titin (aka connectin) is a megaprotein that is approxmetely 3 megadaltons in size. in addition to being the largest protein found in mammalian tissues, it is also the third-most abundant. a single titin molecule is estimated to be between 2 and 2.5 um in length. in striated muscle, titin thus spans fully half of a sarcomere, with its c-terminal end localizing in the m-line at the center of the sarcomere and the n-terminal forming an integral part of the z-line. titin aids in maintaining sarcomeric alignment of myofibril during contraction. titin integrates the z-line and the thick filaments, maintaining the location of the thick filaments between the z-lines. titin is also hyporthesized to play a role in generating at least a portion of the passive tension that is present in skeletal muscle cells. during development of the myofibril, titin is one of the earliest proteins expressed, and it is thought to act as a"molecular ruler" by providing a scaffolding or template for the developing myofibril(clark et al. 2002).