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Myosin light chain 2 modulates MgADP-induced contraction in rabbit skeletal and bovine cardiac skinned muscle

Hideaki Fujita *, Daisuke Sasaki *, Kenji Fukuda * and Shin'ichi Ishiwata *†

Skinned skeletal and cardiac muscle fibres can be activated by MgADP in the presence of MgATP without Ca²⁺; the isometric tension is developed in a sigmoidal manner with the addition of MgADP under relaxing conditions. The critical concentrations of MgADP for this MgADP-induced contraction are about 7.5 and 2.6 mM for skeletal and cardiac muscle fibres, respectively. To investigate whether muscle regulatory proteins, myosin light chain 2 (LC₂) and troponin C (TnC), play a part in the MgADP-induced contraction, these proteins were partly extracted by treatment with trans-1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid (CDTA), a chelater of divalent cations, and the MgADP-tension relationship was examined in rabbit psoas and bovine cardiac skinned fibres. We found that the sigmoidal MgADP-tension relationship became hyperbolic after a partial extraction of LC₂ (about 30 %) and TnC (about 70 %). Reconstitution with LC₂ restored the sigmoidal MgADP-tension relationship of control fibres almost fully in both skeletal and cardiac fibres, whereas reconstitution with TnC alone had no effect. Furthermore, cardiac fibres reconstituted with skeletal LC₂ exhibited an MgADP-tension relationship intermediate between skeletal and cardiac fibres. The partial extraction of LC₂ and TnC resulted in a reduction of the inhibitory effect of inorganic phosphate (P_i) on the MgADP-activated tension. Reconstitution with LC₂ restored the original P_i-tension relationship, whereas reconstitution with TnC had no effect. In other words, extraction of LC₂ apparently increased the affinity of myosin for MgADP but decreased the affinity for P_i. These results demonstrate that LC₂ modulates MgADP-induced activation of actomyosin interaction.

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