Poster Presentation The 42nd Lorne Conference on Protein Structure and Function 2017

Structure and characterization of a key epitope in the conserved C-terminal domain of the malaria vaccine candidate MSP2 (#233)

Jeffrey Seow 1 , Rodrigo A.V. Morales 1 , Chris A. MacRaild 1 , Krishnarjuna Bankala 1 , Sheena McGowan 2 , Tamir Dingjan 1 , Garima Jaipuria 3 , Karyn L. Wilde 4 , Hanudatta S. Atreya 3 , Robin F. Anders 5 , Daniel Christ 6 , Nyssa Drinkwater 2 , Raymond S. Norton 1
  1. Medicinal Chemistry, Monash Institute of Pharmaceutical Sciences, Melbourne, VIC, Australia
  2. Department of Microbiology, Monash University, Melbourne, Victoria, Australia
  3. NMR Research Centre, Indian Institute of Science, Bangalore, India
  4. National Deuteration Facility, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia
  5. Department of Biochemistry and Genetics, La Trobe Institute for Molecular Science, Melbourne, VIC, Australia
  6. Garvan Institute of Medical Research, Darlinghurst, NSW, Australia

Merozoite surface protein 2 (MSP2) is an intrinsically disordered antigen that is abundant on the surface of the malaria parasite Plasmodium falciparum. The two allelic families of MSP2, 3D7 and FC27, differ in their central variable regions, which are flanked by highly conserved C-terminal and N-terminal regions. In a vaccine trial full-length 3D7 MSP2 induced a strain-specific protective immune response despite the detectable presence of conserved region antibodies. This work focuses on the conserved C-terminal region of MSP2 that encompasses the only disulfide bond in the protein and is a key epitope recognised by the mouse monoclonal antibodies 4D11 and 9H4. Although their epitopes are overlapping, immunofluorescence assays (IFA) have shown that 4D11 binds to MSP2 on the merozoite surface with a much stronger signal than 9H4. Understanding the structural basis for this antigenic difference between these antibodies will assist in the design of a broad-spectrum MSP2-based malaria vaccine. 4D11 and 9H4 were re-engineered into antibody fragments (scFv and Fvs) and validated as suitable models for their full-size IgG counterparts by surface plasmon resonance and isothermal titration calorimetry. An alanine scan of the 13-residue epitope 3D7-MSP2207-222 identified the minimal binding epitope of 4D11 and the key residues involved in binding. A 2.2 Å crystal structure of 4D11 Fv bound to the 8-residue epitope NKENCGAA provided valuable insight into the possible conformation of the C-terminal region of MSP2 on the parasite. This work underpins continued efforts to optimise recombinant MSP2 constructs for evaluation as potential vaccine candidates.