Processing of a major parasite surface glycoprotein during the ultimate stages of differentiation in Plasmodium knowlesi.

A monoclonal antibody (13C11) was used to investigate the processing of a Plasmodium knowlesi plasma membrane protein during the late stages of schizogony. 13C11 bound to the surface of merozoites, blocked invasion of erythrocytes and immunoprecipitated a 230 kDa glycoprotein from metabolically labelled schizonts. This protein was a major parasite surface component inserted into the membrane of immature schizonts as shown through the study of saponin-freed schizonts which bound 13C11 to their surface (indirect immunofluorescence and immunoelectron microscopy); in addition, the 230 kDa protein on saponin-freed schizonts was susceptible to trypsin treatment. Cleavage of the protein in pulse-chase experiments was followed by immunoprecipitation with 13C11. As schizogony proceeded, the 230 kDa protein was cleaved to 200, 145 and 110 kDa polypeptides. However, this cleavage did not reflect processing but occurred in vitro during detergent extraction and was due to a proteolytic activity which appeared in the parasite during the later stages of schizogony. As schizonts reached maturity and infected erythrocytes lysed, the 230 kDa protein was processed to 75, 57, 50 kDa and 43 kDa polypeptides which were the surface labelled components on purified merozoites immunoprecipitated by 13C11.