An X-ray diffraction study of alpha-tropomyosin magnesium tactoid

Area of Science:

• Structural biology
• Biophysics
• X-ray crystallography

Background:

• Tropomyosin is a key muscle protein involved in muscle contraction.
• Understanding tropomyosin's aggregated structure is crucial for elucidating muscle function.
• Previous models of tropomyosin aggregates were based on electron microscopy.

Purpose of the Study:

• To determine the detailed structure of alpha-tropomyosin aggregates (Mg-tactoids) using X-ray diffraction.
• To compare the X-ray diffraction data with existing models derived from electron microscopy.
• To investigate the molecular arrangement and conformation within Mg-tactoids.

Main Methods:

• X-ray diffraction of magnetically oriented Mg-tactoid specimens.
• Isomorphous replacement using p-chloromercuribenzoate for phasing reflections.
• Analysis of meridional and equatorial reflections to determine structural parameters.
• Comparison of obtained axial electron density profile with electron microscopy models.

Main Results:

• A satisfactory agreement was found between the X-ray diffraction data and the proposed model when adequate phase combinations were used.
• The C-terminus of tropomyosin appears to have an extended conformation, penetrating the N-N overlap region.
• A principal repeat length of 39.0 nm was observed, indicating a 5% shortening likely due to supercoiling.
• Equatorial reflections revealed a large, low-crystallinity structural unit and suggested an inter-molecular spacing of approximately 2.3 nm.

Conclusions:

• The study provides a detailed structural model of alpha-tropomyosin Mg-tactoids consistent with X-ray diffraction data.
• Evidence suggests a supercoiled, possibly helical, arrangement of tropomyosin molecules within the tactoid.
• The findings offer insights into the molecular basis of tropomyosin aggregation and its role in muscle structure.