RESUMEN
A ternary complex of the black-eyed pea trypsin and chymotrypsin inhibitor (BTCI) with trypsin and chymotrypsin was crystallized by the sitting-drop vapour-diffusion method with 0.1 M HEPES pH 7.5, 10%(w/v) polyethylene glycol 6000 and 5%(v/v) 2-methyl-2,4-pentanediol as precipitant. BTCI is a small protein with 83 amino-acid residues isolated from Vigna unguiculata seeds and is able to inhibit trypsin and chymotrypsin simultaneously by forming a stable ternary complex. X-ray data were collected from a single crystal of the trypsin-BTCI-chymotrypsin ternary complex to 2.7 A resolution under cryogenic conditions. The structure of the ternary complex was solved by molecular replacement using the crystal structures of the BTCI-trypsin binary complex (PDB code 2g81) and chymotrypsin (PDB code 4cha) as search models.