Titin
Titin, or Connectin, is the largest protein that has so far been discovered, undertaking the role of a structural and mechanical molecule, and it is commonly found in the myofibrils of skeletal muscle fibres. Within each sarcomere they act as stable, long, elastic filaments that run from the Z disc to the M line region [1]. Its main function is to help stop muscles from over-stretching, or over-relaxing, by acting as a flexible and dynamic shock absorber [2]. They are able to contract, tighten or even shrink in order to redirect the energy output whenever muscles stretch.
The titin protein is encoded by a single gene, and in humans is known to have 363 exons that code for around 38,000 amino acids [3].
In the myofibril, titin helps place and align the thick myosin filaments between the Z discs within the middle of the sarcomere. It contains long chains of specific domains that resemble immunoglobulins – it is these domains that give the protein its characteristic spring-like elasticity. They can fold and unfold to maintain the structural arrangement of the thick filaments over the M line when the protein is undergoing stress or compression [4], thus acting as an entropic spring [5].
In recent experiments, it has been implied that titin shows traits of its function as a molecular ruler – where the sarcomere is longer, for example in the vertebrate C. Elegans, the titin filaments are also longer, showing a relationship between the length of titin and the length of the sarcomeres [6].
References
- ↑ Nunes J.M., Hensen U, Ge L, Lipinsky M, Helenius J, Grubmüller H, Muller D.J. (2010) Angewandte Chemie International Edition, Volume 49 Issue 20, pp.3528-3531 [Peer Reviewed Journal]. [ONLINE] Available at http://onlinelibrary.wiley.com.libproxy.ncl.ac.uk/doi/10.1002/anie.200906388/full [Accessed 29 November 2012].
- ↑ Benichou I, Givli S (2011) Applied Physics Letters 98, 091904: The Hidden Ingenuity in Titin Structure [Peer Reviewed Journal] The American Institue of Physics. [ONLINE] Available at http://apl.aip.org.libproxy.ncl.ac.uk/resource/1/applab/v98/i9/p091904_s1 [Accessed 29 November 2012].
- ↑ Granzier H.L., Labeit S (2004) Circulation Research 94: page 284-295: The Giant Protein Titin, American Heart Association. [ONLINE] Available at http://circres.ahajournals.org/content/94/3/284.full [Accessed 29 November 2012].
- ↑ Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008) Molecular Biology of The Cell, page 1028, 5th edition, New York: Garland Science.
- ↑ Erickson, H.P. (1997) Science, Volume 276 No 5315, page 1090-1092 Stretching Single Protein Molecules: Titin is a Weird Spring [Peer Reviewed Journal]. [ONLINE] Available at http://www.sciencemag.org.libproxy.ncl.ac.uk/content/276/5315/1090 [Accessed 29 November 2012].
- ↑ Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008) Molecular Biology of The Cell, page 1028, 5th edition, New York: Garland Science.