Identification of the sequence of the regulatory light chain required for the phosphorylation-dependent regulation of actomyosin.

The amino acid structure of regulatory light chain which is essential to express the phosphorylation-mediated regulation of smooth muscle actomyosin ATPase was studied. Regulatory light chain of smooth muscle heavy meromyosin (HMM) was truncated by either lysylendopeptidase or trypsin. Lysylendopeptidase cleaved the regulatory light chain initially at the C-terminal side of lysine 6 (Lys C(1)-HMM) and subsequently at the C-terminal side of lysine 12 (Lys C(2)-HMM). On the other hand, trypsin cleaved at the C-terminal side of arginine 16 (tryp-HMM). While the actin activated ATPase activity of Lys C(1)-HMM and Lys C(2)-HMM was markedly activated by phosphorylation, that of tryp-HMM was not activated by phosphorylation. The exchange of cleaved regulatory light chain of tryp-HMM with undigested regulatory light chain restored the phosphorylation-mediated regulation on the actin activated ATPase activity. The regulatory light chain of the undigested HMM was also exchanged with the trypsin-digested regulatory light chain and this abolished the phosphorylation dependence of acto-HMM ATPase activity. These results show that the amino acid sequence arginine 13-arginine 16 is essential to express the regulation of actin activated ATPase of smooth muscle myosin which is mediated by the phosphorylation at serine 19 of the regulatory light chain.