Filament-forming domain of carp dorsal myosin rod.

Substructure of the myosin rod and its correlation to the filament formation were investigated by using fish myosin rod. It was found that fish rod contains a unique chymotrypsin susceptible site, 40 kDa from the COOH-terminus or 20 kDa downstream from the subfragment-2/light meromyosin junction (S-2/LMM junction). Cleavage at this new site produced subfragment-2 possessing 95 kDa subunit (95k S-2) and light meromyosin possessing 40 kDa subunit (40k LMM). The latter is the shortest unit ever reported to exhibit filament formation. Moreover, the 40k LMM was able to form filaments independently of the presence of Mg2+, while filament formation of rod and ordinary LMM (70k LMM) was promoted by Mg2+ addition. These results indicated that the Mg2+ binding sites are present within the NH2-terminal 20 kDa region of the 70k LMM. We concluded that the COOH-terminal 40 kDa portion of rod is responsible for the self-assembly ability of myosin, while the NH2-terminal 20 kDa region of the 70k LMM is the regulatory domain for thick filament formation through Mg(2+)-binding.