MUSCLE-SPECIFIC CALPAIN, P94, RESPONSIBLE FOR LIMB-GIRDLE MUSCULAR-DYSTROPHY TYPE 2A, ASSOCIATES WITH CONNECTIN THROUGH IS2, A P94-SPECIFICSEQUENCE

p94, a muscle-specific member of calpain family, is unique in that it undergoes rapid and exhaustive autolysis with a half-life of less than 1 h resulting in its disappearance from muscle. Recently, p94 was sho wn to be responsible for limb girdle muscular dystrophy type 2A. To el ucidate the muscular proteolytic system mediated by p94 and to solve t he mystery of its unusually rapid autolysis, we searched for p94-bindi ng proteins by the two hybrid system. Although calpain small subunit p lays a crucial role for regulation of ubiquitous calpains, it did not associate with p94. After a screening of skeletal muscle library, conn ectin (or titin), a gigantic filamentous protein spanning the M- to Z- lines of muscle sarcomere, was found to bind to p94 through a p94-spec ific region, IS2. The connectin-insoluble fraction of washed myofibril s contained full-length intact p94, suggesting that connectin regulate s p94 activity.