Dominic Stone
Muscle titin, also known as connectin, is a protein that is encoded by the TTN gene. It is the third most abundant protein in muscle and is responsible for the passive elasticity of muscle. Titin is important in the contraction of striated muscle tissues and contributes to force transmission at the Z disc 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. Titin's primary functions are to stabilize the thick filament, centre it between the thin filaments, and prevent overstretching of the sarcomere.
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What You'll Learn
Muscle titin is encoded by the TTN gene
Muscle titin is a protein that in humans is encoded by the TTN gene. The TTN gene provides instructions for making a very large protein called titin. This protein plays an important role in skeletal muscles, which the body uses for movement, and in heart (cardiac) muscle. Titin is the third most abundant protein in muscle (after myosin and actin), and an adult human contains approximately 0.5 kg of titin. With its length of ~27,000 to ~35,000 amino acids (depending on the splice isoform), titin is the largest known protein. 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).
Within muscle cells, titin is an essential component of structures called sarcomeres. Sarcomeres are the basic units of muscle tensing (contraction); they are made of proteins that generate the mechanical force needed for muscles to contract. Titin has several functions within sarcomeres. One of the protein's main jobs is to provide structure, flexibility, and stability to these cell structures. Titin interacts with other muscle proteins, including actin and myosin, to keep the components of sarcomeres in place as muscles contract and relax. Titin also contains a spring-like region that allows muscles to stretch. Titin's primary functions are to stabilize the thick filament, center it between the thin filaments, prevent overstretching of the sarcomere, and to recoil the sarcomere like a spring after it is stretched.
The C-terminal region also contains a serine kinase domain that is primarily known for adapting the muscle to mechanical strain. It is “stretch-sensitive” and helps repair overstretching of the sarcomere. The N-terminal (the Z-disc end) contains a "Z repeat" that recognizes Actinin alpha 2. The elasticity of the PEVK region has both entropic and enthalpic contributions and is characterized by a polymer persistence length and a stretch modulus. At low to moderate extensions, the PEVK region's elasticity can be modelled with a standard worm-like chain (WLC) model of entropic elasticity. At high extensions, PEVK stretching can be modelled with a modified WLC model that incorporates enthalpic elasticity.
Several variants (also known as mutations) in the TTN gene have been found to cause centronuclear myopathy, a condition that is characterized by muscle weakness (myopathy) in the skeletal muscles. Most of these variants alter the way the gene's instructions are used to produce titin, resulting in the production of an abnormal protein with reduced or altered activity in muscle cells. Other variants prevent the production of the titin protein. It is unclear how TTN gene variants cause centronuclear myopathy, but it is likely that a shortage of normal titin protein leads to dysfunction of the sarcomere. Abnormal sarcomeres prevent muscle cells from contracting and relaxing normally, resulting in the muscle weakness that is characteristic of centronuclear myopathy.
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It is a molecular spring responsible for muscle elasticity
Titin, also known as connectin, is a protein that is encoded by the TTN gene. It is the third most abundant protein in muscle and is essential for muscle contraction and force production. With a length of ~27,000 to ~35,000 amino acids, titin is the largest known protein.
The protein functions as a molecular spring that is responsible for the passive elasticity of muscle. Titin comprises 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. This spring-like behaviour allows muscles to stretch and is essential for the contraction of striated muscle tissues.
Titin spans half of the sarcomere, connecting the Z disc to the M line. The sarcomere is the basic unit of muscle tensing and is composed of proteins that generate the mechanical force needed for muscles to contract. Titin provides structure, flexibility, and stability to these cell structures.
The I-band region of titin is highly folded when the muscle is relaxed, and gradually lengthens and functions as a spring when the muscle is stretched, developing passive tension. The extensible I-band region is composed of multiple segments with differing spring properties, including the tandem Ig segment and the PEVK segment. The PEVK region contributes to the elasticity of titin with both entropic and enthalpic effects.
In summary, titin is a molecular spring responsible for muscle elasticity through its ability to extend and develop passive tension during muscle stretch, providing the necessary flexibility and stability for muscle contraction and force production.
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Titin is involved in muscle contraction and force production
Titin is a protein that is encoded in humans by the TTN gene. It is the third most abundant protein in muscle and is over 1 μm in length. 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 essential for muscle contraction and force production. It provides the elasticity and passive force required for muscle contraction and relaxation by regulating the length of the sarcomere. Titin spans half of the sarcomere, connecting the Z disc to the M line. It contributes to force transmission at the Z disc and resting tension in the I band region. The I-band structure of titin allows for large elongations and passive force production, giving it a spring-like property.
The role of titin in muscle contraction becomes evident when examining its behaviour during the passive stretch of the sarcomere. In its relaxed state, the I-band region of titin is highly folded with minimal length. During stretching, the I-band segment gradually lengthens and functions as a spring, developing passive tension. This spring-like behaviour is due to the unique sequence of the protein, which is rich in proline, glutamate, valine, and lysine residues (PEVK segment).
The contribution of titin to passive muscle force depends on the length of the muscle fibres. In cardiac muscle, titin contributes more significantly to passive forces at shorter sarcomere lengths compared to longer ones. This is particularly important in the beating heart, where titin plays a crucial role. In skeletal muscles, the role of titin is less clear, and it appears that titin passive force may not be functionally significant in certain muscle groups.
Furthermore, titin activates myosin filaments in skeletal muscle by switching from an extensible spring to a mechanical rectifier. This mechanism increases titin stiffness and triggers the activation of myosin motors, allowing them to interact with actin filaments. Overall, titin's functions within the sarcomere, including its role in force transmission and elasticity, are vital for muscle contraction and force production.
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It stabilises thick filaments and prevents overstretching
Titin is a protein that is over 1 μm in length and is encoded by the TTN gene. It is the third most abundant protein in muscle, after myosin and actin, and an adult human body contains approximately 0.5 kg of titin. Titin is a molecular spring that is responsible for the passive elasticity of muscle. It contributes to force transmission at the Z disc and resting tension in the I band region.
The protein's primary functions are to stabilise the thick filament, centre it between the thin filaments, and prevent overstretching of the sarcomere. An N-terminal Z-disc region and a C-terminal M-line region bind to the Z-line and M-line of the sarcomere, respectively, so that a single titin molecule spans half the length of a sarcomere. The C-terminal region also contains a serine kinase domain that is primarily known for adapting the muscle to mechanical strain. It is "stretch-sensitive" and helps repair overstretching of the sarcomere.
The I-band structure of titin allows for large elongations and passive force production, and thus has been termed a "spring-like" molecule. Titin's spring-like region allows muscles to stretch. Within the I-band titin, the distal tandem immunoglobulin-like segment forms a stiff end-filament, while the other two segments account for titin extensibility: the proximal tandem Ig segment and the unique sequence rich in proline (P), glutamate (E), valine (V), and lysine (K) residues (PEVK segment). Both spring-like segments exhibit variable muscle-type-specific lengths, which account for the differences in passive force–sarcomere length relations.
The length of thick filaments is defined by titin, and alterations in titin length affect force generation. In a mouse model where two of titin's C-zone super-repeats were deleted, thick filament length was reduced in cardiac and skeletal muscles. Functional studies revealed reduced force generation and a dilated cardiomyopathy (DCM) phenotype. Thus, regulation of thick filament length depends on titin and is critical for maintaining muscle health.
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Titin mutations can cause muscle weakness and heart disease
Titin is a protein that functions as a molecular spring in the body, contributing to the passive elasticity of muscles. It is encoded by the TTN gene and is the third most abundant protein in muscle, with an adult human containing approximately 0.5 kg of titin. The protein's spring-like quality allows it to be stretched and returned to its resting state, providing stability and preventing overstretching of the sarcomere, which is the contractile unit of muscle cells.
The TTN gene variants can cause muscle problems, including centronuclear myopathy, congenital titinopathy, and early-onset myopathy with fatal cardiomyopathy (EOMFC). These variants lead to the production of an abnormal titin protein, disrupting the function of sarcomeres and preventing skeletal and cardiac muscles from developing and functioning normally. This results in muscle weakness and heart disease, such as dilated cardiomyopathy, which enlarges and weakens the heart, affecting its ability to pump blood efficiently.
Several studies have examined the impact of titin mutations on heart health. Research has found that truncating titin mutations can affect cardiomyocyte signaling and RNA expression, leading to diminished expression of growth factors. These mutations prevent the development of a normal cardiomyocyte structure and impair its contractile performance, increasing the risk for arrhythmias. Additionally, the location of the mutation matters, with truncating mutations in the titin A-band region strongly associated with dilated cardiomyopathy.
While having a titin mutation does not guarantee the development of heart disease, it indicates an increased risk. This knowledge can help doctors and patients take a proactive approach to managing risk factors, such as better blood pressure management and closer monitoring during pregnancy, a stressful time for the heart and vascular system. Understanding titin mutations also opens up possibilities for future therapeutic interventions, such as pharmacologic agents that could enhance titin gene expression or stimulate cardiomyocyte signals.
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Frequently asked questions
Titin is a protein that functions as a molecular spring in muscles. It is responsible for the passive elasticity of muscle tissues, allowing them to stretch and recoil.
Muscle titin provides structure, flexibility, and stability to muscle cells. It interacts with other muscle proteins, such as actin and myosin, to facilitate muscle contraction and relaxation. Titin also contributes to force transmission and regulates the length of the sarcomere, which is the basic unit of muscle contraction.
The existence of an elastic structure in muscle fibres was first proposed by Reiji Natori in 1954. Later, in the 1970s, Koscak Maruyama and his team isolated an elastic protein from muscle fibre and named it "connectin". Kuan Wang and their colleagues further identified a high molecular weight, elastic protein, which they called "titin".
Sarcomeres are the basic units of muscle contraction, and titin is an essential component within these structures. Titin spans half the length of a sarcomere, connecting the Z disc to the M line. It helps maintain the stability and flexibility of sarcomeres and contributes to force transmission during muscle contractions.
Variations and mutations in the TTN gene, which codes for titin, have been associated with various muscle-related conditions. These include centronuclear myopathy, familial dilated cardiomyopathy, and hereditary myopathy with early respiratory failure. Abnormalities in titin can lead to dysfunction of sarcomeres, resulting in muscle weakness and heart-related issues.
