Processing of the Plasmodium falciparum major merozoite surface protein-1: identification of a 33-kilodalton secondary processing product which is shed prior to erythrocyte invasion

We have previously shown that only a single 19-kDa fragment of the Plasmodium falciparum major merozoite surface protein (MSP1) is carried with an invading merozoite into the infected red cell. This fragment (MSPI19) is derived from the C-terminal, membrane-bound end of a major product, MSPI42, of the primary stage of MSPI proteolytic processing. Using a monoclonal antibody mapped to an epitope within the N-terminal region of MSPI42, we have shown that a soluble 33-kDa polypeptide (MSPI33) corresponding to the N-terminal region of MSPI42 is shed into culture supernatants during merozoite release and erythrocyte invasion. These observations provide further evidence that the secondary processing of MSPI42 involves a highly site-specific proteolytic activity. © 1991.