A new 185,000-dalton skeletal muscle protein detected by monoclonal antibodies.

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Abstract
The M line, which transverses the center of the thick filament region of [chicken] skeletal muscle sarcomeres, appears to be a complex array of multiple structural elements. To date, 2 proteins are associated with the M line. They are MM-CK [meromyosin-creatine kinase], localized in the M 4,4'' substriations, and a 165,000-dalton (164 kd) protein, referred to as both M-protein and myomesin. The positive identification of a 3rd M-line protein of 185 kd. In the course of making monoclonal antibodies (mAb) against a 165-kd fraction, mAb were obtained that bound to the M line of isolated myofibrils as detected by indirect immunofluorescence, but recognized a protein band of 185 kd in immunoblotting experiments with either the original immunogen or low ionic strength myofibril extracts as antigenic targets. The evidence that the 185- and 165-kd proteins are distinct protein species is based on the separation of the 2 proteins into discrete peaks by ion exchange chromatograpy, the distinctive patterns of their degradation products, and non-cross-reactivity of any of 7 mAb. These mAb recognize 3 unique antigenic determinants on the 185-kd molecule and at least 2, and probably 4 sites on the 165-kd molecule as determined from competitive binding and immunofluorescence experiments. To resolve the problem of multiple nomenclature for the 165-kd protein, the 185-kd protein will be referred to a myomesin and the 165-kd protein as M-protein.