Calcium-induced changes in the location and conformation of troponin in skeletal muscle thin filaments.

Troponin is the regulatory protein of striated muscle. Without Ca2+, the contraction of striated muscle is inhibited. Binding of Ca2+ to troponin activates contraction. The location of troponin on the thin filaments and its relation to the regulatory mechanism has been unknown, though the Ca2+-induced dislocation of tropomyosin has been studied. By binding troponin(C+I) to actin in an almost stoichiometric ratio and reconstituting actin-tropomyosin-troponin(C+I) filaments, we reconstructed the three-dimensional structure of actin-tropomyosin-troponin(C+I) with or without Ca2+ from electron cryomicrographs to about 2.5 or 3 nm resolution, respectively. Without Ca2+, the three-dimensional map reveals the extra-density region due to troponin(C+I), which extends perpendicularly to the helix axis and covers the N-terminal and C-terminal regions of actin. In the presence of Ca2+, the C-terminal region of actin became more exposed, and troponin(C+I) became V-shaped with one arm extending towards the pointed end of the actin filament. This structure can be considered to show the location of troponin(C+I) in at least one of the states of skeletal muscle thin filaments. These Ca2+-induced changes of troponin(C+I) provide a clue to the regulatory mechanism of contraction.