King's College London

The study, published in PNAS, used a new imaging technique to show that activation of the muscle motors is dependent on where they are in the myosin filament. A regulatory protein (proteins that act like a light or dimmer switch for turning cellular processes on and off) called myosin-binding protein C (MyBP-C) was found to play an important role in the control of the OFF/ON state of the myosin motors.

Skeletal muscles are largely made up of bundles of muscle fibres. Each muscle fibre contains bundles of myofilaments, called myofibrils, organised into repeating units called sarcomeres – responsible for muscle contraction. Within each sarcomere are threads known as myosin filaments that have motors attached to them. These myosin motors drive the pulling motion that shortens and contracts the muscle.

Researchers at King’s College London developed a new type of ‘fluorescence polarization microscopy’ that can be used to visualise the orientation of fluorescently tagged proteins across distinct filament regions within an individual sarcomere, offering a powerful new tool for studying muscle structure and function.

Using this technique, the researchers attached fluorescent probes to the myosin motors in muscle myofibrils isolated from animal models and mapped their orientation across regions of the myosin filament containing distinct regulatory proteins.

Dr Luca Fusi, Lecturer in Muscle Physiology at King’s and senior author of the paper, said: “MyBP-C is found only in a specific part of the filament known as the C-zone. We developed a new imaging method that allowed us to directly see how myosin motors in the C-zone behave differently from those in regions that do not contain MyBP-C."

When the researchers applied force to the muscle filaments to simulate mechanical loading of the muscle, they saw that how the motors were switched on depended on where they are located along the filament. Motors near the ends of the filament needed lower levels of force to switch on, whereas motors closer to the centre of the filament in the C-zone, where MyBP-C was present, required higher forces to become active.

Dr Fusi says the imaging technique could provide a valuable tool for investigating muscle function as well as disease. Because mutations in MyBP-C can cause skeletal and heart diseases, the new imaging technique could help to understand how changes in this protein alter the regulation of muscle contractility.

The work was supported by a Sir Henry Dale Fellowship awarded by Wellcome and the Royal Society to Dr Luca Fusi.