AGα1 Is the Structural Gene for the Saccharomyces cerevisiae α-Agglutinin, a Cell Surface Glycoprotein Involved in Cell-Cell Interactions during Mating

Abstract

We have cloned the α-agglutinin structural gene, AGα1, by the isolation of α-specific agglutination-defective mutants, followed by isolation of a complementing plasmid. Independently isolated α-specific agglutination-defective mutations were in a single complementation group, consistent with biochemical results indicating that the α-agglutinin is composed of a single polypeptide. Mapping results suggested that the complementation group identified by these mutants is allelic to the aga1 mutation identified previously. Expression of AGα1 RNA was α specific and inducible by α-factor. Sequences similar to the consensus sequences for positive control by MATα1 and pheromone induction were found upstream of the AGα1 initiation codon. The AGα1 gene could encode a 650-amino-acid protein with a putative signal sequence, 12 possible N-glycosylation sites, and a high proportion of serine and threonine residues, all of which are features expected for the α-agglutinin sequence. Disruption of the AGα1 gene resulted in failure to express α-agglutinin and loss of cellular agglutinability in α cells. An Escherichia coli fusion protein containing 229 amino acids of the AGα1 sequence was recognized by an anti-α-agglutinin antibody. In addition, the ability of this antibody to inhibit agglutination was prevented by this fusion protein. These results indicate that AGα1 encodes α-agglutinin. Features of the AGα1 gene product suggest that the amino-terminal half of the protein contains the a-agglutinin binding domain and that the carboxy-terminal half contains a cell surface localization domain, possibly including a glycosyl phosphati-dylinositol anchor.