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 (MSP1(19)) is derived from the C-terminal, membrane-bound end of a major product, MSP1(42), of the primary stage of MSP1 proteolytic processing. Using a monoclonal antibody mapped to an epitope within the N-terminal region Of MSP1(42), we have shown that a soluble 33-kDa polypeptide (MSP1(33)) Corresponding to the N-terminal region of MSP1(42) is shed into culture supernatants during merozoite release and erythrocyte invasion. These observations provide further evidence that the secondary processing Of MSP1(42) involves a highly site-specific proteolytic activity.