DEFECTS IN THE DROSOPHILA MYOSIN ROD PERMIT SARCOMERE ASSEMBLY BUT CAUSE FLIGHT-MUSCLE DEGENERATION

We have determined the molecular and ultrastructural defects associate d with three homozygous-viable myosin heavy chain mutations of Drosoph ila melanogaster. These mutations cause a dominant flightless phenotyp e but allow relatively normal assembly of indirect flight muscle myofi brils. As adults age, the contents of the indirect flight muscle myofi bers are pulled to one end of the thorax. This apparently results from myofibril ''hyper-contraction'', and leads to sarcomere rupture and r andom myofilament orientation. All three mutations cause single amino acid changes in the light meromyosin region of the myosin rod. Two cha nge the same glutamic acid to a lysine residue and the third affects a n amino acid five residues away, substituting histidine for arginine. Both affected residues are conserved in muscle myosins, cytoplasmic my osins and paramyosins. The mutations are associated with age-dependent , site-specific degradation of myosin heavy chain and failure to accum ulate phosphorylated forms of flightin, an indirect flight muscle-spec ific protein previously localized to the thick filament. Given the rep eating nature of the hydrophobic and charged amino add residues of the myosin rod. and the near-normal assembly of myofibrils in the indirec t flight muscle of these mutants, it is remarkable that single amino a cid changes in the rod cause such severe defects. It is also interesti ng that these severe defects are not apparent in other muscles. These phenomena likely arise from the highly organized nature and rigorous p erformance requirements of indirect flight muscle, and perhaps from th e interaction of myosin with flightin, a protein specific to this musc le type.