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Background  Placental malaria is typified by selective clustering of  Plasmodium falciparum  in the intervillous blood spaces of the placenta. Sequestration of malaria parasite in the human placenta is mediated by interactions between chondroitin sulphate A (CSA) on the syncytiotrophoblasts and proteins expressed on the surface of infected human erythrocytes.  Plasmodium falciparum  Erythrocyte Membrane Protein 1 (PfEMP1) encoded by the  var2CSA  gene is believed to be the main parasite ligand for CSA-mediated placental binding.    Methods  Extensive sequence and structure comparisons of the various CSA-binding and non-binding DBL domains from the  var2CSA  gene from A4 and 3D7 strains of  P. falciparum  were performed. Three-dimensional structural models of various DBL domains were built and analysed with a view to assessing conservation of CSA interaction sites across various DBL domains.    Results  Each of the six DBL domains from  var2CSA  are likely to retain the disulfide linkages evident from previously published DBL domain crystal structures. The number of disulfide linkages between the various DBL domains analysed varies from three to seven, of which two are conserved across all DBL domains. The conserved disulfide linkages are distributed within the respective three sub-domains and only one linkage is shared by sub-domains I and II. Major differences between CSA-binding DBL domains are in the loop regions, which tie the alpha helices together, and in variable length terminal extensions. Intriguingly, a crucial loop from A4 DBL 3X which provides the important Gly and Lys residues that chelate the bound sulphate is missing or significantly altered in all other DBL domains that interact with CSA. Further analysis of the proposed sulphate and predicted CSA-binding site indicates either none or very low level of conservation among the critical interacting residues.    Conclusion  Structural comparisons of the three-dimensional structures of CSA-binding DBL domains indicates that the proposed CSA interaction site on A4 DBL 3X is unlikely to be conserved across the other CSA-binding DBL domains from  var2CSA . Therefore, the 4 CSA-binding DBL domains encoded by  var2CSA  are unlikely to have common architectures to their CSA recognition sites. These structural insights have clear implications in using CSA-binding DBL domains for vaccines against placental malaria as it is proposed that the various CSA-binding DBL domains on  var2CSA  will recognize their CSA ligands differently.

malaria journal

Structural insights into chondroitin sulphate A binding Duffy-binding-like domains from Plasmodium falciparum: implications for intervention strategies against placental malaria