The costamere is a structural-functional component of striated muscle cells which connects the sarcomere of the muscle to the cell membrane.
Costameres are sub-sarcolemmal protein assemblies circumferentially aligned in register with the Z-disk of peripheral myofibrils. They physically couple force-generating sarcomeres with the sarcolemma in striated muscle cells and are thus considered one of several "Achilles' heels" of skeletal muscle, a critical component of striated muscle morphology which, when compromised, is thought to directly contribute to the development of several distinct myopathies.
The dystrophin-associated glycoprotein (DAG) complex contains various integral and peripheral membrane proteins such as dystroglycans and sarcoglycans, which are thought to be responsible for linking the internal cytoskeletal system of individual myofibers to structural proteins within the extracellular matrix (such as collagen and laminin). Therefore it is one of the features of the sarcolemma which helps to couple the sarcomere to the extracellular connective tissue as some experiments have shown. Desmin protein may also bind to the DAG complex, and regions of it are known to be involved in signaling.