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

The latex of <i>Calotropis gigantea</i> and <i>Calotropis procera</i> 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 <i>C. gigantea</i> 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 <i>C. gigantea</i> is orthologous to procerain, CpCp1-3, and SnuCalCp03 from <i>C. procera</i> and rather distinct from procerain B.