Identification and visualization of the sexual agglutinin from the mating-type plus flagellar membrane of Chlamydomonas.

Sexual agglutinins located on the flagellar membranes of Chlamydomonas gametes mediate a mating-type-specific adhesion reaction that brings complementary gametes together for zygotic cell fusion. We identify the mating-type plus agglutinin, using a combination of biochemical and genetic analysis, as a glycopolypeptide with an apparent molecular weight of greater than 10(6) by SDS-polyacrylamide gel electrophoresis. Its core polypeptide migrates as a approximately 480-kd species, and it is estimated to be present in approximately 30 copies per gametic flagellum. The agglutinin is present in the wild type, in a mutant that agglutinates but cannot fuse, and in a complementing diploid, whereas it is absent from four nonagglutinating mutants and from a noncomplementing diploid. Electron microscopy shows the purified agglutinin to be a highly asymmetric molecule, 220 X 4 nm. To our knowledge, this is the first reported purification and visualization of a membrane-associated cell-cell recognition protein.