Effects on Ca²⁺-activated tension due to a synthetic NH₂-terminal actin peptide in single skeletal muscle fibers

Insight into the mechanism of force development in striated muscle will be provided by elucidating the specific regions of the actin molecule that interact with myosin and regulatory subunits of the thin filament during Ca²⁺-activated contraction. There is growing evidence that the acidic NH₂- terminal domain of actin 1) may represent an important binding site for myosin and 2) may interact with the inhibitory region of troponin I. The purpose of this study was to determine the effects of a synthetic peptide corresponding to a specific sequence of the NH₂-terminal domain of skeletal muscle actin on Ca²⁺-activated tension in chemically skinned single psoas skeletal muscle fibers. This study focused on the highly conserved Lys¹⁸- Arg²⁸ amino acid sequence of actin, a region of native actin that is believed to interact with troponin I and myosin. The effects of synthetic actin peptide Lys¹⁸-Arg²⁸ on tension development varied, depending on 1) the concentration of Ca²⁺ in the activating solutions and 2) the peptide concentration. At submaximal concentrations of Ca²⁺, isometric tension was reversibly potentiated in the presence of 100-500 μM synthetic actin peptide Lys¹⁸-Arg²⁸. Importantly, scrambling the sequence of Lys¹⁸-Arg²⁸ fully abolished the increase in Ca²⁺ sensitivity, providing evidence that the observed effects were specific to the sequence of peptide Lys¹⁸- Arg²⁸. In contrast, maximum Ca²⁺-activated tension was inhibited by millimolar concentrations of Lys¹⁸-Arg²⁸ and the scrambled peptide, indicating that this effect was nonspecific. The effect of peptide Lys¹⁸- Arg²⁸ to increase the Ca²⁺ sensitivity of tension is not known but may be due to an effect of the actin peptide to alter thin filament activation, a possibility consistent with proposed interactions between this domain of actin and the inhibitory region of troponin I.