Two Plasmodium falciparum merozoite proteins binding to erythrocyte band 3 form a direct complex

Erythrocyte invasion by malaria parasites requires multiple protein interactions. Our earlier studies showed that erythrocyte band 3 is an invasion receptor binding Plasmodium falciparum merozoite surface protein 1 and 9 (MSP1, MSP9) existing as a co-ligand complex. In this study, we have used biochemical approaches to identify the binding sites within MSP1 and MSP9 involved in the co-ligand complex formation. A major MSP9-binding site is located within the 19kDa C-terminal domain of MSP1 (MSP1(19)). Two specific regions of MSP9 defined as Delta1a and Delta2 interacted with native MSP1(19). The 42 kDa domain of MSP1 (MSP1(42)) bearing MSP1(19) in the C-terminus bound directly to both MSP9/Delta1a and Delta2. Thus, the regions of MSP1 and MSP9 interacting with the erythrocyte band 3 receptor are also responsible for assembling the co-ligand complex. Our evidence suggests a ternary complex is formed between MSP1, MSP9, and band 3 during erythrocyte invasion by P. falciparum.