SOLUTION STRUCTURE OF A POLYPEPTIDE CONTAINING 4 HEPTAD REPEAT UNITS FROM A MEROZOITE SURFACE-ANTIGEN OF PLASMODIUM-FALCIPARUM

The Plasmodium falciparum antigen SPAM (secreted polymorphic antigen a ssociated with merozoites) contains an unusual set of heptad repeat un its with alanine at the a and d positions. Twelve heptads with the con sensus sequence AXXAXXX occur in three blocks of four, linked by short nonrepetitive sequences. A 38-residue polypeptide comprising the firs t block of four heptad units and five flanking residues at either end, SPAM-H1, has been synthesized and its structure in aqueous solution d etermined from H-1 NMR data. Sedimentation equilibrium showed the pept ide to be monomeric in aqueous solution. Its structure was determined from H-1 NMR-derived distance and dihedral angle constraints by using distance geometry calculations, restrained simulated annealing, and co njugate gradient energy minimization in the CHARMm force field. The po lypeptide contains an alpha-helix extending from Ser10 (position e of the first heptad) to at least Lys32 (position f of the fourth heptad) and possibly as far as Val35. The helix is bent, partly as result of a kink around residues 19-20. The conformations of the nine N-terminal residues and the six C-terminal residues are not well defined by the N MR data. The rms deviation from the average of the 20 best structures over the well-defined region (residues 11-31, which have backbone angu lar order parameters > 0.8) was 1.56 Angstrom for backbone heavy atoms (N, C-alpha, and C) and 2.12 Angstrom for all heavy atoms. (H2O)-H-2 exchange experiments identified slowly exchanging amide protons near t he C-terminus and the last two turns of the helix. The unusual stabili ty of the C-terminus reflects the presence of a new C-capping motif, w hich may involve the side chain of an asparagine in a position externa l to the C-cap residue. Possible interactions of the H1 sequence with the other two heptad repeat units in the intact merozoite antigen are discussed.