The sarcomeric M-band during development and in disease.

The C-terminus of connectin/titin at the M-band of the sarcomere interacts with several structural as well as potential signalling proteins. One of these is myomesin, which can also bind to myosin and has been suggested to function as an integral structural linker of the thick filaments into the sarcomere. Recent evidence that myomesin possesses the ability to form antiparallel dimers via its C-terminal domain has prompted us to propose a novel three-dimensional model for the sarcomeric M-band. A splice variant of myomesin, termed EH-myomesin, contains an additional segment that has disordered conformation and functions as an entropic spring. It is expressed in a subset of muscle types that are characterised by a broader operational range and are more resistant to damage caused by eccentric contraction. In addition, it is also re-expressed in dilated cardiomyopathy. DRAL/FHL-2 is another protein that interacts with the M-band portion of connectin/titin and which probably functions as an adaptor for the compartmentalisation of metabolic enzymes. Together these results suggest that the M-band is crucial for sarcomere function and maintenance and that its molecular composition can be adapted to divergent physiological needs in different muscle types, which may help to cope with pathological alterations.