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Palindromic assembly of the giant muscle protein titin in the sarcomeric Z-disk

Abstract

The Z-disk of striated and cardiac muscle sarcomeres is one of the most densely packed cellular structures in eukaryotic cells1. It provides the architectural framework for assembling and anchoring the largest known muscle filament systems by an extensive network of protein–protein interactions, requiring an extraordinary level of mechanical stability. Here we show, using X-ray crystallography, how the amino terminus of the longest filament component, the giant muscle protein titin, is assembled into an antiparallel (2:1) sandwich complex by the Z-disk ligand telethonin. The pseudosymmetric structure of telethonin mediates a unique palindromic arrangement of two titin filaments, a type of molecular assembly previously found only in protein–DNA complexes. We have confirmed its unique architecture in vivo by protein complementation assays, and in vitro by experiments using fluorescence resonance energy transfer. The model proposed may provide a molecular paradigm of how major sarcomeric filaments are crosslinked, anchored and aligned within complex cytoskeletal networks.

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Figure 1: Structure of the palindromic titin–telethonin–titin complex.
Figure 2: Telethonin-mediated assembly and Z-disk anchoring of titin filaments.
Figure 3: Evidence for the formation of the palindromic titin–telethonin complex in vivo and in vitro.
Figure 4: Model outlining the involvement of the titin–telethonin complex in the architecture of the sarcomeric Z-disk.

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Acknowledgements

We thank D. Fürst for the gift of antibodies; R. Kühnemuth for discussions on the FRET experiments; M. Forster for involvement in expression and purification tests; G. Burenkov for assistance during data collection at beamline BW6 at MPG-ASMB/DESY; E. Mandelkow and M. von Bergen for making the fluorimeter at MPG-ASMB/DESY available; and E. Ehler for the preparation of neonatal rat cardiomyocyte cultures. N.P. and S.L. were supported by the EU research and training network CAMKIN to M.W. and M.G., respectively. During involvement at the project, O.M. was supported by an EU Marie-Curie postdoctoral fellowship.

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Correspondence to Matthias Wilmanns.

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Coordinates and structure factors have been deposited in the Protein Data Bank under accession number 1YA5. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Discussion

Summarization of previous data on structural assemblies of immunoglobulin (IG)-like domain containing proteins. (PDF 88 kb)

Supplementary Methods

Details on production of seleno-L-methionine (SeMet)–incorporated telethonin, cellular targeting of telethonin and its mutants, antibodies, confocal microscopy. (PDF 125 kb)

Supplementary Table 1

In vitro/in vivo titin-telethonin assembly and localisation data of four telethonin mutants are listed. (PDF 75 kb)

Supplementary Table 2

X-ray data collection and phasing statistics of the structure of the titin/telethonin complex are summarised. (PDF 147 kb)

Supplementary Figure 1

The structure of the (2:1) titin/telethonin complex is shown schematically. (PDF 59 kb)

Supplementary Figure 2

Confocal images of neonatal rat cardiomyocytes that were transiently transfected with six different variants of telethonin. Those variants in which the structure of one the two telethonin β -hairpins was affected do not properly localise to the Z-disk. (PDF 7255 kb)

Supplementary Figure 3

Titin/telethonin complex formation of several telethonin variants under in vitro conditions, as evidenced by native gel electrophoresis. Those variants in which the structure of one the two telethonin β -hairpins was affected do allow complex formation. (PDF 3304 kb)

Supplementary Figure 4

Titin/telethonin complex formation of several telethonin variants in COS cells using a YFP-complementation assay. Those variants in which the structure of one the two telethonin β -hairpins was affected do not complement. (PDF 1977 kb)

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Zou, P., Pinotsis, N., Lange, S. et al. Palindromic assembly of the giant muscle protein titin in the sarcomeric Z-disk. Nature 439, 229–233 (2006). https://doi.org/10.1038/nature04343

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