| Abstract | X-ray quality crystals of an Fab fragment from a transmission-blocking monoclonal antibody 4B7 (MAb 4B7) against a sexual stage protein Pfs25 of Plasmodium falciparum were grown as uncomplexed and peptide-complexed forms. Initially, the intact immunoglobulin was crystallized because cleavage with pepsin or papain did not produce a homogeneous product. Further proteolytic trials with elastase produced a suitable Fab fragment from which crystals have been obtained, both for the free Fab and in complex with cyclic peptides and in the presence of linear peptides. While linear peptides bind to MAb 4B7, cyclic peptides modeled on a predicted -hairpin loop of the third EGF-like domain of Pfs25 bind better and readily co-crystallize with the Fab. The genes for the variable domain of the Fab have been cloned, sequenced and the primary amino-acid sequence for the complete Fab deduced. This work explores the use of glycerol as an additive and the modification of the peptide sequence outside the epitope for improving in the crystallization. Data sets have been collected from crystals of several Fab-peptide complexes and from uncomplexed Fab to resolutions ranging from 2.4 to 3.3 Å. The packing arrangements of several crystal forms have been determined by molecular replacement, and refinement of their three-dimensional structures is in progress. The three-dimensional structure of this Fab complexed with the various peptides will aid in an understanding of the mode by which this antibody recognizes and prevents transmission of the malaria parasite. |
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