The muscle ultrastructure: a structural perspective of the sarcomere.

Muscle ultrastructure is characterised by a complex arrangement of many protein-protein interactions. The sarcomere is the basic repeating unit of muscle, formed by two transverse filament systems: the thick and thin filaments. While actin and myosin are the main contractile elements of the sarcomere, other proteins act as scaffolds, control ultrastructure composition, regulate muscle contraction, and transmit tension between sarcomeres and hence to the whole myofibril. Elucidation of the structures of muscle proteins by X-ray crystallography and nuclear magnetic resonance spectroscopy has been essential in understanding muscle contraction, enabling us to relate biological to structural information. These structures reveal how components of the muscle interact, how different factors influence conformational changes within these proteins, and how mutant muscle proteins may interfere with the regulatory fine-tuning of the contractile machinery, hence leading to disease in some cases. Here, structures solved within the sarcomere have been reviewed in order to put the numerous components into context.