The AGA1 Product Is Involved in Cell Surface Attachment of the Saccharomyces cerevisiae Cell Adhesion Glycoprotein a-Agglutinin

Abstract

Saccharomyces cerevisiae a and a cells express the complementary cell surface glycoproteins a-agglutinin and α-agglutinin, respectively, which interact with one another to promote cellular aggregation during mating. Treatment of S. cerevisiae a cells with reducing agents releases the binding subunit of α-agglutinin, which has been purified and characterized; little biochemical information on the overall structure of α-agglutinin is available. To characterize α-agglutinin structure and function, we have used a genetic approach to clone an α-agglutinin structural gene (AGA1). Mutants with α-specific agglutination defects were isolated, the majority of which fell into a single complementation group, called aga1. The aga1 mutants showed wild-type pheromone production and response, efficient mating on solid medium, and a mating defect in liquid medium; these phenotypes are characteristic of agglutinin mutants. The AGA1 gene was cloned by complementation; the gene sequence indicated that it could encode a protein of 725 amino acids with high serine and threonine content, a putative N-terminal signal sequence, and a C-terminal hydrophobic sequence similar to signals for the attachment to glycosyl phosphatidylinositol anchors. Active α-agglutinin binding subunit is secreted by aga1 mutants, indicating that AGA1 is involved in cell surface attachment of α-agglutinin. This result suggests that AGA1 encodes a protein with functional similarity to the core subunits of α-agglutinin analogs from other budding yeasts. Unexpectedly, the AGA1 transcript was expressed and induced by pheromone in both a and a cells, suggesting that the α-specific expression of active α-agglutinin results only from α-specific regulation of the α-agglutinin binding subunit.