A NEW SUBFAMILY OF VERTEBRATE CALPAINS LACKING A CALMODULIN-LIKE DOMAIN - IMPLICATIONS FOR CALPAIN REGULATION AND EVOLUTION

Calpains are calcium-dependent intracellular nonlysosomal proteases th at are believed to participate in signal transduction. In vertebrates, five different calpains have so far been identified, of which three, mu-, m-, and mu/m-calpain, are ubiquitously expressed while the other two, nCL-1 (p94) and nCL-2, exhibit a restricted tissue distribution. We have identified two new vertebrate calpain genes, Capn5 and Capn6. The human and mouse amino acid sequences of these new calpains are the most divergent of the vertebrate calpains identified. They possess mo st of the residues conserved in calpain family members but the C-termi nal region lacks any homology to the calmodulin-like domain of other v ertebrate calpains. They both exhibit significant homology over the en tire coding region to the protein encoded by the gene tra-3, involved in nematode sex determination, and Capn5 may represent its vertebrate orthologue. The predicted Capn6 protein lacks critical active site res idues and may not be proteolytically active. Both genes are differenti ally expressed in human tissues with highest RNA levels for Capn5 ocur ring in the testis, liver, trachea, colon, and kidney, while Capn6 is highly expressed only in the placenta sample of the 50 tissues examine d. Phylogenetic analysis suggests that the vertebrate calpains arose t hrough a series of gene duplication events that began before the initi al divergence of the vertebrate and invertebrate lineages. The discove ry of these two new calpains highlights a hitherto unknown complexity of the calpain family with subclasses perhaps possessing different mod es of regulation. (C) 1997 Academic Press.