The Structure of the Relaxed Thick Filaments from Lethocerus Asynchronous Flight Muscle

Abstract

The structure of relaxed muscle thick filaments is a key element for understanding muscle physiology and the effect of mutations on muscle function. Previous thick filament structures by 3D-EM resolved individual myosin heads and revealed an interacting head motif (IHM) first identified in smooth muscle myosin. A later model based on X-ray fiber diffraction of Lethocerus flight muscle showed a different, intermolecular, interaction between myosin heads. Almost nothing is known about the structure of the long α-helical coiled-coil myosin rod in thick filaments. Here we show the relaxed structure of Lethocerus thick filaments by cryoEM at sufficient resolution to reveal the relative placement of individual myosin heads at ∼20 A resolution, as well as the myosin rod α-helices which can be traced from the head-rod junction all the way to their C-termini at a resolution of 5.5A. In contrast to the model suggested by X-ray diffraction, we find an IHM somewhat different from IHMs of other relaxed thick filaments. The myosin rod follows a tortuous path from the outside of the thick filament to the inside across a region we call the myosin rod annulus. The pitch of the rod coiled-coil is variable with long pitch regions in the approximate location of skip residues. Surprisingly, threading their way among the forest of myosin rods are four copies, matching the 4-fold symmetry of the filament, of four polypeptide chains. Their presence in this location could modulate the mechanical properties of the myosin backbone. Our results demonstrate the ubiquity of the IHM in relaxed myosin filaments but with a variation that might shed light on the mechanism of stretch activation and shortening deactivation. Supported by NIH and AHA.