Annotating the x-ray diffraction pattern of vertebrate striated muscle

Area of Science:

• Biophysics
• Structural Biology
• Muscle Physiology

Background:

• Low-angle x-ray diffraction (LAXD) is crucial for analyzing myofilament molecular structure in striated muscle.
• LAXD has advanced understanding of myosin head organization in thick filaments within skeletal and cardiac muscle.
• Interpreting LAXD patterns requires modeling, often limited by low-resolution structural data and complex contributions from multiple filament components.

Purpose of the Study:

• To utilize an atomic model of the human cardiac thick filament C-zone to compute component contributions to the diffraction pattern.
• To objectively assess the roles of myosin heads, tails, titin, and cMyBP-C in LAXD patterns.
• To refine models of muscle contraction by clarifying structural contributions to x-ray reflections.

Main Methods:

• Employed an atomic model of the human cardiac thick filament C-zone derived from cryo-electron microscopy (cryo-EM).
• Utilized the myosin inhibitor mavacamten during model derivation.
• Performed computational analysis by including/excluding specific filament components (myosin heads, tails, titin, cMyBP-C) to calculate their diffraction contributions.

Main Results:

• Confirmed myosin heads are the primary source of intensity on myosin layer lines, including the M3 meridional reflection.
• Found myosin tails contribute minimally to the diffraction pattern, including the M6 meridional reflection, which mainly originates from heads and other components.
• Determined the M11 layer line (39-Å spacing) arises predominantly from the titin structure, enabling its use as a strain measure in the myosin filament backbone.

Conclusions:

• The study provides a more accurate deconvolution of structural contributions to muscle x-ray diffraction patterns.
• Findings challenge previous assumptions about myosin tail contributions and highlight titin's role in the M11 layer line.
• These insights will enhance the interpretation of x-ray patterns from intact muscle under various physiological and pharmacological conditions.