CRYSTALLIZATION, SEQUENCE AND PRELIMINARY CRYSTALLOGRAPHIC DATA FOR TRANSMISSION-BLOCKING ANTI-MALARIA FAB 4B7 WITH CYCLIC-PEPTIDES FROM THE PFS25 PROTEIN OF PLASMODIUM-FALCIPARUM

X-ray quality crystals of an Fab fragment from a transmission-blocking monoclonal antibody 4B7 (MAb 4B7) against a sexual stage protein Pfs2 5 of Plasmodium falciparum were grown as uncomplexed and peptide-compl exed forms. Initially, the intact immunoglobulin was crystallized beca use cleavage with pepsin or papain did not produce a homogeneous produ ct. Further proteolytic trials with elastase produced a suitable Fab f ragment from which crystals have been obtained, both for the fire Fab and in complex with cyclic peptides and in the presence of linear pept ides. While linear peptides bind to MAb 4B7, cyclic peptides modeled o n a predicted beta-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 primar y amino-acid sequence for the complete Fab deduced. This work explores the use of glycerol as an additive and the modification of the peptid e sequence outside the epitope for improving in the crystallization. D ata sets have been collected from crystals of several Fab-peptide comp lexes and from uncomplexed Fab to resolutions ranging from 2.4 to 3.3 angstrom. The packing arrangements of several crystal forms have been determined by molecular replacement, and refinement of their three-dim ensional structures is in progress. The three-dimensional structure of this Fab complexed with the various peptides will aid in an understan ding of the mode by which this antibody recognizes and prevents transm ission of the malaria parasite.