Receptor binding subunit of the a-agglutinin heterodimer. S.cerevisiae a and alpha cells express the complementary cell surface glycoproteins a-agglutinin and alpha-agglutinin, respectively, which interact with one another to promote cellular aggregation during mating. Heterodimer; disulfide-linked (Probable). Interacts with SAG1. Present with 1160 molecules/cell in log phase SD medium.