Cardiac sarcomere structure, featuring titinReconstruction of the thin (green) and thick filament from mammalian cardiac tissue. Myosin is in blue, MyBP-C is in yellow, and titin is in two shades of red (dark red for titin-alpha and light red for titin-beta).
Titin[2]/ˈtaɪtɪn/ (contraction for Titan protein) (also called connectin) is a protein that in humans is encoded by the TTNgene.[3][4] The protein, which is over 1 μm in length,[5] 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.[6] These domains unfold when the protein is stretched and refold when the tension is removed.[7]
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.[8] 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.[3][9]
Titin is the third most abundant protein in muscle (after myosin and actin), and an adult human contains approximately 0.5 kg of titin.[10] With its length of ~27,000 to ~35,000 amino acids (depending on the splice isoform), titin is the largest known protein.[11] Furthermore, the gene for titin contains the largest number of exons (363) discovered in any single gene,[12] as well as the longest single exon (17,106 bp).
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Cite error: The named reference Wang_1979 was invoked but never defined (see the help page).
^Lee EH. "The Chain-like Elasticity of Titin". Theoretical and Computational Biophysics Group, University of Illinois. Archived from the original on 13 February 2021. Retrieved 25 September 2014.
^Itoh-Satoh M, Hayashi T, Nishi H, Koga Y, Arimura T, Koyanagi T, et al. (February 2002). "Titin mutations as the molecular basis for dilated cardiomyopathy". Biochemical and Biophysical Research Communications. 291 (2): 385–393. doi:10.1006/bbrc.2002.6448. PMID11846417.
^Labeit S, Kolmerer B, Linke WA (February 1997). "The giant protein titin. Emerging roles in physiology and pathophysiology". Circulation Research. 80 (2): 290–294. doi:10.1161/01.RES.80.2.290. PMID9012751.