Site directed mutagenesis of Drosophila flightin disrupts phosphorylation and impairs flight muscle structure and mechanics

摘要

Flightin is a myosin rod binding protein that in Drosophila melanogaster is expressed exclusively in the asynchronous indirect flight muscles (IFM). Hyperphosphorylation of flightin coincides with the completion of myofibril assembly and precedes the emergence of flight competency in young adults. To investigate the role of flightin phosphorylation in vivo we generated three flightin null (fln 0 ) Drosophila strains that express a mutant flightin transgene with two (Thr158, Ser 162), three (Ser139, Ser141, Ser145) or all five potential phosphorylation sites mutated to alanines. These amino acid substitutions result in lower than normal levels of flightin accumulation and transgenic strains that are unable to beat their wings. On two dimensional gels of IFM proteins, the transgenic strain with five mutant sites (fln 5STA ) is devoid of all phosphovariants, the transgenic strain with two mutant sites (fln 2TSA ) expresses only the two least acidic of the nine phosphovariants, and the transgenic strain with three mutant sites (fln 3SA ) expresses all nine phosphovariants, as the wild-type strain. These results suggest that phosphorylation of Thr158 and/or Ser162 is necessary for subsequent phosphorylation of other sites. All three transgenic strains show normal, albeit long, IFM sarcomeres in newly eclosed adults. In contrast, sarcomeres in fully mature fln 5STA and fln 2TSA adults show extensive breakdown while those in fln 3SA are not as disordered. The fiber hypercontraction phenotype that characterizes fln 0 is fully evident in fln 5STA and fln 2TSA but partially rescued in fln 3SA . Mechanics on skinned fibers from newly eclosed flies show alterations in viscous modulus for fln 5STA and fln 2TSA that result in a significant reduction in oscillatory power output. Expression of fln 5STA and fln 2TSA , but not fln 3SA , in a wild-type (fln + /fln + ) background resulted in a dominant negative effect manifested as flight impairments and hypercontracted IFM fibers. Our studies indicate that Thr158 and/or Ser162 are (is) indispensable for flightin function and suggest that phosphorylation of one or both residues fulfills an essential role in IFM structural stability and mechanics.