Requirement of the 20-kDa light chain for the papain-resistant conformation of gizzard myosin.

The limited chymotryptic digestion of unphosphorylated gizzard myosin in 0.15 M NaCl converted a papain-insensitive myosin in ATP to a papain-sensitive one. This conversion without phosphorylation of its 20-kDa light chain was accompanied with truncation of a 200-kDa heavy chain to a 195-kDa fragment and with the degradation of a 20-kDa light chain. Papain also yielded the 195-kDa fragment from the heavy chain, irrespective of the presence or absence of ATP. However, the ATP-induced protection of unphosphorylated myosin from the papain-digestion disappeared concurrently with degradation of the 20-kDa light chain by papain rather than the truncation of heavy chain. Papers from two laboratories [Onishi, H. & Watanabe, S. (1984) J. Biochem. (Tokyo) 95, 903-905; Kumon, A., Yasuda, S., Murakami, N., and Matsumura, S. (1984) Eur. J. Biochem. 140, 265-271] have reported that the ATP-protection of unphosphorylated myosin against papain is not observed after the 20-kDa light chain has been phosphorylated. The present results might indicate that the ATP-induced protection is also abolished through the chymotryptic degradation of the 20-kDa light chain.