Properties of the muscle proteins--a comparative approach.

The differences in performance that exist between skeletal muscles are in part determined by the presence of different forms of most of the contractile and regulatory proteins of the myofibril - isoforms. These isoforms have common properties but their amino acid sequences are not identical and they exhibit slight differences in biological activities, such as ATPase, affinity for calcium, etc., that are appropriate for the physiological properties of the muscle in which they are present. With the exception of actin, all the major proteins present in the I and A filaments of skeletal muscle have been shown to exist in two or more isoforms. Whereas proteins such as troponin I and troponin C are present as a single isoform in each fibre type in normal muscle, others such as myosin and tropomyosin are present as two or more isoforms, usually in relative amounts characteristic for the fibre type. Type I and type II muscle fibres possess the capacity of synthesizing all the skeletal muscle isoforms of the myofibrillar proteins. The complement of isoforms present in a muscle fibre, however, depends on a number of factors such as the stage of development or regeneration, type of innervation, hormonal effects, etc. Complex mechanisms involving the coordinated control of gene expression must operate to ensure that the set of isoforms of the myofibrillar proteins present is characteristic for the cell type.