Crystallization of the bifunctional proteinase/amylase inhibitor PKI-3 and of its complex with proteinase K.

One of the three wheat germ inhibitors of proteinase K is bifunctional and inhibits simultaneously proteinase K (or subtilisin but not enzymes of the trypsin family) and insect alpha-amylase. The molecular mass of this inhibitor called PKI-3 is 21 kDa, and the binding constant for proteinase K is 0.8 nM at pH 8.2, 25 degrees C, in 1:1 molar ratio. PKI-3 was crystallized by microdialysis against 10-12% polyethylene glycol 6000, 50 mM NaH2PO4, pH 6.7. The crystals have monoclinic space group P2(1) with a = 42.5, b = 65.3, c = 31.5 A, beta = 110 degrees, and diffract beyond 2.0 A resolution. The complex proteinase K X PKI-3 was crystallized by equilibrium vapor diffusion under the same conditions. The crystals are needle-shaped and still too small for X-ray analysis. Gel electrophoresis established the composition of the crystals.