Crystallization and preliminary X-ray crystallographic studies on a Kunitz-type potato serine protease inhibitor

Interest in protease inhibitors has been renewed because of their potent activity in preventing carcinogenesis in a wide variety of in vivo and in vitro model systems. Potato tubers contain a wide range of such protease inhibitors. In cv. Elkana potato tubers, protease inhibitors represent about 50% of the total amount of soluble protein. Potato serine protease inhibitor (PSPI), one of the isoforms of the most abundant group of protease inhibitors, is a dimeric double-headed Kunitz-type inhibitor. No high-resolution structural information on this type of inhibitor has so far been obtained, as all currently known structures are of the monomeric single-headed or monomeric double-headed types. Crystals were grown in 0.1 M HEPES pH 7.5, 10% PEG 8000 and 8% ethylene glycol complemented with 9 mM 1-s-octyl-beta-D-thioglucoside or 0.1 M glycine. Data were collected from a single crystal under cryoconditions to 1.8 A resolution. The protein crystallized in space group P2(1), with unit-cell parameters a = 54.82, b = 93.92, c = 55.44 A, beta = 100.7 degrees; the scaling Rsym is 0.044 for 45,456 unique reflections.