THE CRYSTAL-STRUCTURE OF RECOMBINANT HUMAN NEUTROPHIL-ACTIVATING PEPTIDE-2 (M6L) AT 1.9-ANGSTROM RESOLUTION

Neutrophil-activating peptide-2 (NAP-2) is a 70-residue carboxyl-termi nal fragment of platelet basic protein, which is found in the alpha-gr anules of human platelets. NAP-2, which belongs to the CXC family of c hemokines that includes interleukin-8 and platelet factor 4, binds to the interleukin-8 type II receptor and induces a rise in cytosolic cal cium, chemotaxis of neutrophils, and exocytosis. Crystals of recombina nt NAP-2 in which the single methionine gt position 6 was replaced by leucine to facilitate expression belong to space group Pi (unit cell p arameters a = 40.8, b = 43.8, and c = 44.7 Angstrom and alpha = 98.4 d egrees, beta = 120.3 degrees, and gamma = 92.8 degrees), with 4 molecu les of NAP-2 (M(r) = 7600) in the asymmetric unit. The molecular repla cement solution calculated with bovine platelet factor 4 as the starti ng model was refined using rigid body refinement, manual fitting in so lvent-leveled electron density maps, simulated annealing, and restrain ed least squares to an R-factor of 0.188 for 2 sigma data between 7.0- and 1.9-Angstrom resolution. The final refined crystal structure incl udes 265 solvent molecules. The overall tertiary structure, which is s imilar to that of platelet factor 4 and interleukin-8, includes an ext ended amino terminal loop, three strands of antiparallel beta-sheet ar ranged in a Greek key fold, and one alpha-helix at the carboxyl termin us. The Glu-Leu-Arg sequence that is critical for receptor binding is fully defined by electron density and exhibits multiple conformations.