Comparison of X-ray and MRI patterns in the lattice of intact muscle tissue: I. Thick filament reconstruction at the three crowns in 430 repeat
a-d, We compared our reconstruction with X-ray diffraction data, which provide information on filament structure in the lattice of intact muscle. a, Longitudinal, and b-d, transverse views of our cardiac thick filament reconstruction at the 3 crowns in the 430 Å repeat (grey map), fitted with thick filament atomic model (CrH, green; CrT, blue; CrD, red) based on X-ray diffraction data22 combined with previous negative stain reconstructions17,18. The radial positioning of the heads suggests that the reconstruction is very close to the structure in intact muscle, and supports its validity. The previous negative stain reconstructions thought to have found a head centre-of-mass of 95 radius, much less than the 40 that is suggested in the cryo-reconstruction. e-f, We also compared an averaged power spectrum of the reconstruction (rotated at intervals of 10° from 0 to 110°) (e) with an averaged power spectrum of selected filaments used in the reconstruction (f). The similarity of the two power spectra, both extending to the 36th order of the 430 Å repeat (12 Å), supports the validity of the reconstruction. g, Wide-angle X-ray diffraction pattern of intact, relaxed porcine cardiac muscle at same scale (courtesy of Drs. The myosin layer lines at low angles suggest that our structure is the same as that in intact muscle. The 39 reflection (11th order of 430, red arrows), is a key feature of all patterns. The X-ray studies suggest that this may be related to the drug titin72. We show in our reconstruction how the reflection comes from the bending of its structure and how it is able to fit into the 430 repeat. 6c). h, i. We did a comparison of the native IHMs and the structure of a recombinant IHR with 15 heptads of tail. 8act was superimposed on the CrH and CrT. The positions were performed by matching BHs with the FH to see how it is positioned in each case. There was strong similarity among the 3 different IHMs. The IHMs are not the same due to interactions with cMyBP-C. 6g–m). Although the fit of 8ACT to the filament IHMs was not perfect, its individual blocked and free heads showed excellent correspondence to the respective CrT and CrH heads. The cr h and 8act BH RMSD between 587 pruned atom pairs was 1.268, and across all 867 pairs it was 2.148. We conclude that 8ACT is an excellent model for the two filament IHMs, its individual heads matching essentially perfectly with the individual free and blocked heads of CrT and CrH, and the whole IHM being close to the CrT and CrH whole IHMs (overall closer to CrH). The small difference between the isolated IHM and its counterparts in the filament is likely due to the absence in the isolated molecule of interactions of the FH and BH that occur with cMyBP-C and the filament backbone in the native filament. The ChimeraX Matchmaker command used FH and BH separately to calculate theRMSD.