Ultrastructure and properties of the sexual agglutinins of the biflagellate green alga Chlamydomonas moewusii

The mt− agglutinins of the interfertile species Chlamydomonas moewusii and Chlamydomonas eugametos are very similar fibrous molecules. The mt− agglutinin of C. moewusii has the same Stokes radius (39 nm) and sedimentation coefficient (9.3 S) as its counterpart in C. eugametos; its length (336 nm) and its ultrastructure, including the position of four kinks are also the same as in C. eugametos. The sugar compositions of both agglutinins are very similar, and they react equally well with the monoclonal antibody Mab 66.3 raised against the mt− agglutinin of C. eugametos. Finally, they are equally thermoresistant, with half-lives at 100 °C of 50 min (C. moewusii) and 57 min (C. eugametos). The mt+ agglutinins of both species are different. Both are fibrous molecules with a terminal head, but the fibrous part of the molecule in C. moewusii is shorter (210 nm compared to 276 nm). The mt+ agglutinin of C. moewusii is also significantly more sensitive to heating with a half-life of 6 min at 40 °C compared to the 20 min shown by the mt+ agglutinin of C. eugametos. Their sugar compositions are, however, very similar, and they react equally well with Mab 66.3. The mt+ agglutinin of C. moewusii is sensitive to denaturing reagents and proteolytic attack, whereas the mt− agglutinin is highly resistant. It is proposed that the globular head of the mt+ agglutinin acts as its recognition domain and interacts with a carbohydrate ligand on the mt− agglutinin.