**24. Merozoite surface proteins and host- parasite interaction**

The initial interaction between the merozoite and the erythrocyte is probably a random collision and presumably involves reversible interactions between proteins on the merozoite surface and the host erythrocyte. Several merozoite surface proteins have been described. The best characterized is merozoite surface protein – 1 (MSP-1). Circumstantial evidence implicating MSP – I in erythrocyte invasion include its uniform distribution over the merozoite surface and the observation that antibodies against MSP-I inhibit invasion.[54] In addition, MSP-I does bind to band 3.[55] However, a role for MSP-I in invasion has not been definitively demonstrated. Similarly, the circumsporozite protein (CSP) probably plays a role in targeting sporozoites to hepatocytes by interacting with heparin sulfate proteoglycans . [56]

Another interacting aspect of MSP-I is the proteolytic processing that is coincident with merozoite maturation and invasion.[57] A primary processing occurs at the time of merozite maturation and result in the formation of several polypeptides held together in a noncovalent complex. A secondary processing occurs coincident with merozoite invasion at a site near the C-terminus. The non-covalent complex of MSP-I polypeptide fragments is shed from the merozoite surface following proteolysis and only a small C-terminal fragment is carried into the erythrocyte. This loss of the MSP-I complex may correlate with the loss of the 'fuzzy' coat during merozoite invasion. The C-terminal fragment is attached to the merozoite surface by a GPI anchor and consists of two EGF-like modules. EGF-like modules are found in a variety of protein and are usually implicated in protein-protein interactions. One possibility is that the secondary proteolytic processing functions to expose the EGF-like modules which strengthen the interactions between merozoite and erythrocyte. The importance of MSP-I and its processing are implied from the following observations:


The exact role(s) which MSP-I and its processing play in the merozoite invasion process are not known.
