Structural and immunological correlations between the variable blocks of the VAR2CSA domain DBL6ε from two Plasmodium falciparum parasite lines.

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), a family of adhesins of the falciparum species of the malaria parasite, is exposed on the surface of the infected erythrocyte. In general, only one PfEMP1 variant is expressed at a time but switching between variants occurs, changing both host-cell receptor specificity and serotype. The PfEMP1 variant VAR2CSA causes sequestration of infected erythrocytes in the intervillous spaces of the placenta via the glycosaminoglycan chondroitin sulfate A. This leads to pregnancy-associated malaria, which has severe consequences for the fetus and mother. The extracellular region of VAR2CSA comprises six DBL (Duffy-binding-like) domains and a single CIDR (cysteine-rich inter-domain region) domain. The C-terminal domain DBL6ε, the most polymorphic domain of VAR2CSA, has seven regions of high variability termed variable blocks (VBs). Here we have determined the crystal structure of DBL6ε from the FCR3 parasite line and have compared it with the previously determined structure of that from the 3D7 line. We found significant differences particularly in the N-terminal region, which contains the first VB (VB1). Although DBL6ε is the most variable VAR2CSA domain, DBL6ε-FCR3 and DBL6ε-3D7 react with IgG purified from immune sera of pregnant women. Furthermore, IgG purified on one domain cross-reacts with the other, confirming the presence of cross-reactive epitopes. We also examined reactivity of immune sera to the four least variable VB (VB1, VB2, VB4 and VB5) using peptides with the consensus sequence closest, in turn, to the FCR3 or 3D7 domain. These results provide new molecular insights into immune escape by parasites expressing the VAR2CSA variant.

Production and purification of large quantities of the functional N-terminal ectodomain of human VPAC1 receptor.

Vasoactive intestinal peptide (VIP) is implicated in many physiological and pathophysiological processes, and its receptors are promising targets for the development of new drugs. The human VPAC1 receptor for VIP and pituitary adenylate cyclase-activating polypeptide is a class II G protein coupled receptor. The N-terminal ectodomain (N-ted) of the VPAC1 receptor is a major VIP binding site. To determinate the high resolution structure of the VPAC1 receptor N-ted, large quantities of purified recombinant N-ted produced are required. The N-ted sequence (31-144), which is fused to thioredoxin protein and 6xHis tag, was expressed into Origami Escherichia coli strain. Purification of recombinant N-ted using Ni-NTA affinity column associated to Nu-polyacrylamide gel electrophoresis analysis reveals the presence of one single band of Mw 19,000 corresponding to the purified recombinant N-ted. The purified N-ted was able to recognize VIP and the selective antagonist PG96-269. About 5-10 mg of functional purified protein/liter of bacterial culture is currently produced. This is a crucial step to determine the structure of functional human VPAC1 receptor N-ted by nuclear magnetic resonance.