title: Distinct substrate specificities and functional roles for the 78- and 76-kDa forms of mu-calpain in human platelets.
abstract: The intracellular thiol protease mu-calpain exists as a heterodimeric proenzyme, consisting of a large 80-kDa catalytic subunit and a smaller 30-kDa regulatory subunit. Activation of mu-calpain requires calcium influx across the plasma membrane and the subsequent autoproteolytic conversion of the 80-kDa large subunit to a 78-kDa "intermediate" and a 76-kDa fully autolyzed form. Currently, there is limited information on the substrate specificities and functional roles of these distinct active forms of mu-calpain within the cell. Using antibodies that can distinguish among the 80-, 78-, and 76-kDa forms of mu-calpain, we have demonstrated a close correlation between the autolytic generation of the 78-kDa enzyme and the proteolysis of the non-receptor tyrosine phosphatase, PTP-1B, in ionophore A23187-stimulated platelets. Time course studies revealed that pp60(c-)src proteolysis lagged well behind that of PTP-1B and correlated closely with the generation of the fully proteolyzed form of mu-calpain (76 kDa). In vitro proteolysis experiments with purified mu-calpain and immunoprecipitated PTP-1B or pp60(c-)src confirmed selective proteolysis of pp60(c-)src by the 76-kDa enzyme, whereas PTP-1B cleavage was mediated by both the 76- and 78-kDa forms of mu-calpain. Studies using selective pharmacological inhibitors against the different autolytic forms of mu-calpain have demonstrated that the initial conversion of the mu-calpain large subunit to the 78-kDa form is responsible for the reduction in platelet-mediated clot retraction, whereas complete proteolytic activation of mu-calpain (76 kDa) is responsible for the shedding of procoagulant-rich membrane vesicles from the cell surface. These studies demonstrate the existence of multiple active forms of mu-calpain within the cell, that have unique substrate specificities and distinct functional roles.
authors.affiliation: Department of Medicine, Monash Medical School, the Australian Centre for Blood Diseases, Victoria 3128, Australia.
authors.forename: S M
authors.initials: SM
authors.lastname: Schoenwaelder
authors.forename: S
authors.initials: S
authors.lastname: Kulkarni
authors.forename: H H
authors.initials: HH
authors.lastname: Salem
authors.forename: S
authors.initials: S
authors.lastname: Imajoh-Ohmi
authors.forename: W
authors.initials: W
authors.lastname: Yamao-Harigaya
authors.forename: T C
authors.initials: TC
authors.lastname: Saido
authors.forename: S P
authors.initials: SP
authors.lastname: Jackson
mesh_terms: D000906:Antibodies; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D000001:Calcimycin / Q000494:pharmacology; D002154:Calpain / Q000097:blood*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D019281:Dimerization; D004151:Dipeptides / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000097:blood*; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D008970:Molecular Weight; D015388:Organelles / Q000201:enzymology; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism; D016392:Proto-Oncogene Proteins pp60(c-src) / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D000906:Antibodies; D015853:Cysteine Proteinase Inhibitors; D004151:Dipeptides; D004792:Enzyme Precursors; D046911:Macromolecular Substances; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; C071482:calpeptin; D000001:Calcimycin; D004533:Egtazic Acid; D016392:Proto-Oncogene Proteins pp60(c-src); D017027:Protein Tyrosine Phosphatases; D002154:Calpain; C098484:mu-calpain proenzyme