Localization of the Elastic Proteins in the Flight Muscle of Manduca sexta

The flight muscle of Manduca sexta (DLM(1)) is an emerging model system for biophysical studies of muscle contraction. Unlike the well-studied indirect flight muscle of Lethocerus and Drosophila, the DLM(1) of Manduca is a synchronous muscle, as are the vertebrate cardiac and skeletal muscles. Very little has been published regarding the ultrastructure and protein composition of this muscle. Previous studies have demonstrated that DLM(1) express two projectin isoform, two kettin isoforms, and two large Salimus (Sls) isoforms. Such large Sls isoforms have not been observed in the asynchronous flight muscles of Lethocerus and Drosophila. The spatial localization of these proteins was unknown. Here, immuno-localization was used to show that the N-termini of projectin and Salimus are inserted into the Z-band. Projectin spans across the I-band, and the C-terminus is attached to the thick filament in the A-band. The C-terminus of Sls was also located in the A-band. Using confocal microscopy and experimental force-length curves, thin filament lengths were estimated as ~1.5 µm and thick filament lengths were measured as ~2.5 µm. This structural information may help provide an interpretive framework for future studies using this muscle system.