Flexibility and extensibility in the titin molecule: Analysis of electron microscope data

Muscle elasticity derives directly from titin extensibility, which stems fr om three distinct types of spring-like behaviour of the I-band portion of t he molecule. With progressively greater forces and sarcomere lengths, the m olecule straightens and then unfolds, first in the PEVK-region and then in individual immunoglobulin domains. Here, we report quantitative analysis of flexibility and extensibility in isolated titin molecules visualized by el ectron microscopy. Conformations displayed by molecules dried on a substrat e vary from a random coil to rod-like, demonstrating highly flexible and ea sily deformable tertiary structure. The particular conformation observed de pends on the "wettability" of the substrate during specimen preparation: hi gher wettability favours coiled conformations, whereas lower wettability re sults in more extended molecules. Extension is shown to occur during liquid dewetting. Statistical methods of conformational analysis applied to a pop ulation of coiled molecules gave an average persistence length 13.5(+/-4.5) nm. The close correspondence of this value to an earlier one from light-sc attering studies confirms that conformations observed by microscopy closely reflected the equilibrium conformation in solution. Analysis of hydrodynam ic forces exerted during dewetting also indicates that the force causing st raightening of the molecules and extension of the PEVK-region is in the pic oNewton range, whereas unfolding of the immunoglobulin and fibronectin doma ins may require forces about tenfold higher. The microscope data directly i llustrate the relationship between titin conformation and the magnitude of applied force. They also suggest the presence of torsional stiffness in the molecule, which may affect considerations of elasticity. (C) 2001 Academic Press.