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.
Ishikawa
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T
Photosynthesis Research Laboratory, the Institute of Physical and Chemical Research (RIKEN), Wako, Saitama, Japan.
Wakabayashi
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T
D008841:Actin Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D000199:Actins / Q000378:metabolism / Q000648:ultrastructure; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020285:Cryoelectron Microscopy; D007089:Image Enhancement; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000166:cytology* / Q000378:metabolism; D011487:Protein Conformation; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism / Q000648:ultrastructure; D014336:Troponin / Q000737:chemistry* / Q000378:metabolism* / Q000648:ultrastructure; D019209:Troponin C / Q000737:chemistry / Q000378:metabolism; D019210:Troponin I / Q000737:chemistry / Q000378:metabolism
D000199:Actins; D014335:Tropomyosin; D014336:Troponin; D019209:Troponin C; D019210:Troponin I; D002118:Calcium