Properties of easily releasable myofilaments: are they the first step in myofibrillar protein turnover?

Myofibrillar proteins must be removed from the myofibril before they can be turned over metabolically in functioning muscle cells. It is uncertain how this removal is accomplished without disruption of the contractile function of the myofibril. It has been proposed that the calpains could remove the outer layer of filaments from myofibrils as a first step in myofibrillar protein turnover. Several studies have found that myofilaments can be removed from myofibrils by trituration in the presence of ATP. These easily releasable myofilaments (ERMs) were proposed to be intermediates in myofibrillar protein turnover. It was unclear, however, whether the ERMs were an identifiable entity in muscle or whether additional trituration would remove more myofilaments until the myofibril was gone and whether calpains could release ERMs from intact myofibrils. The present study shows that few ERMs could be obtained from the residue after the first removal of ERMs, and the yield of ERMs from well-washed myofibrils was reduced, probably because some ERMs had been removed by the washing process. Mild calpain treatment of myofibrils released filaments that had a polypeptide composition and were ultrastructurally similar to ERMs. The yield of calpain-released ERMs was two- to threefold greater than the normal yield. Hence, ERMs are an identifiable entity in myofibrils, and calpain releases filaments that are similar to ERMs. The role of ERMs in myofibrillar protein turnover is unclear, because only filaments on the surface of the myofibril would turn over, and changes in myofibrillar protein isoforms during development could not occur via the ERM mechanism.