Reconstitution of biological activity in isoagglutinins fromChlamydomonas eugametos.

Gametes ofChlamydomonas eugametos produce membrane vesicles, called isoagglutinins, which are shed into the culture fluid. It is assumed that they originate from the flagellar membrane for, like flagella, they can bind to the flagellar surface of gametes of the opposite mating type (mt). The composition ofmt (-) isoagglutinin was investigated with respect to this agglutinability. When the agglutination factor present on the surface ofmt (-) isoagglutinins (PAS-1.2) was removed, together with other membrane bound glycoproteins, the membrane vesicles were rendered inactive. They could be reactivated however by incubation with the extracted glycoproteins in a time-and concentration-dependent manner. The agglutination factor proved to be necessary yet sufficient in itself for the reactivation process to occur. Experiments with CsCl density gradients showed that the agglutination factor truly bound to the vesicles during reactivation. Inactivated vesicles derived frommt (+) gametes could be reactivated to gainmt (-) properties. Reactivation was inhibited by prior treatment with trypsin. The results indicate that the agglutination factor inmt (-) isoagglutinins is an extrinsic membrane protein bound to an intrinsic proteinaceous receptor.