Group:MUZIC:Titin: Difference between revisions

At the N-terminal end of titin Ig-domains Z1/Z2 interact with small 19 kDa protein, called telethonin <ref> Pinotsis N, Petoukhov M, Lange S, Svergun D, Zou P, Gautel M, Wilmanns M. (2006)  Evidence for a dimeric assembly of two titin/telethonin complexes induced by the telethonin C-terminus. J Struct Biol. 155(2):239-50. Epub 2006 Apr 27.PMID 16713295 </ref>  <ref> see http://www.uniprot.org/uniprot/O15273 </ref>(also called  “T-Cap”). It connects titin molecules, according to current models from same half of the sarcomere, into an antiparallel “sandwich”. This complex has 2:1 stoichiometry, which means that one telethonin molecule allows an antiparallel arrangement of two titin molecules.Due to numerous hydrogen bonds that connect the β-strands of two molecules, the titin-telethonin complex is extremely resistant to mechanical forces along one axis of the complex. Telethonin is thus likely to be responsible for anchoring titin molecules in the Z-disc. It has also been proposed to play a role as a mechanosensor, as well as participating in targeting other sarcomeric proteins. For example, telethonin seems to be connected to membrane-associated proteins like small ankyrin-1 (sANK1) and the potassium channel subunit minK  <ref> see http://www.uniprot.org/uniprot/P15382 </ref>, that are localized in the sarcoplasmic reticulum and T-tubules, respectively. In turn, small-ankyrin-1 is associated with spectrin, desmin and obscurin. It was proposed that indirect interactions of small ankyrin with the titin amino-terminus promote correct positioning of the sarcoplasmic reticulum (SR) around the Z-disc. Titin could also participate in SR organization via the interaction of Ig-like domains Z8/Z9 with obscurin  <ref> see http://www.uniprot.org/uniprot/Q5VST9 </ref>. Binding of telethonin to minK would place T-tubules in proximity of the Z-disc and might influence functioning of potassium channel depending on myocyte stretch. However, minK is not expressed in skeletal muscle (where T-tubules are localized at the A/I junction) or ventricular myocytes <ref> Kupershmidt S, Yang T, Anderson ME, Wessels A, Niswender KD, Magnuson MA, Roden DM. (1999) Replacement by homologous recombination of the minK gene with lacZ reveals restriction of minK expression to the mouse cardiac conduction system. Circ Res. 84(2):146-52. PubMed PMID 9933245. </ref>, therefore this proposed mechanism is unlikely to be of general relevance. Telethonin was also shown to interact the TGF-beta related growth factor myostatin and the calcineurin inhibitor calsarcin-3. Finally, the NH2-terminus of titin was reported to interact via telethonin with muscle LIM protein (MLP or CSRP3). In striated muscles, MLP localizes partly to the Z-disc, however it is found also at costameres, in the I-band and in the nucleus. Translocation of MLP to the nucleus may co-regulate myogenic transcription factors and upregulate protein expression. Another related cascade, the “Telethonin-MLP-calcineurin- nuclear factor of activated T cells (NFAT)” signaling pathway is implicated in mechanosensing and leads to physiological hypertrophy. It has been proposed that mechanical stress on the Z-disc activates this signaling cascade, however, the precise mechanism of signal transduction and the role of titin domains Z1/Z2 remain to be investigated.Another telethonin-mediated interaction of titin’s amino terminus is with MDM2 (mouse double minute-2). This protein is an E3 ubiquitin ligase that ubiquitylates the tumor suppressor p53/TP53, leading to its proteasomal degradation. Other sites of titin mechanosensing, not resident in the Z-disc, include the MARP-Myopalladin complex that interacts with titin's N2A-domain in the I-band and the nbr1 complex interacting with the titin kinase domain at the M-band.  

Interaction with actin was reported for titin's domains Z9-I1. Additional binding partners of titin at the Z-disc are nebulin and filamin C, which both interact direct with titin by their carboxy-terminal parts. <br/>

 

Essential interactions of α-actinin and the Z-repeats of titin within Z-disc were shown experimentally <ref> Ohtsuka H, Yajima H, Maruyama K, Kimura S. (1997)  Binding of the N-terminal 63 kDa portion of connectin/titin to alpha-actinin as revealed by the yeast two-hybrid system. FEBS Lett. 401(1):65-7. PMID 9003807 </ref>  <ref> Sorimachi H, Freiburg A, Kolmerer B, Ishiura S, Stier G, Gregorio CC, Labeit D, Linke WA, Suzuki K, Labeit S. (1997) Tissue-specific expression and alpha-actinin binding properties of the Z-disc titin: implications for the nature of vertebrate Z-discs. J Mol Biol. 270(5):688-95 PMID 9245597 </ref> <ref> Atkinson RA, Joseph C, Kelly G, Muskett FW, Frenkiel TA, Nietlispach D, Pastore A. (2001) Ca2+-independent binding of an EF-hand domain to a novel motif in the alpha-actinin-titin complex. Nat Struct Biol. 8(10):853-7. PMID 11573089</ref>.

This interaction was reported for Z-repeats 1 and 7 and the calmodulin-like domains (syn. EF-hand domains) at the C-terminus of α-actinin. A third of interaction site is located between Z-repeat 7 and the adjacent Ig-domain of titin and involves the spectrin-like domains 2 and 3 of the α-actinin homodimer.

Strong interactions between actin, α-actinin and titin form a spatial scaffold, thus enabling the correct placement of other proteins inside the Z-disc. In addition, it was also shown that two muscle proteins, LIM and FATZ, are interacting with both telethonin and α-actinin, reinforcing the titin/telethonin and titin/α-actinin networks.  

The Z-disk connects all elastic and contractile components of sarcomere and enables transduction of tensile forces. Some of these components take part in different signaling pathways, others are responsible for direct mechanosensing. Within the Z-disk several layers of actin crosslinked by α-actinin are usually visible on electron microscopy images. The thicker is the Z-disk, the more layers it has.

 

The Z- disc thus connects major elastic and contractile components of the sarcomere and enables the transduction of active and passive forces along actin and titin filaments, respectively. Some of these components take part in different signaling pathways, others are responsible for direct mechanosensing. Within the Z-disc, several layers of actin crosslinks by α-actinin are visible in electron microscopy images. The thicker is the Z-disc, the more layers it has. The thickness of Z-discs varies significantly between different types of muscles due to adaptation to variable levels of mechanic stress. It was proposed that titin Z-repeats are the major determinant of Z-disc thickness. The number of Z-repeats and the number of crosslink layers in the Z-disc correlate tightly (i.e. sarcomeres with the full range of Z-repeats have the thickest disc, and the chicken pectoralis Z-disc with only 2 layers of crosslinks has only two Z-repeats, <ref> Peckham M, Young P, Gautel M. (1997)  Constitutive and variable regions of Z-disk titin/connectin in myofibril formation: a dominant-negative screen. Cell Struct Funct. 22(1):95-101.PMID 9113395</ref>). However, major questions about the ultrastructure and molecular assembly of the Z-disc remain, as it appears that the length of a single repeat may be less than the thickness of single layer inside Z-disc (19 nm). Clearly, highly resolved ultrastructural analysis avoiding the problem of EM sample shrinkage will be needed to complement the structure of individual Z-disk protein complexes.  

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