Geometrical constraints affecting crossbridge formation in insect flight muscle

Area of Science:

• Muscle physiology
• Biophysics
• Structural biology

Background:

• Insect flight muscle structure is complex, involving interactions between thick (myosin) and thin (actin) filaments.
• Understanding the precise arrangement and interaction of myosin crossbridges with actin is crucial for explaining muscle contraction mechanics.

Purpose of the Study:

• To investigate how the structural arrangement of insect flight muscle filaments influences rigor crossbridge interactions.
• To model the formation of crossbridge lattices and their characteristic appearances in electron microscopy.

Main Methods:

• Computer-modelling simulations of myosin crossbridge interactions with actin filaments.
• Analysis of actin-labelling patterns and crossbridge arrangements based on varying axial and azimuthal positions.
• Comparison of model predictions with experimental data from electron micrographs of insect flight muscle.

Main Results:

• Simulations accurately reproduced actin-labelling patterns observed in electron microscopy.
• The number of attached crossbridges varied significantly based on relative filament positioning.
• Optimal filament positioning maximized attached bridges and explained the 'double chevron' and 'flared X' appearances.

Conclusions:

• Filamentous annealing to positions of maximal crossbridge attachment likely generates the observed crossbridge lattice.
• Actin filament orientation on a P64 lattice, with specific azimuthal positioning, leads to the 'flared X' crossbridge formation.
• Rigor crossbridge lattice formation is primarily determined by axial and azimuthal positioning, not necessarily lateral filament register.