Easily releasable myofilaments and myofibrillar protein turnover

Myofibrillar proteins present a unique system for metabolic turnover of proteins because organization of the myofibrillar lattice must be maintained if the myofibril is to remain functional. The proteosome is involved in turnover of myofibrillar proteins, but intact myofibrils that are 10–100 μm in diameter cannot enter the central catalytic cavity of the proteosome. Hence, some intermediate form of myofibrillar proteins exists between the intact myofibril and proteosomal degradation. It has been proposed that ~ 10–15% of the protein in striated muscle myofibrils can be released in the form of easily releasable myofilaments (ERM) and that these ERM are intermediates between the intact myofibril and proteins that can be accessed by the proteosome for degradation. ERM are released by gentle agitation of myofibrils in the presence of ATP and contain the major myofibrillar proteins, actin and myosin, but do not contain α‐actinin, desmin, M‐protein, or other proteins that are not associated with the actin and myosin filaments. We have obtained ERM from rat and bovine skeletal muscle; the ERM from either species contain myosin, actin, titin, tropomyosin, and troponin, but do not contain α‐actinin or desmin. The yields of ERM were less than reported: 0.18–0.44% of total myofibrillar protein. Incubation with purified calpain increases the yield of ERM by 10%. Anti‐ubiquitin antibodies from several sources labeled myosin and actin in both ERM and in intact myofibrils, but did not label any other proteins in either the ERM or the myofibrils. It is not clear that the ERM we obtained are intermediates in myofibrillar protein turnover. Supported by NRI 202‐35206‐11630, 2004‐04338, 2001‐35503‐10776, and the MDA.