In situ titin properties at long lengths when Ig domain folding/unfolding is prevented

Titin, also known as connectin, is the largest known protein and was discovered in the mid-1970s and consists of Immunoglobulin (Ig) domains that flank N2A and PEVK segments. At long lengths and high force these Ig domains unfold. There is growing evidence that titin may change its stiffness by attaching its proximal portion to actin upon activation. If so folding/unfolding of Ig domains may become physiologically relevant. This is of interest because Ig domain folding/unfolding is associated with a great loss of energy, but preventing such folding/unfolding has been shown to result in a virtually elastic behaviour with no energy loss in isolated titin molecules. Therefore, the purpose of this study was to test titin properties in situ within sarcomeres at lengths where Ig domain unfolding has occurred while preventing folding/unfolding of Ig domains during small stretch shortening cycles