por What is titin?
Titin is the term used to describe a pair of giant polypeptide chains that make up the third most abundant protein in the sarcomeres of a wide range of cardiac and skeletal muscles.
Titin is one of the largest known proteins in terms of number of amino acid residues and therefore in terms of molecular weight. This protein is also known as connectin and is present in both vertebrates and invertebrates.
It was first described under this name (connectin) in 1977, and in 1979 it was defined as the double-band at the top of a polyacrylamide gel electrophoresis gel under denaturing conditions (with sodium dodecyl sulfate). In 1989 its location was established by immunoelectron microscopy.
Together with another large protein, nebulin, titin is one of the main components of the elastic framework of the muscle cell cytoskeleton, which coexists with thick filaments (myosin) and thin filaments (actin) within sarcomeres. So much so, that it is known as the third filament system of muscle fibers.
The thick and thin filaments are responsible for generating the active force, while the titin filaments determine the viscoelasticity of the sarcomeres.
A sarcomere is the repeating unit of myofibrils (muscle fibers). It is approximately 2 μm long and is delimited by “plaques” or lines called Z lines, which segment each myofibril into striated fragments of defined size.
Titin molecules assemble into extremely long, flexible, thin, and extensible filamentous strands. Titin is responsible for the elasticity of skeletal muscle and is believed to function as a molecular scaffold that specifies the correct assembly of sarcomeres on myofibrils.
Titin structure
In vertebrates, titin has about 27,000 amino acid residues and a molecular weight of around 3 MDa (3,000 kDa). It is composed of two polypeptide chains known as T1 and T2, which have similar chemical compositions and similar antigenic properties.
Mini-titins are found in invertebrate muscle, with a molecular weight of between 0.7 and 1.2 MDa. Included in this group of proteins are the twitchin protein from Caenorhabditis elegans and the projectin protein found in the genus Drosophila.
Vertebrate titin is a modular protein composed primarily of immunoglobulin and fibronectin III-like (FNIII-like) domains arranged in batch. It has an elastic region rich in proline, glutamic acid, valine, and lysine residues known as the PEVK domain, and another serine kinase domain at its carboxyl terminus.
Each of the domains is approximately 100 amino acids in length and are known as class I titin (the fibronectin III-like domain) and class II titin (the immunoglobulin-like domain). Both domains fold into 4 nm long sandwich structures, composed of antiparallel β sheets.
The cardiac connectin molecule contains 132 immunoglobulin domain repeat motifs and 112 fibronectin III-like domain repeat motifs.
The coding gene for these proteins (TTN) is the «champion» of introns, since it has almost 180 of them inside.
The transcripts of the subunits are differentially processed, especially the coding regions of the immunoglobulin (Ig) and PEVK-like domains, giving rise to isoforms with different extensible properties.
titin functions
The function of titin in sarcomeres depends on its association with different structures: its C-terminus is attached to the M line, while the N-terminus of each titin is attached to the Z line.
The proteins nebulin and titin act as «molecular rulers» that regulate the length of thick and thin filaments, respectively. Titin, as mentioned, extends from the Z disc to beyond the M line, in the center of the sarcomere, and regulates the length of the sarcomere, preventing overstretching of the muscle fiber.
It has been shown that the folding and unfolding of titin assists the process of muscle contraction, that is, it generates the mechanical work that achieves the shortening or extension of the sarcomeres, while the thick and thin fibers are the molecular motors of movement.
Titin participates in the maintenance of the thick filaments in the center of the sarcomere and its fibers are responsible for the generation of passive tension during stretching of the sarcomeres.
other functions
In addition to its participation in the generation of viscoelastic force, titin has other functions, among which are:
Participation in mechano-chemical signaling events through its association with other sarcomeric and non-sarcomeric proteins.
Length-dependent activation of the contractile apparatus.
Sarcomere assembly.
Contribution to the structure and function of the cytoskeleton in vertebrates, among others.
Studies have shown that in human cells and Drosophila embryos, titin has another function as a chromosomal protein. The elastic properties of the purified protein correspond perfectly with the elastic properties of chromosomes both in living cells and in vitro assembled chromosomes.
The participation of this protein in the compaction of chromosomes has been demonstrated thanks to site-directed mutagenesis experiments of the gene that encodes it, which results in both muscular and chromosomal defects.
Studies carried out in 2005 showed that the titin kinase domain is involved in the complex expression system of muscle genes, a fact demonstrated by the mutation of this domain that causes hereditary muscle diseases.
Titin-related pathologies
Some heart diseases have to do with alterations in the elasticity of titin. Such alterations greatly affect the extensibility and diastolic passive stiffness of the myocardium and, presumably, the mechanosensitivity.
The TTN gene has been identified as one of the main genes involved in human diseases, therefore the properties and functions of the heart protein have been extensively studied in recent years.
Dilated cardiomyopathy and hypertrophic cardiomyopathy are also the product of mutations in several genes, including the TTN gene.
References
Despopoulos, A., & Silbernagl, S. Color Atlas of Physiology (5th ed.). New York: Thieme.
Herman, D., Lam, L., Taylor, M., Wang, L., Teekakirikul, P., Christodoulou, D. Truncations of Titin Causing Dilated Cardiomyopathy. The New England Journal of Medicine.
Keller, T. Structure and function of titin and nebulin. Current Opinion in Biology.
Lange, S., Lange, S., Xiang, F., Yakovenko, A., Vihola, A., Hackman, P. The Kinase Domain of Titin Controls Muscle Gene Expression and Protein Turnover. Science.
Linke, WA, & Hamdani, N. Gigantic Business: Titin Properties and Function Through Thick and Thin. Circulation Research.