The Influence of Five Alternative Myosin Converter Domains on Drosophila Muscle Mechanical Properties

Abstract

In Drosophila, alternatively spliced myosin isoforms containing five different converter sequences (11A-11E) are expressed in a diverse range of muscle types including the superfast indirect flight muscle (IFM), jump muscle and slow embryonic muscles. We measured the converter's influence on IFM fiber mechanical properties such as maximum power output (Pmax), frequency of maximum power output (Fmax), strain for maximum power output (AMP), stretch activated tension (Fsa) and mechanical rate constants 2pib and 2pic. Using the work loop technique to impose muscle strain amplitudes that simulate in vivo conditions, we found that transgenically replacing the IFM-11A converter with the embryonic-11D version significantly reduced Pmax by 41%, decreased Fmax by 63%, and increased AMP by 100%. However, no change in Fsa was detected for the IFM fibers expressing the 11D converter. Preliminary results from transgenic substitution of the IFM-11A converter with the embryonic-11E version showed a 50% reduction in Pmax and a 57% reduction in Fmax compared to control muscle fibers. The mechanical rate constant 2pib, determined by sinusoidal analysis, was reduced by 55% by substituting the 11D converter and by 68% by substituting the 11E converter into the IFM myosin isoform. The mechanical rate constant 2pic, increased by more than 50% after substitution of 11D and about 100% with the 11E converter switched into the IFM myosin isoform. These results are physiologically significant as we observed decreased wing beat frequencies of 11% for the 11D, 15% for 11E and decreased flight performance for the 11D fly line at 15°C. We conclude that the five converter versions provide for a wide variation in muscle mechanical properties between muscle fiber types.