We have cloned the a-agglutinin structural gene, AGal, by the isolation of a-specific agglutination-defective mutants, followed by isolation of a complementing plasmid. Independently isolated a-specific agglutinationdefective mutations were in a single complementation group, consistent with biochemical results indicating that the a-agglutinin is composed of a single polypeptide. Mapping results suggested that the complementation group identified by these mutants is allelic to the agal mutation identified previously. Expression ofAGal RNA was a specific and inducible by a-factor. Sequences similar to the consensus sequences for positive control by MATal and pheromone induction were found upstream of the AGaJ initiation codon. The AGal 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 a-agglutinin sequence. cells of other yeast species (7,54). Therefore, the glycoprotein-glycoprotein interactions are quite specific even though the analogous agglutinins of different yeast species share biochemical characteristics.Biochemical analysis has indicated that the a-agglutinin of S. cerevisiae is a glycoprotein of approximate 160 kilodaltons (kDa) (45). Removal of N-linked carbohydrate generates a major species of 72 kDa and several minor species. Structural analysis and determination of agglutinin binding determinants have been hampered by low yields and carbohydrate-generated heterogeneity of the agglutinins. A genetic approach allowed the isolation of a putative a-agglutinin structural mutant, agal (43). The agal mutation results in an a-specific agglutination defect, resulting from the lack of expression of active a-agglutinin, and a decrease in a mating efficiency. Additional mutants have been isolated that result in agglutination defects in both a and a cells and also affect pheromone production and response (17); the pleiotropic effects of these mutations indicate that they do not identify agglutinin structural genes.We have initiated a genetic and molecular approach to the study of agglutinin structure and function, which will complement the biochemical approach that has been used previ-