REGULATORY DOMAINS OF THE HUMAN CALPAIN FAMILY
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Calpains are intracellular enzymes that merge cysteine protease and calcium sensing activities together in one molecule. They respond to Ca2+ signals and modify the activity of their targets by selective proteolysis. Calpains are involved in normal cellular process like cell migration and apoptosis. The over-activation of calpain due to disturbances in Ca2+ homeostasis or inactivation due to mutations, contribute to diseases like ischemic injury and muscular dystrophy. The classical calpains 1 and 2 are heterodimeric enzymes containing a large (80 kDa) subunit and a small subunit (28 kDa). Dimerization occurs through the 5th EF-hand of penta-EF-hand (PEF) domains present in both large and small subunits. In this study, I have used structural genomics approaches to explore the PEF and C2-like regulatory domains of some of the other 12 human calpain isoforms. I have shown that recombinant PEF domain of skeletal muscle-specific calpain 3 exists as a stable homodimer when produced alone. Modelling studies suggest that there would be no barriers for dimerization of the full-length enzyme through the PEF domains which would place the protease cores at opposite ends of the dimer. Co-expression studies using small subunit were performed with PEF domains of calpains 1, 3, 8, 9, 11, 12 and 13. A differential tagging system was devised to differentiate heterodimers from homodimers. The PEF domains of calpains 1, 3, 9 and 13 co-expressed with the small subunit, while the others failed to express. The PEF domains of calpains 1 and 9 formed heterodimers. Conversely, the PEF domain of calpain 3 formed a homodimer and that of calpain 13 predominantly formed a homodimer with a small amount of heterodimer. Homodimerization of calpains implies they are less-likely to be inhibited by the endogenous calpain inhibitor, calpastatin. C2-like regulatory domains of calpains 5-13 were also studied. The structure of the distal C2-like domain of calpain 7 was solved. It is markedly different from canonical C2 domains and may not bind Ca2+.