Alterations in flightin phosphorylation in Drosophila flight muscles are associated with myofibrillar defects engendered by actin and myosin heavy-chain mutant alleles.

Flightin is a 20-kD myofibrillar protein found in the stretch-activated flight muscles of Drosophila melanogaster. Nine of the eleven isoelectric variants of flightin are generated in vivo by multiple phosphorylations. The accumulation of these isoelectric variants is affected differently by mutations that eliminate thick filaments or thin filaments. Mutations in the myosin heavy-chain gene that prevent thick filament assembly block accumulation of all flightin variants except N1, the unphosphorylated precursor, which is present at much reduced levels. Mutations in the flight muscle-specific actin gene that block actin synthesis and prevent thin filament assembly disrupt the temporal regulation of flightin phosphorylation, resulting in premature phosphorylation and premature accumulation of flightin phosphovariants. Cellular fractionation of fibers that are devoid of thin filaments show that flightin remains associated with the thick filament-rich cytomatrix. These results suggest that flightin is a structural component of the thick filaments whose regulated phosphorylation is dependent upon the presence of thin filaments.