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Titin , also known as connectin, is a protein that, in humans, is encoded by the ''TTN'' gene.〔(【引用サイトリンク】 url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=7273 )〕 Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring which is responsible for the passive elasticity of muscle. It is composed of 244 individually folded protein domains connected by unstructured peptide sequences. These domains unfold when the protein is stretched and refold when the tension is removed. Titin is important in the contraction of striated muscle tissues. It connects the Z line to the M line in the sarcomere. The protein contributes to force transmission at the Z line and resting tension in the I band region. It limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle. Variations in the sequence of titin between different types of muscle (e.g., cardiac or skeletal) have been correlated with differences in the mechanical properties of these muscles.〔 After myosin and actin, titin is the third most abundant protein in muscle and an adult human contains approximately 0.5 kg of titin. With its length of ~27,000 to ~33,000 amino acids (depending on the splice isoform), titin is the largest known protein. Furthermore, the gene for titin contains the largest number of exons (363) discovered in any single gene, as well as the longest single exon (17,106 bp). == Discovery == Reiji Natori in 1954 was the first to propose an elastic structure in muscle fiber to account for the return to the resting state when muscles are stretched and then released. In 1977, Koscak Maruyama and coworkers isolated an elastic protein from muscle fiber which they called connectin. Two years later, Kuan Wang and coworkers identified a doublet band on electrophoresis gel corresponding to a high molecular weight elastic protein which they named titin. Labeit in 1990 isolated a partial cDNA clone of titin.〔 In 1995, Labeit and Kolmerer determined the cDNA sequence of human cardiac titin.〔 Bang and coworkers in 2001 determined the complete sequence of the human titin gene.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「titin」の詳細全文を読む スポンサード リンク
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