Cell-wall synthesis in Chlamydomonas reinhardtii: an immunological study on the wild type and wall-less mutants cw2 and cw15.

Cells of Chlamydomonas reinhardtii Dang. wild type and the cell-wall mutants cw2 and cw15 were grown synchronously. The two mutants secreted copious amounts of cell-wall-like glycoproteins into the culture medium in contrast to the wild type which released only minor quantities. Both the secreted proteins as well as those present in the lumen of the endoplasmic reticulum (ER) and Golgi apparatus (GA) were tested for crossreactivity against a number of monoclonal antibodies (MACs) prepared against the 2BII glycoprotein cell-wall complex of the wild type (E. Smith et al., 1984, Planta 161, 330-338). Of the four monoclonals applied, one, (MAC 6), did not react in either dot blot or Western blots with any of the luminal and medium proteins. By dot blotting, MAC 2 recognized polypeptides only in the wild-type medium. Neither MAC 2 nor MAC 6 were capable of recognizing polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, MAC 2 recognized one of the 2BII polypeptides (135 kDa) as well as a large number of other polypeptides in wild-type and mutant media. The 135-kDa polypeptide was also detected in the luminal extracts of ER and GA membranes from the wild type and cw2 mutant. It was also present in the GA fraction of the cw15 mutant. If, as previously claimed, these monoclonal antibodies are indeed directed against the carbohydrate portion of the 2BII complex, our results would indicate that protein O-glycosylation is not restricted to the GA but may start in the ER. They also confirm inferences made by others that the cell-wall mutants cw2 and cw15 possess the capacity to synthesize and secrete the major glycoproteins of the cell wall, but, due to the lack of the W2 wall layer, are unable to assemble these components into a coherent, crystalline wall.