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Structure of muscle filaments from immunohistochemical and ultrastructural studies.

In this presentation I will deal with the muscle filaments which are responsible for contraction, that is, the thin actin-containing filaments and the thick myosin-containing filaments. I will deal primarily with the use of antibody-labeling methods in studying the structure and composition of these filaments. Although the structural protein of the thin filament is actin and of the thick filament is myosin, other proteins are associated with them. The proteins tropomyosin and troponin are associated with actin in the thin filaments and they serve to regulate the interaction between actin and myosin (81). These proteins are present along the entire length of the thin filaments. The proteins associated with myosin in the thick filament are M-band protein (5, 34, 46), C-protein (41, 53, 55) and other less well studied proteins (55).These proteins are not present along the entire length of the myosin filament but are limited to specific regions along the filament. The function of these proteins is presently the object of speculation. Antibody-labeling studies of muscle in fluoresence microscopy have primarily involved the use of fluorescein as the fluorescent tag. Although other fluorescent tags have been used in antibody-labeling studies, fluorescein is most popular (10, 37). In electron microscopy, unaltered antibodies (46-48, 55, 57, 58) and ferritintagged antibodies (44, 66) have been successfully used for antibody-labeling studies of muscle. The use of ferritin-tagged antibodies presents a difficulty in the study of striated muscle inthat the tagged antibody isprobably too large to penetrate between the actin and myosin filaments (50). To my knowledge horseradish peroxidase-tagged antibodies (38, 39) have not yet been used in studies of muscle.