Cell wall anchoring 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. Covalently-linked GPI-modified cell wall protein (GPI-CWP). The number of the intragenic tandem repeats varies between different S.cerevisiae strains. Extensively O-glycosylated by PMT1 and PMT2. The GPI-anchor is attached to the protein in the endoplasmic reticulum and serves to target the protein to the cell surface. There, the glucosamine-inositol phospholipid moiety is cleaved off and the GPI-modified mannoprotein is covalently attached via its lipidless GPI glycan remnant to the 1,6-beta-glucan of the outer cell wall layer.