Characterization of Chlamydomonas sexual agglutinins

Molecules (agglutinins) mediating mating-type-specific adhesion of Chlamydomonas reinhardtii plus and minus gametes have been purified and shown to be homologous by several criteria. Both are large, extrinsic glycoproteins, rich in hydroxyproline and serine, which to a variety of inactivating agents. Direct visualization of agglutinins by the rapid-freeze deep-etch technique of Heuser reveals that both are extremely asymmetric, fibrous molecules with similar but distinct topographies. Each has a globular head, a rigid domain, a relatively flexible region and a terminal hook. In situ images show that the molecules associate with the flagellar surface via the latter (hook) domain, projecting the globular head distally. A functional role for the head is implied from inactivation studies (thermolysin digestion and reduction/alkylation) showing a selective modification of this domain. A library of monoclonal antibodies, raised against the plus agglutinin, has been used to probe the structural organization of this molecule by immunotopographic mapping. Two Two distinct classes of monoclonal antibodies are shown to react with carbohydrate epitopes localized to the two termini (head and hook) of the plus agglutinin while a third recognizes a polypeptide determinant that repeats along the rod-like portion of the molecule. The latter class is gamete (but not mating-type)-specific while the other two classes display distinct patterns of cross reactivity. Finally, the Chlamydomonas agglutinins show a striking compositional and structural resemblance to the major cell wall glycoproteins of this organism. Wall glycoproteins, which are also rich in hydroxyproline and serine and capable of self-assembly, may therefore share a common evolutionary heritage with the agglutinins.