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

Prophenoloxidase (proPO) activating factor-I (PPAF-I) is a catalytically active clip-domain SP, cleaves which proPO. The results of crystallization and preliminary X-ray analysis of the SP domain of PPAF-I are reported here. 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