Direct visualization of extensibility in isolated titin molecules.

"Molecular combing" induced by a receding meniscus has been shown to extend individual titin molecules. Electron microscopy reveals that both ends of the molecule tend to attach to a mica substrate, probably due to their local positive charges. This leaves the remainder of the molecule free to be straightened and extended by forces of up to approximately 800 pN. A small region in the I-band part of the molecule, which probably corresponds to sequence high in P, E, V and K residues, is the most compliant and appears to extend by an unfolding of the polypeptide chain. Other parts of the molecule are also capable of extension. These mechanical extensions in titin are probably reversible.