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 cation-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 Å and belong to the monoclinic space group P2₁, with unit-cell parameters a = 57.36, b = 90.45, c = 80.39 Å, β = 92.64°. The asymmetric unit contains four molecules of mexicain, with a corresponding crystal volume per protein weight (V_M) of 2.24 ų Da^-1 and a solvent content of 45% by volume. A molecular-replacement model has been determined and refinement is in progress.