CYTOSKELETAL PROTEINS OF INSECT MUSCLE - LOCATION OF ZEELINS IN LETHOCERUS FLIGHT AND LEG MUSCLE

Asynchronous insect flight muscles produce oscillatory contractions an d can contract at high frequency because they are activated by stretch as well as by Ca2+. Stretch activation depends on the high stiffness of the fibres and the regular structure of the filament lattice. Cytos keletal proteins may be important in stabilising the lattice. Two prot eins, zeelin 1 (35 kDa) and zeelin 2 (23 kDa), have been isolated from the cytoskeletal fraction of Lethocerus us flight muscle. Both zeelin s have multiple isoforms of the same molecular mass and different char ge. Zeelin 1 forms micelles and zeelin 2 forms filaments when renature d in low ionic strength solutions. Filaments of zeelin 2 are ribbons 1 0 nm wide and 3 nm thick. The position of zeelins in fibres from Letho cerus flight and leg muscle was determined by immunofluorescence and i mmunoelectron microscopy. Zeelin 1 is found in flight and leg fibres a nd zeelin 2 only in flight fibres. In flight myofibrils, both zeelins are in discrete regions of the A-band in each half sarcomere. Zeelin 1 is across the whole A-band in leg myofibrils. Zeelins are not in the Z-disc, as was thought previously, but migrate to the Z-disc in glycer inated fibres. Zeelins are associated with thick filaments and analysi s of oblique sections showed that zeelin 1 is closer to the filament s haft than zeelin 2. The antibody labelling pattern is consistent with zeelin molecules associated with myosin near the end of the rod region . Alternatively, the position of zeelins may be determined by other A- band proteins. There are about 2.0 to 2.5 moles of myosin per mole of each zeelin. The function of these cytoskeletal proteins may be to mai ntain the ordered structure of the thick filament.