Telethonin, also known as Tcap, is a protein that in humans is encoded by the TCAP gene. [5] [6] [7] Telethonin is expressed in cardiac and skeletal muscle at Z-discs and functions to regulate sarcomere assembly, T-tubule function and apoptosis. Telethonin has been implicated in several diseases, including limb-girdle muscular dystrophy, hypertrophic cardiomyopathy, dilated cardiomyopathy and idiopathic cardiomyopathy.
Telethonin is a 19.0 kDa protein composed of 167 amino acids. [8] Telethonin has a unique β-sheet structure, which enables antiparallel association with the Titin Z1-Z2 domains in cardiac and skeletal muscle. [9] Structural analysis of full-length Telethonin with the N-terminal region of Titin indicate that the C-terminus of Telethonin is critical for the dimerization of two Telethonin/Titin complexes into a higher oligomeric structure. [10]
Telethonin expression is developmentally regulated in both cardiac and skeletal muscle and is thought to be critical to sarcomere assembly. [11] Telethonin was found to be a late assembling protein only present in mature myofibrils at Z-discs. [12]
Telethonin forms a complex with muscle LIM protein (MLP) at sarcomere Z-discs, which constitutes part of the cardiomyocyte stretch sensory mechanism. [13] It has also been shown that Telethonin binds to the beta-subunit of the slow activating component of the delayed rectifier potassium channel, MinK, in areas localized to T-tubule membranes surrounding Z-lines in the inner myocardium. [14] In addition, Telethonin interacts with the sodium channel Na(v)1.5, and alters the activation kinetics via doubling the window current. [15] These data suggest that Telethonin may constitute a mechano-electrical links between Z-lines and T-tubules. Further functional evidence for this has come from studies utilizing a Telethonin-knockout mouse (KO), which have shown that Telethonin is involved in T-tubule structure and function, as well as apoptosis in the heart. Telethonin KO animals showed preserved Titin anchoring at baseline, and instead showed a profound deficit during nuclear biomechanical stress in modulating the turnover of the proapoptotic p53 protein. [16] Telethonin KO animals also displayed calcium transient dysynchrony, T-tubule loss and depressed L-type calcium channel function. [17]
Telethonin is a substrate of titin kinase, [18] protein kinase D (PKD) and CaM Kinase II. [19] Telethonin, as well as TNNI3, MYBPC3 and MYOM2 are phosphorylated by PKD in cardiomyocytes, and this leads to a reduction in calcium sensitivity of myofilaments, as well as accelerated crossbridge kinetics. [20] Bis-phosphorylation of Telethonin specifically at sites Serine-157 and Serine-161 has been shown to be essential for normal T-tubule organization and intracellular calcium transient kinetics. [19]
The intracellular degradation of Telethonin is regulated by MDM2 in a proteasomal-dependent yet ubiquitin-independent manner. [21] Telethonin specifically interacts with the pro-apoptotic protein Siva, suggesting that Telethonin may be involved in the mechanism underlying Coxsackievirus B3 infection in acute and chronic myocarditis [22]
Telethonin was also identified to be targeted and regulated by transcriptional activators CLOCK and BMAL1, thus demonstrating that TCAP is a circadian regulated gene. [23]
Mutations in this gene are associated with limb-girdle muscular dystrophy type R7 (previously 2G), [24] hypertrophic cardiomyopathy, [25] [26] [27] dilated cardiomyopathy, [28] [29] idiopathic cardiomyopathy, [30] and gastrointestinal smooth muscle-related diseases. [15]
Two mutations in Telethonin, Thr137Ile and Arg153His have been associated with hypertrophic cardiomyopathy, which enhance the binding of Telethonin with Titin and MYOZ2. The Glu132Gln mutation has been associated with dilated cardiomyopathy, which has the opposite effect in that it impairs the binding of Telethonin with Titin and MYOZ2. [31] Mutations in Titin associated with dilated cardiomyopathy, including Val54Met, have been shown specifically to impair binding of Titin with Telethonin. [32] In a mouse model of dilated cardiomyopathy, recapitulating the human dilated cardiomyopathy mutation in MLP, Trp4Arg, studies have found that this mutation disrupts normal binding and localization of MLP with Telethonin. [13] In a rat model of hypertension-induced cardiomyopathy, a human variant of BMP10, Thr326Ile, showed decreased binding to Telethonin and increased extracellular secretion. [33]
Telethonin has been shown to interact with:
Desmin is a protein that in humans is encoded by the DES gene. Desmin is a muscle-specific, type III intermediate filament that integrates the sarcolemma, Z disk, and nuclear membrane in sarcomeres and regulates sarcomere architecture.
Titin is a protein that in humans is encoded by the TTN gene. Titin is a giant protein, greater than 1 µm in length, that 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. These domains unfold when the protein is stretched and refold when the tension is removed.
Myomesin is a protein family found in the M-line of the sarcomere structure. Myomesin has various forms throughout the body in striated muscles with specialized functions. This includes both slow and fast muscle fibers. Myomesin are made of 13 domains including a unique N-terminal followed by two immunoglobulin-like (Ig) domains, five fibronectin type III (Fn) domains, five more Ig domains. These domains all promote binding which indicates that myomesin is regulated through binding.
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.
ACTC1 encodes cardiac muscle alpha actin. This isoform differs from the alpha actin that is expressed in skeletal muscle, ACTA1. Alpha cardiac actin is the major protein of the thin filament in cardiac sarcomeres, which are responsible for muscle contraction and generation of force to support the pump function of the heart.
Alpha-actinin-2 is a protein which in humans is encoded by the ACTN2 gene. This gene encodes an alpha-actinin isoform that is expressed in both skeletal and cardiac muscles and functions to anchor myofibrillar actin thin filaments and titin to Z-discs.
The myosin-binding protein C, cardiac-type is a protein that in humans is encoded by the MYBPC3 gene. This isoform is expressed exclusively in heart muscle during human and mouse development, and is distinct from those expressed in slow skeletal muscle (MYBPC1) and fast skeletal muscle (MYBPC2).
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.
Filamin-C (FLN-C) also known as actin-binding-like protein (ABPL) or filamin-2 (FLN2) is a protein that in humans is encoded by the FLNC gene. Filamin-C is mainly expressed in cardiac and skeletal muscles, and functions at Z-discs and in subsarcolemmal regions.
Myosin regulatory light chain 2, ventricular/cardiac muscle isoform (MLC-2) also known as the regulatory light chain of myosin (RLC) is a protein that in humans is encoded by the MYL2 gene. This cardiac ventricular RLC isoform is distinct from that expressed in skeletal muscle (MYLPF), smooth muscle (MYL12B) and cardiac atrial muscle (MYL7).
Myosin heavy chain, α isoform (MHC-α) is a protein that in humans is encoded by the MYH6 gene. This isoform is distinct from the ventricular/slow myosin heavy chain isoform, MYH7, referred to as MHC-β. MHC-α isoform is expressed predominantly in human cardiac atria, exhibiting only minor expression in human cardiac ventricles. It is the major protein comprising the cardiac muscle thick filament, and functions in cardiac muscle contraction. Mutations in MYH6 have been associated with late-onset hypertrophic cardiomyopathy, atrial septal defects and sick sinus syndrome.
Obscurin is a protein that in humans is encoded by the OBSCN gene. Obscurin belongs to the family of giant sarcomeric signaling proteins that includes titin and nebulin. Obscurin is expressed in cardiac and skeletal muscle, and plays a role in the organization of myofibrils during sarcomere assembly. A mutation in the OBSCN gene has been associated with hypertrophic cardiomyopathy and altered obscurin protein properties have been associated with other muscle diseases.
Ankyrin Repeat, PEST sequence and Proline-rich region (ARPP), also known as Ankyrin repeat domain-containing protein 2 is a protein that in humans is encoded by the ANKRD2 gene. ARPP is a member of the muscle ankyrin repeat proteins (MARP), which also includes CARP and DARP, and is highly expressed in cardiac and skeletal muscle and in other tissues. Expression of ARPP has been shown to be altered in patients with dilated cardiomyopathy and amyotrophic lateral sclerosis. A role for Ankrd2 in tumor progression and metastases spreading has also been described.
Ankyrin repeat domain-containing protein 1, or Cardiac ankyrin repeat protein is a protein that in humans is encoded by the ANKRD1 gene also known as CARP. CARP is highly expressed in cardiac and skeletal muscle, and is a transcription factor involved in development and under conditions of stress. CARP has been implicated in several diseases, including dilated cardiomyopathy, hypertrophic cardiomyopathy, and several skeletal muscle myopathies.
Cysteine and glycine-rich protein 3 also known as cardiac LIM protein (CLP) or muscle LIM protein (MLP) is a protein that in humans is encoded by the CSRP3 gene.
Myozenin-2, also referred to as Calsarcin-1, is a protein that in humans is encoded by the MYOZ2 gene. The Calsarcin-1 isoform is a muscle protein expressed in cardiac muscle and slow-twitch skeletal muscle, which functions to tether calcineurin to alpha-actinin at Z-discs, and inhibit the pathological cardiac hypertrophic response. This differs from the fast-skeletal muscle isoform, calsarcin-2.
E3 ubiquitin-protein ligase TRIM63, also known as "MuRF1", is an enzyme that in humans is encoded by the TRIM63 gene.
Myopalladin is a protein that in humans is encoded by the MYPN gene. Myopalladin is a muscle protein responsible for tethering proteins at the Z-disc and for communicating between the sarcomere and the nucleus in cardiac and skeletal muscle
Myomesin-1 is a protein that in humans is encoded by the MYOM1 gene. Myomesin-1 is expressed in muscle cells and functions to stabilize the three-dimensional conformation of the thick filament. Embryonic forms of Myomesin-1 have been detected in dilated cardiomyopathy.
Myosin binding protein C, fast type is a protein that in humans is encoded by the MYBPC2 gene.
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