The Essential Light Chain N-terminal Extension Alters Force and Fiber Kinetics in Mouse Cardiac Muscle

Abstract

The functional significance of the actin-binding region at the N terminus of the cardiac myosin essential light chain (ELC) remains elusive. In a previous experiment, the endogenous ventricular ELC was replaced with a protein containing a 10-amino acid deletion at positions 5-14 (ELC1v⌬5-14, referred to as 1v⌬5-14), a region that interacts with actin (Sanbe, A., Gulick, J., Fewell, J., and Robbins, J. (2001) J. Biol. Chem. 276, 32682-32686). 1v⌬5-14 mice showed no discernable mutant phenotype in skinned ventricular strips. However, because the myofilament lattice swells upon skinning, the mutant phenotype may have been concealed by the inability of the ELC to reach the actin-binding site. Using the same mouse model, we repeated earlier measurements and performed additional experiments on skinned strips osmotically compressed to the intact lattice spacing as determined by x-ray diffraction. 1v⌬5-14 mice exhibited decreased maximum isometric tension without a change in calcium sensitivity. The decreased force was most evident in 5-6-month-old mice compared with 13-15-month-old mice and may account for the greater ventricular wall thickness in young 1v⌬5-14 mice compared with age-matched controls. No differences were observed in unloaded shortening velocity at maximum calcium activation. However, 1v⌬5-14 mice exhibited a significant difference in the frequency at which minimum complex modulus amplitude occurred, indicating a change in cross-bridge kinetics. We hypothesize that the ELC N-terminal extension interaction with actin inhibits the reversal of the power stroke, thereby increasing isometric force. Our results strongly suggest that an interaction between residues 5-14 of the ELC N terminus and the C-terminal residues of actin enhances cardiac performance.

Influence of fast and slow alkali myosin light chain isoforms on the kinetics of stretch-induced force transients of fast-twitch type IIA fibres of rat

“…The fact that the MLC1f/MLC3f ratio does not influence the myosin head kinetics [5] excludes this possibility at first glance. However, studies on heart preparations showed that the myosin head kinetics is decelerated by the interaction of the N-terminal extension of the ventricular alkali MLC isoform MLC1v with the actin filament [35,37]. MLC1v is completely identical with skeletal muscle MLC1s, and therefore, it is expected that the N-terminal extension of MLC1s also binds to the actin filament thereby decelerating the myosin head kinetics.…”

Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium

“…This result implies, therefore, that the amount of osmotic compression that is required to induce the apparent switch may depend on sarcomere length. In a recent study in isolated skinned murine myocardium, it was reported that much higher levels of dextran application (Ͼ2% dextran) were required to induce compression of the myofilament lattice (28). It should be noted, however, that no attempt was made in that study to limit activation of the PKA system.…”

Myosin essential light chain in health and disease

“…However, when skinned muscle strips from ELC v⌬5-14 mice were osmotically compressed to intact lattice spacing, a decrease in maximum isometric tension without a change in calcium sensitivity was observed (50). These results suggested that the interaction between actin and the NH 2 -terminus of ELC and specifically with amino acid residues 5-14 plays an important role in cardiac muscle performance (50).…”

“…The movement of myosin S2 together with the myosin head (S1) on the surface of the synthetic myosin filaments was only observed for the native compared with truncated ELC mutant proteins (68 -70). Using transgenesis, Miller et al demonstrated that a ELC v missing the NH 2 -terminal residues 5-14 resulted in decreased maximum isometric tension without any change in Ca 2ϩ sensitivity measured in skinned ventricular strips that were osmotically compressed to intact lattice spacing (50). However, no alterations in either unloaded shortening or maximum shortening velocities were observed when muscle strips were not compressed (80).…”