title: Calpain subunits remain associated during catalysis.
abstract: The Ca2+ dependent cysteine proteases, calpains are heterodimers containing a large (ca. 80 kDa) catalytic subunit and a 25-30 kDa small subunit. Whether calpains remain dimers while catalyzing hydrolysis of protein substrates has been controversial. Now by doing subunit co-immunoprecipitation, we provide direct evidence to resolve this argument. In the presence of Ca2+ concentrations which permit catalytic activity, both subunits of either m- or mu-calpain are co-immunoprecipitated by monoclonal antibodies directed against a single subunit. Furthermore, both subunits can be co-immunoprecipitated during calpain-catalyzed proteolysis of the substrate casein. These results indicate that both major calpain isozymes maintain their heterodimeric form during the catalytic cycle. Thus, small subunit might have a direct role in regulating the physiologic function of either major calpain isozyme.
authors.affiliation: Department of Pharmacology and Therapeutics, Medical College of Ohio, Toledo 43699-0008, USA.
authors.forename: W
authors.initials: W
authors.lastname: Zhang
authors.forename: R L
authors.initials: RL
authors.lastname: Mellgren
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002154:Calpain / Q000737:chemistry / Q000378:metabolism*; D002364:Caseins / Q000378:metabolism; D002384:Catalysis; D002417:Cattle; D006801:Humans; D006868:Hydrolysis
chemical_list: D002364:Caseins; D002154:Calpain; D002118:Calcium