Limitations of coarse grain models of actin filament on prediction of X-ray diffraction patterns in contracting skeletal muscle

Abstract

X-ray diffraction from contracting skeletal muscle provides critical structural information about myofilament organization and conformational changes during contraction. However, interpretation is challenging because observed patterns arise from the superposition of signals from multiple structural components with varying orientations and dynamics. Computational models based on explicit 3D sarcomere structures offer a way to separate these contributions and assign features in the diffraction pattern to specific molecular origins. Here, we use the MUSICO platform to generate stochastic, explicit 3D sarcomere structures and simulate X-ray diffraction patterns from all-atom and coarse-grained (CG) actin filament models with 1, 9, and 47 spheres per actin monomer. MUSICO fiber simulations were run for relaxed skeletal muscle (pCa 9), incorporating experimental values for sarcomere and interfilament spacing, temperature, actin filament length, and thin filament regulation with a 9-state crossbridge model. Comparison of simulated diffraction patterns revealed two principal CG-induced artifacts: (i) attenuation of scattering intensity due to the sphere form factor; and (ii) changes in the helical radius distribution, distorting radial intensity profiles and shifting peaks towards center. While meridional peak shapes were preserved across models, intensity differences were observed, though normalization largely removed these discrepancies. In contrast, equatorial and off- meridional profiles were more sensitive to CG level, with the 47-sphere model only valid up to the first actin meridional reflection. These findings quantify CG limitations in actin diffraction modeling and inform optimal model resolution for specific structural analyses.