Crystal Structure of Papain-Like Cysteine Protease, Calotropain FI, Purified from the Latex of Calotropis gigantea

The latex of Calotropis gigantea and Calotropis procera elute as two distinct peaks on cation exchange chromatography. One of the peaks is reported to possess multiple papain-like cysteine proteases with different biochemical properties that have been identified with different names. This is mainly due to the absence of a primary sequence for the proteases. Here, we report the crystal structures of calotropain FI from C. gigantea bound to the inhibitor E64 at pH 6 and 9 at 1.25 and 1.4 Å, respectively. Both structures are identical and are very similar to ervatamin B and papain structures. The high quality of electron density maps revealed the primary sequence of calotropain FI. The sequence comparison shows that calotropain FI from C. gigantea is orthologous to procerain, CpCp1-3, and SnuCalCp03 from C. procera and rather distinct from procerain B.