Chicken skeletal muscle tropomodulin: novel localization and characterization

Tropomodulin is a 40.6-kDa isoform-specific tropomyosin-binding protein which inhibits actin filament elongation from the slow-growing (pointed) end and localizes at or near the pointed ends of thin filaments in rat skeletal muscle. Immunofluorescent localization using affinity-purified anti-tropomodulin antibodies in avian myofibril preparations demonstrates novel immunoreactivity at the Z-disc in addition to the previously reported localization at the periphery of I-Z-I brushes where actin filaments terminate. Identical results were obtained using antibody preparations generated against either bacterially expressed tropomodulin or human erythrocyte tropomodulin. Chicken muscle preparations contain Mr 43000 polypeptides which bind antibodies generated against tropomodulin in Western blot analysis, as well as 125I-labeled tropomyosin in blot overlays. Tropomodulin mRNA expression in adult muscle was confirmed by RNase protection assays, and the sequence of our tropomodulin cDNA amplified from chicken muscle mRNA preparations by polymerase chain reaction closely matches clones selected by chicken muscle cDNA library screening. The novel immunolocalization we report raises new possibilities for the role of tropomodulin in the organization of avian skeletal muscle at the Z-disc. We conclude that tropomodulin is likely to be important in striated muscle biology as a structural component in the Z-disc region which participates in the process of thin filament organization and assembly.