Cryo-EM structure of tarantula myosin filament: Implications for analysis of myopathy mutations.

Abstract

The high-resolution structure of myosin thick filaments of invertebrates has aided understanding of key muscle functions and analysis of disease mutations in humans. We previously published a low-resolution tarantula skeletal thick filament cryo-EM structure (Woodhead et al., 2005), which was used to map hypertrophic and dilated cardiomyopathy mutations (Alamo et al., 2017, Nag et al., 2017), present mostly in the head-head and head-tail interaction sites of the interacting-heads motif (IHM). This study suggested that the myosin coiled-coil tails in the filament backbone were arranged in groups of three, forming 40-Å-diameter subfilaments (Wray, 1979), but it lacked the detail required to resolve individual tails and to map the 40% of myopathy mutations that occur in the tail region of the molecule. Here we report a 5.5 Å resolution cryo-EM map of the tarantula thick filament backbone that resolves the densities of the full length of the myosin tails and their packing interactions. The map reveals that the tails do not form subfilaments but are arranged individually in layers forming a “molecular crystal,” in which the myosin coiled-coils gradually move towards the center of the filament, near the paramyosin core, while slowly curving around the filament axis. A similar arrangement was found in insect flight muscle thick filaments (Hu et al., 2016), suggesting it could be a general packing scheme in thick filaments, as originally proposed by Squire (1973). The positions of skip residues predict longer pitch regions and bending of the coiled-coils, as seen in the cryo-EM density. The structure also suggests new interactions of the blocked-head converter region with the backbone of the filament, implying hyperstability of IHMs. Improved resolution, and solution of the vertebrate thick filament structure, should enable precise mapping of tail mutations.