Membrane glycoproteins of Chlamydomonas eugametos flagella

Monoclonal antibodies to surface glycoconjugates in Chlamydomonas eugametos recognize strain-specific O-methyl sugars

Monoclonal antibodies are described that are directed against cell surface components of the unicellular green alga Chlamydomonas eugametos. These antibodies recognize strain-specific epitopes which occur at the surface of vegetative and gametic cells. Two different groups of epitopes are distinguished that are never detectable together in one clonal cell culture. Evidence is presented showing that the antigenicity of cell surface molecules is a consequence of the presence of particular O-methylated sugars. Monoclonal antibodies reacting with one group of epitopes were studied in more detail, and immunoprecipitation and Western-blot studies showed that these epitopes can be arranged into four classes. The use of these monoclonal antibodies as strain-specific markers in light- and electron-microscopical techniques is illustrated.

Light dependence of sexual agglutinability in Chlamydomonas eugametos

The mating activity of mating-type plus gametes of Chlamydomonas eugametos depends on light. Cells lost their ability to agglutinate with mating-type minus gametes after a dark period of 30 min. They regained their agglutinability after 10 min exposure to light. Other mating reactions, such as tipping and flagellar tip activation, were not dependent upon light. Since cycloheximide and tunicamycin did not affect the light-induced activation of flagellar agglutinability, no protein synthesis or glycosylation is involved in this process. Equal amounts of biologically active agglutination factor could be extracted from cells placed either in light or in darkness. A minor portion of the active material was found to be located on the flagellar surface of illuminated cells. No active material was found on the flagellar surface of dark-exposed cells, whereas their cell bodies contained the same amount of active material as the cell bodies of illuminated cells. Since a light-induced flow of agglutination factors from the cell body to the flagella could not be detected and dark-exposed cells could be slightly activated by amputation or fixation by glutaraldehyde, we propose that light affects flagellar agglutinability by an in-situ modification of the agglutination factor on the flagella. When mt (+) and mt (-) strains were crossed and the progeny examined for light-sensitivity, it was apparent that this phenomenon is not mating type-linked.

Sex-specific binding and inactivation of agglutination factor in Chlamydomonas eugametos

Gametes of opposite mating type (mt (+) and mt (-)) of the green alga Chlamydomonas eugametos agglutinate via their flagella as a prelude to sexual fusion. To quantitate sexual agglutination, an in vitro assay has been developed using (35)S-labeled flagella and the isolated mt (-)agglutination factor. It is shown that not only isolated flagella, but also the mt (-)agglutination factor rapidly bind to the flagella of intact gametes of the opposite mating type. This confirms the role of the mt (-)agglutination factor in determining the sexual agglutinability of mt (-)gametes. As a function of binding, the agglutinative power of the flagella of both mating types is destroyed by a temperature-sensitive process. Likewise, the mt (-)agglutination factor can be completely inactivated.

Preferential turnover of membrane proteins in the intact Chlamydomonas flagellum

Radioactive labeling studies demonstrate a continuous incorporation of newly synthesized proteins and glycoproteins into the intact flagella of Chlamydomonas. This apparent turnover is preferentially occurring for membrane components. In particular, two classes of flagellar membrane components, one a high molecular weight (HMW) group of closely migrating glycoproteins and the other a protein with a MW around 65 kD, are continuously turning over in the vegetative cell. This selective protein turnover may explain the ability of Chlamydomonas to rapidly recover from proteolytic modification of the flagellar surface and to change its flagellar surface properties during the early events in mating.

Topography of Chlamydomonas: fine structure and polypeptide components of the gametic flagellar membrane surface and the cell wall

Surface polypeptide components of the flagellar membrane of Chlamydomonas reinhardi Dang. gametes are identified by their accessibility to in-vivo vectoral labeling by glucose oxidase-coupled lactoperoxidase-dependent (125)I iodination. Vectoral labeling is accomplished without observable adverse effects on cell viability or gametic function. Flagella isolated from labeled wild-type cells carry about 3% of the total incorporated label, which is found by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be distributed among 16 identifiable polypeptide bands. The most prominent surface-labeled species migrates in the Mr (relative molecular weight) 350 k region of the gel; each of the remaining iodinated polypeptides, which range in Mr from 25 k to 500 k, carries only a small proportion of incorporated label. To determine which polypeptides are unique to the flagellum and which are contaminants from the cell wall, wild-type profiles were compared with those of mutant strains and of mechanically isolated cell walls. Identification of contaminants was also facilitated by two-dimensional peptide mapping. We conclude that only 11 of the labeled bands are contributed by flagellar polypeptides; the remaining five bands are shown to be contaminants from the cell wall, and additional cell-wall polypeptides are found to co-migrate with flagellar species. A polypeptide designated as a possible membrane tubulin in preliminary studies is shown here to be different from tubulin in its peptide map. The 11 polypeptides assigned as specific flagellar surface components are candidate participants in such biological events as sexual adhesion, flagellar surface motility, and sensory signalling.

The cell wall of Chlamydomonas eugametos. Immunological aspects

An antiserum was raised against the major cell wall glycoprotein of Chlamydomonas eugametos which after purification reacted specifically with all individual wall components but not with intact cell walls. The antigenic sites in intact walls appear to be cryptic but become exposed on partial enzymatic degradation or in situ during daughter-cell release when the insoluble component is digested. Using the antiserum as a specific label for cell walls in various stages of disintegration, cell wall digestion during asexual and sexual reproduction was studied. It is also shown that while cell wall material is associated with isolated flagella, it is not normally associated with the flagella of intact cells.

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.

Sexual agglutination factor from Chlamydomonas eugametos

Chlamydomonas eugametos gametes can sexually agglutinate via their flagellar surfaces whereas vegetative cells cannot. Therefore, flagellar glycoproteins, present in gamete cells but absent from vegetative cells, were investigated as prospective mt (-)agglutination factors. They were identified as periodic acid Schiff (PAS) stained bands separated in sodium dodecyl sulphate-polyacrylamide electrophoresis gels. Gamete-specific bands were determined by comparison with equivalent gels of vegetative flagella and by immunological techniques using antisera raised against isolated mt (-) gamete flagella. Four high molecular weight flagellar glycoproteins proved to be gamete specific (PAS-1.2, PAS-1.3, PAS-3 and PAS-4). They were extracted from flagella by 3 M guanidine thiocyanate, separated in a column of Sepharose 2B, and tested for in vitro agglutination activity on mt (+) gametes. A single peak of activity was found to be correlated with the presence of the PAS-1.2 band. It is shown that mt (-) agglutination activity is related to the concentration of this glycoprotein in flagellar membranes.

Concanavalin A binding to Chlamydomonas eugametos flagellar proteins and its effect on sexual reproduction

The relative amounts of Concanavalin A (Con A) bound by gamete and vegetative flagella of both mating types (mt (+) and mt (-)) of Chlamydomonas eugametos were determined using (125)I-Con A. Con A agglutinated all cell types by cross-linking their flagella in a random manner. No correlation was found between the extent of Con A-binding and Con A-mediated isoagglutination. Con A inhibited the sexual interaction between gametes at various levels. In mt (+) gametes it blocked sexual agglutination, whereas in mt (-) gametes it prevented papillar fusion. By SDS-gel electrophoresis nine Con A-binding components were found to be present in flagella. However, it was not possible to allocate a role in sexual agglutination to any of these components since they were present in all cell types, including vegetative cells which are not able to sexually agglutinate.