MOLECULAR AND FUNCTIONAL-PROPERTIES OF A CALPAIN ACTIVATOR PROTEIN-SPECIFIC FOR MU-ISOFORMS

A natural calpain activator protein has been isolated from bovine brai n and characterized in its properties and molecular structure. The pro tein is a homodimer with a molecular mass of about 30 kDa and results in being almost identical to UK114 goat liver protein. Significant sim ilarities with mouse HR12 protein were also observed, whereas at lower degree of similarity was found with a family of heat-responsive prote ins named YJGF and YABJ from Haemophilus influenzae and Bacillus subti lis, respectively. The brain activator expresses a strict specificity for the mu-calpain isoform, being completely ineffective on the m-calp ain form. As expected, also UK114 was found to possess calpain-activat ing properties, indistinguishable from those of bovine brain activator . A protein showing the same calpain-activating activity has been also isolated from human red cells, indicating that this factor is widely expressed, All these activators are efficient on CL-calpain independen tly from the source of the proteinase. The high degree of specificity of the calpain activator for a single calpain isoform may be relevant for the understanding of sophisticated intracellular mechanisms underl ying intracellular proteolysis, These data are indicating the existenc e of a new component of the Ca2+ dependent proteolytic system, constit uted of members of a chaperonin-like protein family and capable of pro moting intracellular calpain activation.