title: Dissociation of m-calpain subunits occurs after autolysis of the N-terminus of the catalytic subunit, and is not required for activation.
abstract: Calpain is a heterodimeric, intracellular Ca(2+)-dependent, "bio-modulator" that alters the properties of substrates through site-specific proteolysis. It has been proposed that calpains are activated by autolysis of the N-terminus of the large subunit and/or its dissociation into the subunits. It is, however, unclear whether the dissociation into subunits is required for the expression of protease activity and/or for in vivo function. Recently, the crystal structure of m-calpain in the absence of Ca(2+) has been resolved. The 3D structure clearly shows that the N-terminus of the m-calpain large subunit (mCL) makes contact with the 30K subunit, suggesting that autolysis of the N-terminus of mCL changes the interaction of both subunits. To examine the relationship between autolysis, dissociation, and activation, we made and analysed a series of N-terminal mutants of mCL that mimic the autolysed forms or have substituted amino acid residue(s) interacting with 30K. As a result, the mutant m-calpains, which are incapable of autolysis, did not dissociate into subunits, whereas those lacking the N-terminal 19 residues (Delta 19), but not those lacking only nine residues (Delta 9), dissociated into subunits even in the absence of Ca(2+). Moreover, both Delta 9 and Delta 19 mutants showed an equivalent reduced Ca(2+) requirement for protease activity. These results indicate that autolysis is necessary for the dissociation of the m-calpain subunits, and that the dissociation occurs after, but is not necessary for, activation.
authors.affiliation: Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nakagawa
authors.forename: H
authors.initials: H
authors.lastname: Masumoto
authors.forename: H
authors.initials: H
authors.lastname: Sorimachi
authors.forename: K
authors.initials: K
authors.lastname: Suzuki
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001329:Autolysis; D002118:Calcium / Q000378:metabolism*; D002154:Calpain / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020134:Catalytic Domain; D002460:Cell Line; D004789:Enzyme Activation; D004795:Enzyme Stability; D006801:Humans; D007313:Insecta / Q000166:cytology; D008958:Models, Molecular; D008970:Molecular Weight; D016297:Mutagenesis, Site-Directed; D021122:Protein Subunits
chemical_list: D021122:Protein Subunits; D002154:Calpain; D002118:Calcium