The long C-terminal extension of insect flight muscle-specific troponin-I isoform is not required for stretch activation

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 Ca2+-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.

► Bumblebee flight muscle fibers were treated by a specific endoproteinase (Igase).
► Igase effectively removed the long extension of flight muscle-specific troponin-I.
► Extension-free flight muscle fibers were still capable of stretch activation.
► Stretch activation does not depend on this structure unique to insect flight muscle.