To execute the rapid wingbeats that allow them to fly, many small insects rely on an elastic-filament system that makes their flight muscles extremely stiff. Although the I-band of indirect flight muscle sarcomeres has been shown to contain projectin and kettin, two large titin-like proteins, it was not known whether these proteins functioned like titin, which in vertebrates provides elasticity in striated muscle. Kulke et al. (page 1045) now provide strong evidence that kettin is functionally equivalent to titin, and suggest that this protein and projectin are the main contributors to the high stiffness of myofibrils in insects.
Using a novel technique to analyze the mechanical properties of individual myofibrils from Drosophila indirect flight muscle, the authors show that disrupting kettin immediately decreases myofibril stiffness. Additional analysis demonstrates that kettin in myofibrils is attached to both actin and the myosin thick filament, and suggests that kettin and projectin are the major constituents of the elastic-filament system.
Different isoforms of kettin may be alternatively spliced products of the large D-titin gene. Large kettin isoforms are associated with muscle tissues that are more extensible and less stiff, leading the authors to propose that insect myofibril stiffness is determined by the specific isoforms of projectin and kettin expressed in different muscle types. This closely parallels current models for the activity of titin in mammals, in which different titin isoforms exhibit muscle type-specific expression.