ROLE OF 17-KDA ESSENTIAL LIGHT CHAIN ISOFORMS OF AORTA SMOOTH-MUSCLE MYOSIN

Aorta smooth muscle myosin contains two kinds of 17-kDa essential light chain, LC17nm (nonmuscle-type) and LC17gi (gizzard-type) [Hasegawa, Y., Ueda, Y., Watanabe, M., & Morita, F. (1992) J. Biochem. 111, 798-803]. The LC17 isoforms were released from porcine aorta myosin by incubation at 46-degrees-C. The rate of release was 1.5 to 2 times higher with LC17gi than with LC17nm. Aorta myosins containing the two LC17 isoforms in various ratios could be reconstituted. The actin-activated ATPase activity was measured as a function of LC17nm content. The V(m) value was lower with myosin which contained more LC17nm. The apparent dissociation constant for F-actin, K(m) was 20-fold less with myosin which contained 81% LC17nm than myosin which contained 23% LC17nm. A similar difference in the dissociation constants of myosin for F-actin was observed in the presence of adenylyl imidodiphosphate. The role of LC17nm appears to be to make aorta myosin suitable for maintaining the muscle tension with a low expenditure of energy. The isoform-dependent difference in the F-actin-binding affinities of myosin seems partly due to the difference in the affinities of LC17 isoforms themselves for F-actin. We found that the isolated LC17nm itself could bind with F-actin with a dissociation constant of 64-mu-M, but LC17gi could not. The C-terminal region where the amino acid residues are substituted in the two isoforms [Hasegawa, Y., Ueda, Y., Watanabe,M., & Morita,F.(1992) J. Biochem. 111, 798-803; Lash, J.A., Helper,D.J., Klug,M., Nicolozakes,A.W., & Hathaway,D.R. (1990) Nucleic Acids Res, 18, 7176] appears to be involved in actin-binding, heavy chain-binding, and ATPase activity.