title: The long C-terminal extension of insect flight muscle-specific troponin-I isoform is not required for stretch activation.
abstract: Stretch-induced enhancement of active force (stretch activation, SA) is observed in striated muscles in general, and most conspicuously in insect flight muscle (IFM). It remains unclear whether a common mechanism underlies the SA of all muscle types, or the SA of IFM relies on its highly specialized features. Recent studies suggest that IFM-specific isoforms of thin filament regulatory proteins (troponin and tropomyosin) are implicated in SA. Among others, IFM-specific troponin-I (troponin-H or TnH), with an unusually long Pro-Ala-rich extension at the C-terminus, has been speculated to transmit the mechanical signal of stretch to the troponin complex. To verify this hypothesis, it was removed by a specific endoproteinase in bumblebee IFM, expecting that it would eliminate SA while leaving intact the capacity for Ca(2+)-activated isometric force. Electrophoretic data showed that the extension was almost completely (97%) removed from IFM fibers after treatment. Unexpectedly, SA force was still conspicuous, and its rate of rise was not affected. Therefore, the results preclude the possibility that the extension is a main part of the mechanism of SA. This leaves open the possibility that SAs of IFM and vertebrate striated muscles, which lack the extension, operate under common basic mechanisms.
authors.affiliation: Research and Utilization Div., SPring-8, Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-6198, Japan. iwamoto@spring8.or.jp
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Iwamoto
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D004330:Drosophila; D005426:Flight, Animal / Q000502:physiology*; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000502:physiology*; D019210:Troponin I / Q000737:chemistry / Q000235:genetics / Q000502:physiology*
chemical_list: D019210:Troponin I