Troponin-Tropomyosin Control of Thin Filament Activity Revealed by Electron Microscopy and 3-D Reconstruction.

Abstract

Muscle contraction and the actomyosin ATPase that drives the contractile process are switched on and off by changes in sarcoplasmic free Ca2+ -concentration. In skeletal and cardiac muscles, on-off switching is mediated by the actinassociated protein tropomyosin and by the troponin complex. While the details of this mechanism are still subject to debate, it is well-accepted that tropomyosin strands move to sterically block and unblock myosin binding sites on actin, thereby controlling actomyosin ATPase and consequently contraction. It is also well known that the Ca2+- dependency of the movement of tropomyosin on actin is governed by troponin.As a means of studying tropomyosin movement and the influence of troponin, we have used cryo-EM, negative staining and 3-dimensional helical reconstruction to define the positions of tropomyosin and troponin on thin filaments. We examined various preparations of native isolated filaments and filaments reconstituted with wild-type and mutant proteins.