Structure of a Fab Complex with the C-terminal fragment of merozoite surface protein-1 of Plasmodium vivax determined by Computational Docking

Abstract

One current vaccine candidate against Plasmodium vivax, targeting asexual blood stages, is the major merozoite surface protein-1 of P. vivax (PvMSP-1). Vaccine trials with PvMSP-1$_{19}$ and PvMSP-1$_{33}$ have succeeded in protecting monkeys and it has been shown that a large proportion of individuals naturally exposed to P. vivax infection, develop specific antibodies to PvMSP-1$_{19}$. In the present study, computational protein-protein docking was used to predict the structure of the antigen--antibody complex between PvMSP1$_{19}$ and the Fab region of the G17.12 monoclonal antibody. This antibody does not inhibit erythrocyte invasion or MSP1 processing, but it recognises a discontinuous epitope on PfMSP1$_{19}$ that has been mapped to regions recognised by invasion-inhibiting antibodies. The molecular simulations were performed using, as starting structures, the Fab fragment of the P. falciparum MSP1$_{19}$-mAbG17.12 complex (pdb:1ob1) and the structure of the P. vivax MSP1$_{19}$ previously determined by homology modeling. The mAb was submitted to a docking procedure with antigen PvMSP1$_{19}$ using the programs PatchDock and FireDock to obtain an initial structure for the complex. A final optimization was performed with RosettaDock using a Monte Carlo algorithm. The final structure of the PvMP1$_{19}$-mAb17.12 complex shows that the antibody recognizes a discontinuous epitope that include segments on the first domain and some residues at the end of the second domain. The model provides valuable guidelines for future experimental work devoted to the identification of B-epitopes and synthesis of peptides with antigenic activity.