Muscle contraction is regulated by the intracellular Ca2+concentration. In vertebrate striated muscle, troponin and tropomyosin on actin filaments comprise a Ca2+-sensitive switch that controls contraction. Ca2+binds to troponin and triggers a series of changes in actin-containing filaments that lead to cyclic interactions with myosin that generate contraction. However, the precise location of troponin relative to actin and tropomyosin and how its structure changes with Ca2+have been not determined. To understand the regulatory mechanism, we visualized the location of troponin by determining the three-dimensional structure of thin filaments from electron cryo-micrographs without imposing helical symmetry to ∼35 Å resolution. With Ca2+the globular domain of troponin was gourd-shaped and was located over the inner domain of actin. Without Ca2+the main body of troponin was shifted by ∼30 Å towards the outer domain and bifurcated, with a horizontal branch (troponin arm) covering the N and C-terminal regions of actin. The C-terminal one-third of tropomyosin shifted towards the outer domain of actin by ∼35 Å supporting the steric blocking model, however it is surprising that the N-terminal half of tropomyosin shifted less than ∼12 Å. Therefore tropomyosin shifted differentially without Ca2+With Ca2+tropomyosin was located entirely over the inner domain thereby allowing greater access of myosin for force generation. The interpretation of three-dimensional maps was facilitated by determining the three-dimensional positions of fluorophores labelled on specific sites of troponin or tropomyosin by applying probabilistic distance geometry to data from fluorescence resonance energy transfer measurements.
All Science Journal Classification (ASJC) codes
- Structural Biology
- Molecular Biology