Structures of malaria vaccine candidates
ZP Feng, DW Keizer, S Yao, JJ Babon, RA Stevenson, RS Norton, in collaboration with RF Anders, VJ Murphy, CG Adda (La Trobe University) [J Mol Biol. Jul 22;350(4):641-56 PMID:15964019 220. Zhang X, Perugini MA, Yao S, Adda CG, Murphy VJ, Low A, Anders RF & Norton RS (2008) Solution conformation, backbone dynamics and lipid interactions of the intrinsically unstructured malaria surface protein MSP2. Journal of Molecular Biology 379, 105–121.
Apical membrane antigen 1 (AMA1), a merozoite surface protein found in all species of Plasmodium, is a strong candidate for inclusion in a malarial vaccine. The 62-kDa ectodomain of AMA1 consists of three disulfide-stabilised domains, and the disulfide-bond stabilised conformation is essential for protection, as the antigen is not an effective vaccine after reduction and alkylation. Determining the structure of the ectodomain will provide a basis for understanding its interaction with inhibitory molecules and enhancing its properties as a vaccine.
We have determined the structures of domains II and III of AMA1. In domain II (Figure) two disulfide bonds link the N- and C-terminal regions of the molecule, which form a four-stranded beta-sheet linked to a short helix. A long loop linking the N- and C-terminal regions contains four other alpha-helices, the locations of which are not fixed relative to the beta-sheet core, even though they are well-defined locally. Importantly, this region of domain II contains the epitope recognised by the invasion-inhibitory antibody 4G2, even though it does not contain any of the polymorphisms that are regarded as having arisen in response to the pressure of immune recognition.
We are currently developing small molecule and peptide inhibitors of AMA1.
Merozoite surface protein 2, MSP2, is also a vaccine candidate. It is an intrinsically unstructured protein that forms amyloid-like fibrils upon storage. We have used NMR to examine peptides corresponding to the conserved N-terminal region of MSP2 for the presence of well-ordered local structure and the ability to form fibrils related to those formed by full-length MSP2. In the N-terminal region residues 8-15 formed a hairpin-like structure, with the remainder of this region being unstructured. An 8-residue peptide corresponding to residues 8-15 also formed this hairpin-like structure and produced amyloid fibrils similar to those formed by the N-terminal region of MSP2.