Electrostatic potential around actin.

Abstract

We presume that tension of contracting muscle originates from electrostatic force experienced by actin and myosin. We suppose that a high-energy state of myosin-ADP-Pi interacts with actin, transferring the stored energy to actin, and that the actin excited in this way develops around itself electric field which exerts sliding force against charged myosin heads. To explore the idea, first we conjectured how electric charges on actin produce electric field in the axial direction, and second we experimentally examined electrostatic circumstances around actin in solution and in muscle fibers by optimizing diffusion-enhanced fluorescence energy transfer. In the experiments, Tb ion, which has a long excited-state lifetime, was used as donor. To introduce Tb to actin, Tb-DTPA-phalloin and Tb-DTPA-maleimide were synthesized. As acceptors with electric charges (Za = -3 to +2), rhodamine B that was conjugated with various amino acids or their derivatives was used. The fluorescence energy transfer efficiency (ET) was estimated from the shortening in the lifetime of Tb. The electrostatic circumstances around actin were inferred from the ET-Za relation. When Tb was introduced at Cys-374 of actin, the Tb-site was found in negative electric potential. S-1 binding to the labeled actin neutralized the electric potential almost completely. Tb-DTPA-phalloin bound to actin seemed to reside in the vicinity of tryptophan residue(s). Electric potential around the phalloin site was negative. S-1 binding to the actin slightly reduced the negativity. In glycerinated fibers in the rigor state, the phalloin site was again found in negative potential. When fibers were transferred from an ADP-rigor solution to an active solution, the negative electric potential was neutralized to some extent. The direction of this change could not be explained by detachment of crossbridges from actin, since the detachment should have given an opposite direction of changes in the electric potential. Thus, this observation may indicate that electric potential characteristic of the active state occurs at actin surfaces.