminal hydrophobic regions which have the hallmarks of secretory signal sequences and glycosylphosphatidylinositol addition sites, respectively. Heterologous expression of an N-terminal fragment of Als1p in <jats:italic>Saccharomyces cerevisiae</jats:italic> demonstrated function of the putative signal sequence with cleavage following Ala17. This signal sequence cleavage site was conserved in the four other Als proteins analyzed, suggesting identical processing of each protein. Primary-structure features of the five Als proteins suggested a cell-surface localization, which was confirmed by indirect immunofluorescence with an anti-Als antiserum. Staining was observed on mother yeasts and germ tubes, although the intensity of staining on the mother yeast decreased with elongation of the germ tube. Similar to other <jats:italic>ALS</jats:italic> genes, <jats:italic>ALS2</jats:italic> and <jats:italic>ALS4</jats:italic> were differentially regulated. <jats:italic>ALS4</jats:italic> expression was correlated with the growth phase of the culture; <jats:italic>ALS2</jats:italic> expression was not observed under many different in vitro growth conditions. The data presented here demonstrate that <jats:italic>ALS</jats:italic> genes encode cell-surface proteins and support the conclusion that the size and number of Als proteins on the <jats:italic>C. albicans</jats:italic> cell surface vary with strain and growth conditions. </jats:p>