The Crystal Structure of Recombinant Human Neutrophil-activating Peptide-2 (M6L) at 1.9-Å Resolution (∗)

Neutrophil-activating peptide-2 (NAP-2) is a 70-residue carboxyl-terminal fragment of platelet basic protein, which is found in the α-granules of human platelets. NAP-2, which belongs to the CXC family of chemokines that includes interleukin-8 and platelet factor 4, binds to the interleukin-8 type II receptor and induces a rise in cytosolic calcium, chemotaxis of neutrophils, and exocytosis. Crystals of recombinant NAP-2 in which the single methionine at position 6 was replaced by leucine to facilitate expression belong to space group P1 (unit cell parameters a = 40.8, b = 43.8, and c = 44.7 Å and α = 98.4°, β = 120.3°, and γ = 92.8°), with 4 molecules of NAP-2 (Mr = 7600) in the asymmetric unit. The molecular replacement solution calculated with bovine platelet factor 4 as the starting model was refined using rigid body refinement, manual fitting in solvent-leveled electron density maps, simulated annealing, and restrained least squares to an R-factor of 0.188 for 2 σ data between 7.0- and 1.9-Å resolution. The final refined crystal structure includes 265 solvent molecules. The overall tertiary structure, which is similar to that of platelet factor 4 and interleukin-8, includes an extended amino-terminal loop, three strands of antiparallel β-sheet arranged in a Greek key fold, and one α-helix at the carboxyl terminus. The Glu-Leu-Arg sequence that is critical for receptor binding is fully defined by electron density and exhibits multiple conformations.