Stretching molecular springs: elasticity of titin filaments in vertebrate striated muscle W.A. Linke Institute of Physiology II, University of Heidelberg, Heidelberg, Germany Offprint requests to: W.A. Linke, Institute of Physiology II, University of Heidelberg, Im Neuenheimer Feld 326, Heidelberg, Germany. Fax: +49-6221-544049. e-mail: wolfgang.linke@urz.uni-heidelberg.de
Summary. Titin,
the giant protein of striated muscle, provides a continuous link
between the Z-disk and the M-line of a sarcomere. The elastic
I-band section of titin comprises two main structural elements,
stretches of immunoglobulin-like domains and a unique sequence,
the PEVK segment. Both elements contribute to the extensibility
and passive force development of nonactivated muscle. Extensibility
of the titin segments in skeletal muscle has been determined by
immuno-fluorescence/immunoelectron microscopy of sarcomeres stained
with sequence-assigned titin antibodies. The force developed upon
stretch of titin has been measured on isolated molecules or recombinant
titin fragments with the help of optical tweezers and the atomic
force microscope. Force has also been measured in single isolated
myofibrils. The force-extension relation of titin could be readily
fitted with models of biopolymer elasticity. For physiologically
relevant extensions, the elasticity of the titin segments was
largely explainable by an entropic-spring mechanism. The modelling
explains why during stretch of titin, the Ig-domain regions (with
folded modules) extend before the PEVK domain. Key words: Connectin,
Skeletal muscle, Cardiac muscle, Passive tension, Wormlike chain |