muscle to mechanical strain. It is “stretch-sensitive” and helps repair overstretching of the sarcomere. The N-terminal (the Z-disc end) contains a "Z repeat" that recognizes Actinin alpha 2. The elasticity of the PEVK region has both entropic and enthalpic contributions and is characterized by a polymer persistence length and a stretch modulus. At low to moderate extensions PEVK elasticity can be modeled with a standard worm-like chain (WLC) model of entropic elasticity. At high extensions PEVK stretching can be modeled with a modified WLC model that incorporates enthalpic elasticity. The difference between low-and high- stretch elasticity is due to electrostatic stiffening and hydrophobic effects. Embedded between the PEVK and Ig residues are N2A domains. Evolution The titin domains have evolved from a common ancestor through many gene duplication events. Domain duplication was facilitated by the fact that most domains are encoded by single exons. Other giant sarcomeric proteins made out of Fn3/Ig repeats include obscurin and myomesin. Throughout evolution, titin mechanical strength appears to decrease through the loss of disulfide bonds as the organism becomes heavier. Titin A-band has homologs in invertebrates, such as twitchin (unc-22) and projectin, which also contain Ig and FNIII repeats and a protein kinase domain. The gene duplication events took place independently but were from the same ancestral Ig and FNIII domains. It is said that the protein titin was the first to diverge out of the family. Drosophila projectin, officially known as bent (bt), is associated with lethality by failing to escape the egg in some mutations as well as dominant changes in wing angles. Drosophila Titin, also known as Kettin or sallimus (sls), is kinase-free. It has roles in the elasticity of both muscle and chromosomes. It is homologous to vertebrate titin I-band and contains Ig PEVK domains, the many repeats being a hot target for splicing. There also exists a titin homologue, ttn-1, in C. elegans. It has a kinase domain, some Ig/Fn3 repeats, and PEVT repeats that are similarly elastic. Function Titin is a large abundant protein of striated muscle. Titin's primary functions are to stabilize the thick filament, center it between the thin filaments, prevent overstretching of the sarcomere, and to recoil the sarcomere like a spring after it is stretched. An N-terminal Z-disc region and a C-terminal M-line region bind to the Z-line and M-line of the sarcomere, respectively, so that a single titin molecule spans half the length