Structures of Phage-Display Peptides that Bind to the Malarial Surface Protein, Apical Membrane Antigen 1, and Block Erythrocyte Invasion

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Abstract
Apical membrane antigen 1 (AMA1) of the human malaria parasite Plasmodium falciparum is synthesized by schizont stage parasites and has been implicated in merozoite invasion of host erythrocytes. Phage-display techniques have recently been used to identify two 15-residue peptides, F1 and F2, which bind specifically to P. falciparum AMA1 and inhibit parasite invasion of erythrocytes [Li, F., et al. (2002) J. Biol. Chem.277, 50303−50310]. We have synthesized F1, F2, and three peptides with high levels of sequence identity, determined their relative binding affinities for P. falciparum AMA1 with a competition ELISA, and investigated their solution structures by NMR spectroscopy. The strongest binding peptide, F1, contains a β-turn that includes residues identified via an alanine scan as being critical for binding to AMA1 and inhibition of merozoite invasion of erythrocytes. The three F1 analogues include a 10-residue analogue of F1 truncated at the C-terminus (tF1), a partially scrambled 15-mer (sF1), and a disulfide-constrained 14-mer (F1tbp) which is related to F1 but has a sequence identical to that of a disulfide-constrained loop in the first epidermal growth factor module of the latent transforming growth factor-β binding protein. tF1 and F1tbp bound competitively with F1 to AMA1, and all three contain a type I β-turn encompassing key residues involved in F1 binding. In contrast, sF1 lacked this structural motif, and did not compete for binding to AMA1 with F1; rather, sF1 contained a type III β-turn involving a different part of the sequence. Although F2 was able to bind to AMA1, it was unstructured in solution, consistent with its weak invasion inhibitory effects. Thus, the secondary structure elements observed for these peptides in solution correlate well with their potency in binding to AMA1 and inhibiting merozoite invasion. The structures provide a valuable starting point for the development of peptidomimetics as antimalarial antagonists directed at AMA1.