Purification and characterization of nebulin subfragments produced by 0.1 mM CaCl2

Nebulin, which forms a long inextensible filament in sarcomeres, was fragmented into 200-, 180-, 40-, 33-, and 23-kDa subfragments on treatment with 0.1 mM CaCl2. The subfragments released from myofibrils were successfully purified by immunoaffinity column chromatography. The 200-, 40-, 33-, and 23-kDa subfragments were released from myofibrils and occupied 80% of the nebulin filaments. The remainder comprised the 180-kDa subfragment bound to the myofibrils. There is a possibility that an entire nebulin filament is constructed from the 200-, 180-, 40-, 33-, and 23-kDa subfragments. We have developed a new "fluorescence-method" to detect the binding of calcium ions to a protein using quin2, and clarified that nebulin is a calcium-binding protein, and that calcium ions bind to the 200-, 40-, and 23-kDa subfragments. Nebulin filaments are probably fragmented on the binding of large amounts of calcium ions to the 200-, 40-, and 23-kDa subfragments.