Structure, expression, and function of the Xenopus laevis caspase family

Caspases, a family of cysteine proteases, have been recognized as the central executors of programmed cell death. Nonetheless, the information on the caspase family has been limited to mammals, Drosophila, and nematodes. To examine the structure and characterization of the Xenopus caspase family, we have cloned the cDNAs encoding caspase-2 and -6-10 in addition to caspase-1 and -3, which we characterized previously (Yaoita, Y., and Nakajima, K. (1997) J. Biol. Chem. 272, 5122-5127). First, the existence of these caspases in frog suggests that the caspase cascades clarified in mammals are conserved at least from Amphibia. Interestingly, Xenopus caspase-1, -8, and -10 (especially caspase-8) showed a lower degree of identity to human equivalents than the other caspases. Second, mRNAs of many caspases increased during the climax of metamorphosis in regressing organs, tail, and intestine, where programmed cell death occurs, but not in apoptotic tail-derived cultured cells (XLT-15-11) treated with thyroid hormone, showing that new RNA synthesis of caspases is dispensable to programmed cell death. Third, comparison of human and Xenopus caspase sequences implies that some proposed regulations of human caspases are not conserved in frog.