The Thin Filament in Insect Flight Muscle

Abstract

In this chapter we describe the special properties of insect muscle thin filament proteins and the way in which they differ from those in vertebrates. As in the vertebrate, the repeating unit of the muscle fibre (sarcomere) contains interdigitated thick (myosin containing) and thin (actin containing) filaments which generate the contractile force. The backbone of the insect muscle thin filament is provided by the helical F-actin polymer. Other proteins along the thin filament are modified versions of proteins present in the vertebrate thin filament. These include arthrin (ubiquitinated actin) and a heavy troponin subunit (TnH). The latter differs in the two insects studied, Drosophila and Lethocerus, and is absent in the vertebrate. The main functional difference in the insect thin insect filaments is between those in indirect flight muscles (IFM) and all other muscles. The IFM can be regulated at much higher frequencies by the process known as “stretch activation”. The mechanism of stretch activation is still not completely understood but it now appears that troponin-C, a normal regulatory component of the thin filaments, is involved. A small amount of high resolution information is now available for the vertebrate troponin complex and we have tried to incorporate this into what other structural data is available for the insect thin filament. The partial X-ray structure for the vertebrate troponin complex would fit within the density envelope found for insect troponin but, as expected, does not account for all the density.