Relaxed and active thin filament structures; a new structural basis for the regulatory mechanism.
The structures of muscle thin filaments reconstituted using skeletal actin and cardiac troponin and tropomyosin have been determined with and without bound Ca2+using electron microscopy and reference-free single particle analysis. The resulting density maps have been fitted with atomic models of actin, tropomyosin and troponin showing that: (i) the polarity of the troponin complex is consistent with our 2009 findings, with large shape changes in troponin between the two states; (ii) without Ca2+the tropomyosin pseudo-repeats all lie at almost equivalent positions in the 'blocked' position on actin (over subdomains 1 and 2); (iii) in the active state the tropomyosin pseudo-repeats are all displaced towards subdomains 3 and 4 of actin, but the extent of displacement varies within the regulatory unit depending upon the axial location of the pseudo-repeats with respect to troponin. Individual pseudo-repeats with Ca2+bound to troponin can be assigned either to the 'closed' state, a partly activated conformation, or the 'M-state', a fully activated conformation which has previously been thought to occur only when myosin heads bind. These results lead to a modified view of the steric blocking model of thin filament regulation in which cooperative activation is governed by troponin-mediated local interactions of the pseudo-repeats of tropomyosin with actin.
Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol BS8 1TD, UK. Electronic address: Danielle.paul@bristol.ac.uk.
Danielle M
DM
Paul
Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol BS8 1TD, UK.
John M
JM
Squire
Division of Structural Biology, The Institute of Cancer Research, London SW3 6JB, UK.
Edward P
EP
Morris
D008841:Actin Cytoskeleton / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D008854:Microscopy, Electron; D011485:Protein Binding; D014335:Tropomyosin / Q000737:chemistry / Q000378:metabolism*; D014336:Troponin / Q000737:chemistry / Q000378:metabolism*
D000199:Actins; D014335:Tropomyosin; D014336:Troponin; D002118:Calcium
Actin; Regulation; Thin filament; Tropomyosin; Troponin