Fast skeletal muscle troponin T (fTnT) is a protein that in humans is encoded by the TNNT3 gene. [5] [6]
The TNNT3 gene is located at 11p15.5 in the human genome, encoding the fast skeletal muscle isoform of troponin T (fsTnT). fsTnT is an ~31-kDa protein consisting of 268 amino acids including the first methionine with an isoelectric point (pI) of 6.21 (embryonic form). fsTnT is the tropomyosin-binding and thin filament anchoring subunit of the troponin complex in the sarcomeres of fast twitch skeletal muscle. [7] [8] [9] TNNT3 gene is specifically expressed in vertebrate fast twitch skeletal muscles. [8] [9] [10]
TNNT3 gene evolved as one of the three TnT isoform genes in vertebrates. Each of the TnT isoform genes is linked to an upstream troponin I (TnI, one of the other two subunits of the troponin complex) isoform gene, and fsTnT is linked with fsTnI genes (Fig. 1). Sequence homology and protein epitope allosteric similarity data suggest that TnT gene was originated by duplication of a TnI-like ancestor gene and fsTnT was the first TnT emerged. [11] Whereas significantly diverged from the slow skeletal muscle TnT (ssTnT encoded by TNNT1) and cardiac TnT (cTnT encoded by TNNT2), Structure of fsTnT is conserved among vertebrate species (Fig. 2), reflecting specialized functional features of the different muscle fiber types. [7] [8] [9]
Mammalian TNNT3 gene contains 19 exons. Alternative RNA splicing of 8 of them significantly increases structural variations of fsTnT. [12] Two variable regions of the fsTnT protein are generated by alternative splicing (Fig. 3).
In the N-terminal region of fsTnT, exons 4, 5, 6, 7 and 8 are alternatively spliced in adult skeletal muscle cells. [12] [13] [14] A fetal fsTnT exon located between exons 8 and 9 is specifically expressed in embryonic muscle (Briggs and Schachat 1993). Exons 16 and 17, previously designated as α and β exons, in the C-terminal region of fsTnT are alternatively spliced in a mutually exclusive manner. [15]
Avian Tnnt3 gene has evolved with additional alternatively spliced exons, w, P1-7(x) and y, encoding the N-terminal variable region (Fig. 3). [16] [17] [18] Reflecting the power of combined alternative splicing of multiple exons to generate fsTnT variants, two-dimensional gel electrophoresis detected more than 40 different fsTnT splice forms in chicken leg muscle. [19]
Through alternative splicing of the fetal exon and other alternative exons in the N-terminal variable region, the expression of fsTnT during mammalian and avian development undergoes a high molecular to low molecular weight isoform switch in both fast and slow fiber dominant skeletal muscles. [20] The inclusion of more N-terminal exons increases the negative charge that tunes the overall molecular conformation of fsTnT and alters interaction with TnI, TnC and tropomyosin. [21] [22] [23] The alternative splicing-based addition of N-terminal negative charge in fsTnT also contributes to the tolerance to acidosis. [24]
Alternative splicing of the two C-terminal mutually exclusive exons 16 and 17 appears also regulated during development. [14] Exon 17 with a sequence more similar to the counterpart segment in ssTnT and cTnT is predominantly expressed in embryonic and neonatal fsTnT. [14] [25] Exon 16 of fsTnT was only found in adult skeletal muscles. Exons 16 and 17 both encode a 14 amino acids peptide fragment residing in the α-helix interfacing with TnI and TnC. Protein interaction studies revealed that incorporation of exon 17 weakened binding of fsTnT to TnC and tropomyosin. [26] Therefore, alternative splicing of exons 16 and 17 regulates the binding of fsTnT with TnI, possibly TnC, and thus tunes the function of the troponin complex and skeletal muscle contractility during development.
Avian Tnnt3 gene with additional alternatively spliced exons has unique expression pattern. The seven P exons are specifically expressed in pectoral muscles but not leg muscles. [24] During post hatch development of the avian pectoral muscles, the segment encoded by the P exons (named Tx from the original annotation of the coding exons as an x exon) is up-regulated and included predominantly in fsTnT of adult pectoral muscles. [27] Each P exon encodes a pentapeptide AHH(A/E)A. The Tx segment of adult fsTnT in avian orders of Galliformes and Craciformes contains 7-9 H(A/E)AAH repeats that possess high affinity binding to transition metal ions Cu(II), Ni(II), Zn(II) and Co(II). [27] The Tx segment of chicken breast muscle fsTnT also a binding capacity for calcium, presumably serves as a calcium reservoir in avian fast pectoral muscles. [28] Together with more N-terminal negative charges, this function may contribute to the higher calcium sensitivity of chicken breast muscle than that of leg muscle. [29]
The switch of high to low molecular weight splice forms occurs in avian leg muscles during post hatching development similar to that in developing mammalian skeletal muscles. Early during post hatch development of chicken pectoral muscles, fsTnT also shows a high to low molecular weight switch. However, around 28 days after hatch, fsTnT with Tx segment spliced-in is rapidly up-regulated and becomes the major fsTnT splice form in adult pectoral muscles. [27]
Deficiency of ssTnT did not affect the developmental switch of fsTnT splice forms in ssTnT-null mice, indicating that the developmental alternative splicing of the fsTnT pre-mRNA is regulated independent of skeletal muscle fiber type abnormality and adaptation. [20]
The 2016 version of this article was updated by an external expert under a dual publication model. The corresponding academic peer reviewed article was published in Gene and can be cited as: Bin Wei; J-P Jin (13 January 2016). "TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships". Gene . Gene Wiki Review Series. 582 (1): 1–13. doi:10.1016/J.GENE.2016.01.006. ISSN 0378-1119. PMC 5325693 . PMID 26774798. Wikidata Q37666010. |
Tropomyosin is a two-stranded alpha-helical, coiled coil protein found in many animal and fungal cells. In animals, it is an important component of the muscular system which works in conjunction with troponin to regulate muscle contraction. It is present in smooth and striated muscle tissues, which can be found in various organs and body systems, including the heart, blood vessels, respiratory system, and digestive system. In fungi, tropomyosin is found in cell walls and helps maintain the structural integrity of cells.
The PAX3 gene encodes a member of the paired box or PAX family of transcription factors. The PAX family consists of nine human (PAX1-PAX9) and nine mouse (Pax1-Pax9) members arranged into four subfamilies. Human PAX3 and mouse Pax3 are present in a subfamily along with the highly homologous human PAX7 and mouse Pax7 genes. The human PAX3 gene is located in the 2q36.1 chromosomal region, and contains 10 exons within a 100 kb region.
Troponin T is a part of the troponin complex, which are proteins integral to the contraction of skeletal and heart muscles. They are expressed in skeletal and cardiac myocytes. Troponin T binds to tropomyosin and helps position it on actin, and together with the rest of the troponin complex, modulates contraction of striated muscle. The cardiac subtype of troponin T is especially useful in the laboratory diagnosis of heart attack because it is released into the blood-stream when damage to heart muscle occurs. It was discovered by the German physician Hugo A. Katus at the University of Heidelberg, who also developed the troponin T assay.
Dystrobrevin is a protein that binds to dystrophin in the costamere of skeletal muscle cells. In humans, there are at least two isoforms of dystrobrevin, dystrobrevin alpha and dystrobrevin beta.
Actin, alpha skeletal muscle is a protein that in humans is encoded by the ACTA1 gene.
Troponin I, cardiac muscle is a protein that in humans is encoded by the TNNI3 gene. It is a tissue-specific subtype of troponin I, which in turn is a part of the troponin complex.
Cardiac muscle troponin T (cTnT) is a protein that in humans is encoded by the TNNT2 gene. Cardiac TnT is the tropomyosin-binding subunit of the troponin complex, which is located on the thin filament of striated muscles and regulates muscle contraction in response to alterations in intracellular calcium ion concentration.
Tropomyosin alpha-1 chain is a protein that in humans is encoded by the TPM1 gene. This gene is a member of the tropomyosin (Tm) family of highly conserved, widely distributed actin-binding proteins involved in the contractile system of striated and smooth muscles and the cytoskeleton of non-muscle cells.
Tropomyosin alpha-3 chain is a protein that in humans is encoded by the TPM3 gene.
Troponin C, also known as TN-C or TnC, is a protein that resides in the troponin complex on actin thin filaments of striated muscle and is responsible for binding calcium to activate muscle contraction. Troponin C is encoded by the TNNC1 gene in humans for both cardiac and slow skeletal muscle.
β-Tropomyosin, also known as tropomyosin beta chain is a protein that in humans is encoded by the TPM2 gene. β-tropomyosin is striated muscle-specific coiled coil dimer that functions to stabilize actin filaments and regulate muscle contraction.
Troponin I, slow skeletal muscle is a protein that in humans is encoded by the TNNI1 gene. It is a tissue-specific subtype of troponin I, which in turn is a part of the troponin complex.
Troponin I, fast skeletal muscle is a protein that in humans is encoded by the TNNI2 gene.
Slow skeletal muscle troponin T (sTnT) is a protein that in humans is encoded by the TNNT1 gene.
Annexin A7 is a protein that in humans is encoded by the ANXA7 gene.
AMP deaminase 3 is an enzyme that in humans is encoded by the AMPD3 gene.
Troponin C, skeletal muscle is a protein that in humans is encoded by the TNNC2 gene.
Enolase 3 (ENO3), more commonly known as beta-enolase (ENO-β), is an enzyme that in humans is encoded by the ENO3 gene.
Tropomodulin 4 (muscle) also known as TMOD4 is a protein which in humans is encoded by the TMOD4 gene.
Calponin 1 is a basic smooth muscle protein that in humans is encoded by the CNN1 gene.