Ca2+-induced switching of troponin and tropomyosin on actin filaments as revealed by electron cryo-microscopy

Muscle contraction is regulated by the intracellular Ca2+ concentration. Ln vertebrate striated muscle, troponin and tropomyosin on actin filaments co mprise a Ca2+-sensitive switch that controls contraction. Ca2+ binds to tro ponin and triggers a series of changes in actin-containing filaments that l ead 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 determinin g the three-dimensional structure of thin filaments from electron cryo-micr ographs without imposing helical symmetry to similar to 35 Angstrom resolut ion With Ca2+, the globular domain of troponin was gourd-shaped and was loc ated over the inner domain of actin. Without Ca2+, the main body of troponi n was shifted by similar to 30 Angstrom towards the outer domain and bifurc ated, 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 similar to 35 Angstrom supporting the steric b locking model, however it is surprising that the N-terminal half of troyomy osin shifted less than similar to 12 Angstrom. Therefore tropomyosin shifte d differentially without Ca-2+. 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 sp ecific sites of troponin or tropomyosin by applying probabilistic distance geometry to data from fluorescence resonance energy transfer measurements. (C) 2001 Academic Press.