The molecular switch that controls muscle contraction: the structure of the troponin protein complex in both the “on” and “off” states. Top panels show the protein model, while in the bottom panels the protein is docked into three dimensional maps obtained from electron microscopy.

The muscles in our body are controlled by calcium levels inside the muscle cells. In their resting state, the calcium level is low, while on excitation, calcium ions are released into the cell, resulting in the activation of the contractile machinery – the proteins, actin and myosin. A calcium sensor protein called troponin detects the change in calcium concentration and releases the blockage that prevents the myosin motor protein interacting with the actin filament. The mechanism by which troponin switches from an “on” state to an “off” state was previously unknown. A collaboration between academics at the University of California San Francisco, Institut Laue Langevin, Grenoble and the Protein Structure group at UNSW has resulted in the first experimental structures of the troponin complex in the on and off states.

Neutrons can be used to gain structural information on biological macromolecules. Multicomponent proteins, such as troponin (which has three subunits) can be probed by selectively deuterating (substituting heavy hydrogen for hydrogen) only a subset of the constituent proteins. In the experiments to probe troponin structure, the deuterated subunits were “visible” to the neutron beam, while the normal protonated subunits were “invisible”. By collecting neutron scattering data from troponin samples with all possible combinations of deuterated and protonated components, the structure of the complex could be modelled.

Using computational methods, Dr Bill King in the Protein Structure group was able to obtain models for the troponin complex in both the on state (+Ca2+) and the off state (-Ca2+). As the calcium ion levels drop, the “head” region of troponin (N-TnC) releases a “tail” region (C-TnI). It is believed that this region then binds to the actin filament and blocks the binding of the myosin motor protein. More recently, the troponin team collaborated with a group at Boston University to interpret three-dimensional reconstructions of troponin containing actin filaments obtained from electron micrographs.

The goal now is to uncover how the “tail” (C-TnI) can block the myosin motor protein interacting with the actin filament. This region is medically significant, as numerous mutations that are responsible for inherited cardiac disease map to this region of the protein.

Paul Curmi