Purification, crystallization and preliminary X-ray analysis of mexicain

Mexicain is a 23.7 kDa papain-like cysteine protease from the tropical plant Jacaratia mexicana. Extracted as a mix of proteases from the latex of the fruit, mexicain is isolated after canon-exchange chromatography as the most abundant product. The purified product inhibited with E-64 was crystallized by sitting-drop vapour diffusion in the presence of ethanolamine. Cryoprotected crystals diffracted X-rays from a home source to 1.98 Angstrom and belong to the monoclinic space group P2(1), with unit-cell parameters a=57.36, b=90.45, c=80.39 Angstrom, P=92.64. The asymmetric unit contains four molecules of mexicain, with a corresponding crystal volume per protein weight (V-M) of 2.24 Angstrom(3) Da(-1) and a solvent content of 45% by volume. A molecular-replacement model has been determined and refinement is in progress.