The Interacting Head Motif Structure Does Not Explain the X-Ray Diffraction Patterns in Relaxed Vertebrate (Bony Fish) Skeletal Muscle and Insect (Lethocerus) Flight Muscle.

Knupp Knupp,Squire Squire,Morris Morris

doi:10.3390/biology8030067

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https://doi.org/10.3390/biology8030067

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Abstract

Unlike electron microscopy, which can achieve very high resolution but to date can only be used to study static structures, time-resolved X-ray diffraction from contracting muscles can, in principle, be used to follow the molecular movements involved in force generation on a millisecond timescale, albeit at moderate resolution. However, previous X-ray diffraction studies of resting muscles have come up with structures for the head arrangements in resting myosin filaments that are different from the apparently ubiquitous interacting head motif (IHM) structures found by single particle analysis of electron micrographs of isolated myosin filaments from a variety of muscle types. This head organization is supposed to represent the super-relaxed state of the myosin filaments where adenosine triphosphate (ATP) usage is minimized. Here we have tested whether the interacting head motif structures will satisfactorily explain the observed low-angle X-ray diffraction patterns from resting vertebrate (bony fish) and invertebrate (insect flight) muscles. We find that the interacting head motif does not, in fact, explain what is observed. Previous X-ray models fit the observations much better. We conclude that the X-ray diffraction evidence has been well interpreted in the past and that there is more than one ordered myosin head state in resting muscle. There is, therefore, no reason to question some of the previous X-ray diffraction results on myosin filaments; time-resolved X-ray diffraction should be a reliable way to follow crossbridge action in active muscle and may be one of the few ways to visualise the molecular changes in myosin heads on a millisecond timescale as force is actually produced.

Highlights

A long-term goal of those working to describe in detail the nature of the crossbridge interaction between myosin and actin filaments that occurs during muscle contraction is to describe the underlying molecular movements using analysis of time-resolved X-ray diffraction patterns [1,2,3]
The heads are oriented with their actin binding sites suitably aligned to interact with the actin filaments with the head orientation observed in 3D reconstructions from electron micrographs of actin filaments labelled with myosin heads in the rigor state
electron microscopy (EM)-fitted interacting head motif head configurations arranged with the appropriate myosin filament symmetry from the EM reconstructions of human heart muscle myosin filaments [26] and Lethocerus myosin filaments [27]

Summary

Introduction

A long-term goal of those working to describe in detail the nature of the crossbridge interaction between myosin and actin filaments that occurs during muscle contraction is to describe the underlying molecular movements using analysis of time-resolved X-ray diffraction patterns [1,2,3] This kind of analysis is made more practicable by using particular types of muscle, namely simple lattice muscles (e.g., bony fish muscle) for vertebrates [4,5,6] and insect flight muscle for invertebrates [7,8,9]. The heads are oriented with their actin binding sites suitably aligned to interact with the actin filaments with the head orientation observed in 3D reconstructions from electron micrographs of actin filaments labelled with myosin heads in the rigor state (no ATP [12,13])

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