Molecular determinants of SR-B1-dependent Plasmodium sporozoite entry into hepatocytic cells

Sporozoite forms of the malaria parasite are transmitted by mosquitoes and first infect the liver for an initial round of replication before parasite proliferation in the blood. The molecular mechanisms involved during sporozoite invasion of hepatocytes remain poorly understood. Two receptors of the Hepatitis C virus (HCV), the tetraspanin CD81 and the scavenger receptor class B type 1 (SR-B1), play an important role during the entry of sporozoites into hepatocytic cells. In contrast to HCV entry, which requires both CD81 and SR-B1 together with additional host factors, CD81 and SR-B1 operate independently during malaria liver infection. Sporozoites from human-infecting and rely respectively on CD81 or SR-B1. Rodent-infecting can use SR-B1 to infect host cells as an alternative pathway to CD81, providing a tractable model to investigate the role of SR-B1 during liver infection. Here we show that mouse SR-B1 is less functional as compared to human SR-B1 during infection. We took advantage of this functional difference to investigate the structural determinants of SR-B1 required for infection. Using a structure-guided strategy and chimeric mouse/human SR-B1 constructs, we could map the functional region of human SR-B1 within apical loops, suggesting that this region of the protein may play a crucial role for interaction of sporozoite ligands with host cells and thus the very first step of infection.