Preliminary X-ray crystallographic analysis of the catalytic domain of prophenoloxidase activating factor-I

Clip‐domain serine proteases (SPs) have been identified in invertebrates as crucial enzymes that are involved in diverse extracellular signalling pathways. Prophenoloxidase (proPO) activating factor‐I (PPAF‐I), a catalytically active clip‐domain SP, cleaves proPO. To date, no crystal structures of a catalytically active clip‐domain SP have been determined. Here, the results of crystallization and preliminary X‐ray analysis of the SP domain of PPAF‐I are reported. The crystal of the PPAF‐I SP domain was obtained using the hanging‐drop vapour‐diffusion method in a precipitant solution containing 0.15 M lithium sulfate, 30% polyethylene glycol 4000 and 0.1 M Tris–HCl pH 8.0. The crystal diffracts X‐rays to 1.7 Å resolution using a synchrotron‐radiation source. The crystal belongs to space group P212121, with one molecule in the asymmetric unit and unit‐cell parameters a = 38.3, b = 53.3, c = 116.6 Å, α = β = γ = 90°. A molecular‐replacement solution has been found using kallikrein as a starting model, resulting in an interpretable electron‐density map.