Русский 
English Structure of super-relaxed myosin heads in relaxed muscle
Background
In a relaxed state in a skeletal or cardiac muscle cell, the globular heads of myosin molecules do not interact with actin but are located near the backbone of a thick filament. Recently the complex nature of this state has been revealed and a new energy-conserving state of myosin called super-relaxed (SRX) has been discovered. Structurally SRX was attributed to an interacting heads motif (IHM) of a pair of myosin heads on the backbone of a thick filament discovered using cryo-electron microscopy. Two heads in the IHM are bound to each other and also have contacts with the neighbourhood.
Aims
Since the cryo-EM technique may cause errors in the positioning of myosin heads on the backbone the correctness of the IHM-based model as a description of the thick filament structure in the relaxed muscle should be verified.
Methods
X-ray diffraction studies of the whole femur muscle of tarantula were combined with the analysis of the diffraction patterns using mathematical modelling.
Results
We compared the intensities of the first six myosin layer lines calculated using IHM-based model with those of the ex vivo and skinned relaxed muscle of tarantula. We also did the comparison with the densities of the 2.0-nm 3D map, used to fit the 3JBH IHM-based model and the higher resolution (1.3 nm) thick-filament frozen-hydrated 3D maps. The R-factor for the IHM-based model and the ex vivo data was 4.9%, while those for the model and the 2.0- and 1.3-nm 3D-map densities were 4.0% and 8.4%, respectively, indicative of generally good fits.
Conclusions
The correctness of the cryo-EM based IHM model of myosin filaments for relaxed intact and chemically permeabilized muscles has been confirmed.