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Published 3 September 2001. doi:10.1083/jcb.200104016
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© The Rockefeller University Press, 0021-9525/2001/9/1045 $5.00
The Journal of Cell Biology, Volume 154, Number 5, September 3, 2001 1045-1058

Article

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

Michael Kulke1, Ciprian Neagoe1, Bernhard Kolmerer2, Ave Minajeva1, Horst Hinssen3, Belinda Bullard2 and Wolfgang A. Linke1

1 Institute of Physiology and Pathophysiology, University of Heidelberg, D-69120 Heidelberg, Germany
2 European Molecular Biology Laboratory, D-69012 Heidelberg, Germany
3 Biochemical Cell Biology, University of Bielefeld, D-33501 Bielefeld, Germany

Address correspondence to Wolfgang A. Linke, Intitute of Physiology and Pathophysiology, University of Heidelberg, Im Neuenheimerfeld 326, D-69120 Heidelberg, Germany. Tel.: 49-6221-544130. Fax: 49-6221-544049. E-mail: wolfgang.linke{at}urz.uni-heidelberg.de

Kettin is a high molecular mass protein of insect muscle that in the sarcomeres binds to actin and {alpha}-actinin. To investigate kettin's functional role, we combined immunolabeling experiments with mechanical and biochemical studies on indirect flight muscle (IFM) myofibrils of Drosophila melanogaster. Micrographs of stretched IFM sarcomeres labeled with kettin antibodies revealed staining of the Z-disc periphery. After extraction of the kettin-associated actin, the A-band edges were also stained. In contrast, the staining pattern of projectin, another IFM–I-band protein, was not altered by actin removal. Force measurements were performed on single IFM myofibrils to establish the passive length-tension relationship and record passive stiffness. Stiffness decreased within seconds during gelsolin incubation and to a similar degree upon kettin digestion with µ-calpain. Immunoblotting demonstrated the presence of kettin isoforms in normal Drosophila IFM myofibrils and in myofibrils from an actin-null mutant. Dotblot analysis revealed binding of COOH-terminal kettin domains to myosin. We conclude that kettin is attached 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 IFM and determine the muscle's high stiffness necessary for stretch activation. Possibly, the two proteins modulate myofibrillar stiffness by expressing different size isoforms.

Key Words: connecting filament; titin; muscle mechanics; projectin; PEVK sequence


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