Calcium ions and the structure of muscle actin filament. An X-ray diffraction study.
In order to investigate the effects of Ca(2+)-binding to troponin on the conformation of the muscle thin filament (consisting of actin, tropomyosin and troponin) in the absence of actomyosin interaction, two series of X-ray diffraction experiments were undertaken. Firstly, the small angle X-ray scattering from filaments in solution indicate the tropomyosin strands are centred at about 3.5 nm from the filament axis and this distance is calcium independent. Secondly, X-ray fibre diffraction patterns from the filaments orientated in glass capillaries were studied. The X-ray intensity of the 2nd actin layer-line increased in a highly co-operative manner at a concentration of free calcium ions [Ca2+] of 10(-6.8) M, which is the range in which muscle contraction is physiologically regulated. However, this intensity increase accounted for some 30% of the total increase observed in diffraction patterns from muscle on activation, suggesting that the Ca(2+)-binding alters the state of the thin filament, which then undergoes further changes upon interaction with myosin.
Popp
D
D
Department of Biophysics, Max Planck Institute for Medical Research, Heidelberg, Germany.
Maéda
Y
Y
D000199:Actins / Q000648:ultrastructure*; D000818:Animals; D002118:Calcium / Q000502:physiology*; D009119:Muscle Contraction; D009132:Muscles / Q000502:physiology / Q000648:ultrastructure*; D014961:X-Ray Diffraction
D000199:Actins; D002118:Calcium