Mexicain is a 23.8 kDa cysteine protease from the tropical plant Jacaratia mexicana. It is isolated as the most abundant product after cation-exchange chromatography of the mix of proteases extracted from the latex of the fruit. The purified enzyme inhibited with E-64 [N-(3-carboxyoxirane-2-carbonyl)-leucyl-amino(4-guanido)butane] was crystallized by sitting-drop vapour diffusion and the structure was solved by molecular replacement at 2.1 A resolution and refined to an R factor of 17.7% (R(free) = 23.8%). The enzyme belongs to the alpha+beta class of proteins and the structure shows the typical papain-like fold composed of two domains, the alpha-helix-rich (L) domain and the beta-barrel-like (R) domain, separated by a groove containing the active site formed by residues Cys25 and His159, one from each domain. The four monomers in the asymmetric unit show one E-64 molecule covalently bound to Cys25 in the active site and differences have been found in the placement of E-64 in each monomer.

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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 A and belong to the monoclinic space group P2(1), with unit-cell parameters a = 57.36, b = 90.45, c = 80.39 A, beta = 92.64 degrees . The asymmetric unit contains four molecules of mexicain, with a corresponding crystal volume per protein weight (V(M)) of 2.24 A(3) Da(-1) and a solvent content of 45% by volume. A molecular-replacement model has been determined and refinement is in progress.

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