Some Functions of Proteins from the Drosophila Sallimus (sls) Gene

Insect flight muscles contract at high frequencies and are activated by periodically stretching the muscles. For the stretch to have an effect, the muscles must be stiff. Two elastic proteins, projectin and kettin, are responsible for a large part of the muscle stiffness. Thin filaments containing actin emerge from Z-discs, which occur at periodic intervals along the myofibril, and thick filaments containing myosin interdigitate with the thin filaments. Both projectin and kettin form a mechanical link between the Z-discs and the ends of thick filaments. Kettin is made up of immunoglobulin-like (Ig) modules separated by linker sequences, and is associated with actin in the region of the Z-disc. The protein is an isoform derived from the Drosophila sallimus (sls) gene. Longer isoforms from the sls gene have additional, more extensible, sequence and these are found in non-flight muscles that are less stiff. Isoforms of the protein Sls have several different functions. Kettin causes thin filaments to align side-by-side in an anti-parallel fashion, which could nucleate Z-disc formation in developing myofibrils. Kettin is in the enlarged Z-discs close to the site of attachment of myofibrils to the cuticle, and may reinforce actin filaments in this region, giving the structure the required stiffness. Sls appears early in development of the Drosophila embryo and is needed for fusion of myoblasts to form myotubes which will become muscle fibres. Sls is associated with the membrane at the site of myoblast fusion, together with other proteins (Duf and Rols) that are needed for fusion. The elastic properties of single molecules of kettin have been measured using optical tweezers. The Ig domains unfold at relatively low stretching forces and refold at high forces. This suggests that kettin could be a folding-based spring, which may be relevant to its function in early muscle development, as well as in the adult myofibril.