Plasmodium sporozoites require the protein B9 to invade hepatocytes
Plasmodium sporozoites are transmitted to a mammalian host during blood feeding by an infected mosquito and invade hepatocytes for initial replication of the parasite in the liver. This leads to the release of thousands of merozoites into the blood circulation and initiation of the pathogenic blood stages of malaria. Merozoite invasion of erythrocytes has been well characterized at the molecular and structural levels. In sharp contrast, the molecular mechanisms of sporozoite invasion of hepatocytes are poorly characterized. Here we report a new role during sporozoite entry for the B9 protein, a member of the 6-cysteine domain protein family. Using genetic tagging and gene deletion approaches in rodent malaria parasites, we show that B9 is secreted from sporozoite micronemes and is required for productive invasion of hepatocytes. Structural modelling indicates that the N-terminus of B9 forms a beta-propeller domain structurally related to CyRPA, a cysteine-rich protein forming an invasion complex with Rh5 and RIPR in P. falciparum merozoites. We provide evidence that the beta-propeller domain of B9 is essential for protein function during sporozoite entry and interacts with P36 and P52, both also essential for productive invasion of hepatocytes. Our results suggest that, despite using distinct sets of parasite and host entry factors, Plasmodium sporozoites and merozoites may share common structural modules to assemble protein complexes for invasion of host cells.