The M-band: an elastic web that crosslinks thick filaments in the center of the sarcomere

The sarcomere of striated muscle is an efficient molecular machine, characterized by perfect structural organization of contractile filaments. This order is ensured by the sarcomere cytoskeleton, an important element of which is the M-band, believed to maintain the thick filament lattice. We review here recent progress in understanding the M-band function and its structural organization. We explain how the M-band might reduce the intrinsic instability of thick filaments and help titin to maintain order in the sarcomeres. The M-band molecular structure has been clarified recently by biochemical and biophysical approaches that focused on the properties of the prominent M-band component myomesin. These have shown that antiparallel myomesin dimers might link the thick filaments in the M-band, a role analogous to that of α-actinin in the Z-disc. Furthermore, similar to titin, myomesin is a molecular spring with complex visco-elastic properties that can be modified by alternative splicing. M-band protein composition correlates with the expression of titin isoforms and appears to be a reliable marker for biomechanical conditions in contracting muscle. We propose that the M-band is in fact a dynamic structure that monitors the stress appearing in the thick filament lattice during contraction and quickly reorganizes to meet new physiological requirements. The sarcomere of striated muscle is an efficient molecular machine, characterized by perfect structural organization of contractile filaments. This order is ensured by the sarcomere cytoskeleton, an important element of which is the M-band, believed to maintain the thick filament lattice. We review here recent progress in understanding the M-band function and its structural organization. We explain how the M-band might reduce the intrinsic instability of thick filaments and help titin to maintain order in the sarcomeres. The M-band molecular structure has been clarified recently by biochemical and biophysical approaches that focused on the properties of the prominent M-band component myomesin. These have shown that antiparallel myomesin dimers might link the thick filaments in the M-band, a role analogous to that of α-actinin in the Z-disc. Furthermore, similar to titin, myomesin is a molecular spring with complex visco-elastic properties that can be modified by alternative splicing. M-band protein composition correlates with the expression of titin isoforms and appears to be a reliable marker for biomechanical conditions in contracting muscle. We propose that the M-band is in fact a dynamic structure that monitors the stress appearing in the thick filament lattice during contraction and quickly reorganizes to meet new physiological requirements. membrane surrounding the muscle cell. elongation of the muscle in the activated state. Although eccentric contractions are physiologically common, they often lead to muscle injury and soreness. external force applied to the muscle in a passive state (i.e. not being stimulated to contract). the force generated by the acto-myosin interaction in the activated muscle. a device allowing measurement of the force–extension characteristics of single molecules (pico/nano-Newton range). protein scaffold that integrates the force-generating filaments into sarcomeres. The major protein components of the sarcomeric cytoskeleton are α-actinin, which crosslinks actin filaments in the Z-disc, myomesin, which connects the myosin filaments in the M-band, and titin.