Muscle force is generated by myosin heads stereospecifically attached to actin.
Muscle force is generated by myosin crossbridges interacting with actin. As estimated from stiffness and equatorial X-ray diffraction of muscle and muscle fibres, most myosin crossbridges are attached to actin during isometric contraction, but a much smaller fraction is bound stereospecifically. To determine the fraction of crossbridges contributing to tension and the structural changes that attached crossbridges undergo when generating force, we monitored the X-ray diffraction pattern during temperature-induced tension rise in fully activated permeabilized frog muscle fibres. Temperature jumps from 5-6 degrees C to 16-19 degrees C initiated a 1.7-fold increase in tension without significantly changing fibre stiffness or the intensities of the (1,1) equatorial and (14.5 nm)(-1) meridional X-ray reflections. However, tension rise was accompanied by a 20% decrease in the intensity of the (1,0) equatorial reflection and an increase in the intensity of the first actin layer line by approximately 13% of that in rigor. Our results show that muscle force is associated with a transition of the crossbridges from a state in which they are nonspecifically attached to actin to one in which stereospecifically bound myosin crossbridges label the actin helix.
Bershitsky
S Y
SY
National Institute for Medical Research, Mill Hill, London, UK.
Tsaturyan
A K
AK
Bershitskaya
O N
ON
Mashanov
G I
GI
Brown
P
P
Burns
R
R
Ferenczi
M A
MA
D000199:Actins / Q000502:physiology*; D000818:Animals; D001696:Biomechanical Phenomena; D066298:In Vitro Techniques; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D009132:Muscles / Q000502:physiology*; D009218:Myosins / Q000502:physiology*; D011896:Rana temporaria; D013696:Temperature; D014961:X-Ray Diffraction
D000199:Actins; D009218:Myosins