Titin
Largest-known protein in human muscles / From Wikipedia, the free encyclopedia
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Titin[5] /ˈtaɪtɪn/ (contraction for Titan protein) (also called connectin) is a protein that in humans is encoded by the TTN gene.[6][7] Titin is a protein, over 1 μm in length,[8] that functions as a molecular spring that is responsible for the passive elasticity of muscle. It comprises 244 individually folded protein domains connected by unstructured peptide sequences.[9] These domains unfold when the protein is stretched and refold when the tension is removed.[10]
Titin is important in the contraction of striated muscle tissues. It connects the Z disc to the M line in the sarcomere. The protein contributes to force transmission at the Z disc and resting tension in the I band region.[11] 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 striated muscle (cardiac or skeletal) have been correlated with differences in the mechanical properties of these muscles.[6][12]
Titin is the third most abundant protein in muscle (after myosin and actin), and an adult human contains approximately 0.5 kg of titin.[13] With its length of ~27,000 to ~35,000 amino acids (depending on the splice isoform), titin is the largest known protein.[14] Furthermore, the gene for titin contains the largest number of exons (363) discovered in any single gene,[15] as well as the longest single exon (17,106 bp).