MOLECULAR DIVERSITY OF MYOFIBRILLAR PROTEINS - GENE-REGULATION AND FUNCTIONAL-SIGNIFICANCE

Myofibrillar proteins exist as multiple isoforms that derive from mult igene (isogene) families. Additional isoforms, including products of t ropomyosin, myosin light chain 1 fast, troponin T, titin, and nebulin genes, can be generated from the same gene through alternative splicin g or use of alternative promoters. Myofibrillar protein isogenes are d ifferentially expressed in various muscle types and fiber types but ca n be coexpressed within the same fiber. Isogenes are regulated by tran scriptional and posttranscriptional ge mechanisms; however, specific r egulatory sequences and transcriptional factors have not yet been iden tified. The pattern of isogene expression varies during muscle develop ment in relation to the different origin of myogenic cells and primary /secondary fiber generations and is affected by neural and hormonal in fluences. The variable expression of myofibrillar protein isoforms is a major determinant of the contractile properties of skeletal muscle f ibers. The diversity among isomyosins is related to the differences in the parameters of chemomechanical transduction as ATP hydrolysis rate and shortening velocity. Troponin and tropomyosin isoforms determine the variable sensitivity to calcium, whereas titin isoforms dictate th e elastic properties of muscle fibers at rest. Both myosin and troponi n isoforms contribute to the differences in the resistance to fatigue of muscle fibers.