A novel member of the calcium-dependent cysteine protease family

In the course of cDNA cloning of the large subunits of human mu- and mCANPs, a novel cDNA clone encoding a putative calcium-dependent cysteine protease homologous to but distinct from both mu- and m-types was found. The encoded protein, designated tentatively as p94, is composed of four domains similar to those found in other CANP large subunits, but includes three unique regions that have no homology to other CANPs. These unique sequences might be involved in regulating the activation and/or determining the intracellular localization of p94. Since the mRNA for p94 is five times more abundant than that for the CANP small subunit in skeletal muscle, it is possible that p94 does not associate with the small subunit in vivo. In contrast to the ubiquitous expression of mu- and m-types, the mRNA for p94 is expressed only in skeletal muscle. Besides acting as a protease, p94 may act as a skeletal muscle specific regulatory protein like troponin C.