Kettin, a major source of myofibrillar stiffness in Drosophila indirect flight muscle

Kettin is a high molecular mass protein of insect muscle that in the sarcom eres binds to actin and a-actinin. To investigate kettin's functional role, we combined immunolabeling experiments with mechanical and biochemical stu dies on indirect flight muscle (IFM) myofibrils of Drosophila melanogaster. Micrographs of stretched IFM sarcomeres labeled with kettin antibodies rev ealed staining of the Z-disc periphery. After extraction of the kettin-asso ciated actin, the A-band edges were also stained. In contrast, the staining pattern of projectin, another IFM-1-band protein, was not altered by actin removal. Force measurements were performed on single IFM myofibrils to est ablish the passive length-tension relationship and record passive stiffness . Stiffness decreased within seconds during gelsolin incubation and to a si milar degree upon kettin digestion with mu -calpain. Immunoblotting demonst rated the presence of kettin isoforms in normal Drosophila IFM myofibrils a nd in myofibrils from an actin-null mutant. Dotblot analysis revealed bindi ng of COOH-terminal kettin domains to myosin. We conclude that kettin is at tached not only to actin but also to the end of the thick filament. Kettin along with projectin may constitute the elastic filament system of insect I FM and determine the muscle's high stiffness necessary for stretch activati on. Possibly, the two proteins modulate myofibrillar stiffness by expressin g different size isoforms.