What we have achieved so far
- In 2021 the group of Stefan Raunser and Mathias Gautel has produced the first high-resolution 3D image of the sarcomere, the basic contractile unit of skeletal and heart muscle cells, by using electron cryo-tomography (cryo-ET). (Cell, 03/2021)
- 2022 the Raunser & Gautel labs have obtained the first high-resolution 3D image of nebulin, a giant actin-binding protein that is an essential component of skeletal muscle. (Science, 02/2022)
- Frank Schnorrer and Dirk Görlich have generated a toolbox of small and highly specific nanobodies to study the precise location of the titin homologs in the different muscles of the fruit fly Drosophila melanogaster. (bioRxiv, 04/2022)
- Frank Schnorrer and Dirk Görlich have revealed the precise titin nanoarchitecture in insects flight muscles. (bioRxiv 04/202)
Structures from intact myofibrils reveal mechanism of thin filament regulation through nebulin
Wang Z, Grange M, Pospich S, Wagner T, Kho A.L, Gautel M, Raunser S (02/2022). Science
Using cryo-tomography a team led by Stefan Raunser in collaboration with Mathias Gautel has obtained the first high-resolution 3D image of nebulin, a giant actin-binding protein that is an essential component of skeletal muscle. This discovery has brought to light the chance to better understand the role of nebulin, as its functions have remained largely nebulous due to its large size and the difficulty in extracting nebulin in a native state from muscle. Their findings could lead to novel therapeutic approaches to treat muscular diseases, as genetic mutations in nebulin are accompanied by a dramatic loss in muscle force known as nemaline myopathy.
3D-Structure of Nebulin in its native environment and its function. Nebulin (pink) was identified by comparing cardiac and skeletal thin filament structures. Actin, nebulin, tropomyosin, myosin heavy chain, myosin essential light chain and myosin regulatory light chain are coloured in green, magenta, light blue, dark blue, yellow, orange and red, respectively. Nebulin maintains the length and the stability of the thin filament. It also regulates muscle contraction.
The molecular basis for sarcomere organization in vertebrate skeletal muscle
Wang Z, Grange M, Wagner T, Khoo AL, Gautel M, Raunser S (03/2021) Cell
Here, the Raunser group, in collaboration with the Gautel group, determines the molecular architecture of native vertebrate skeletal sarcomeres by electron cryo-tomography. Visualizing the mouse sarcomere in the rigor state using electron cryotomography reveals architectural details of the different zones and provides insight into how key factors are arranged within them to support function during muscle contraction.
- Three-dimensional sarcomere organization and plasticity at the molecular level
- Myosin double heads can adopt two different interactions with actin filaments
- Transition between tropomyosin states happens within one tropomyosin unit
- An irregular mesh of α-actinin doublets cross-links antiparallel actin filaments
Sarcomere organization at molecular level. Row 1 shows a schematic representation of the sarcomere. Row 2 shows the three-dimensional sarcomere organization and plasticity at the molecular level (filaments: pink and green, a-Actinin: blue). Row 3: Interaction of muscle proteins in detail. First two bubbles show the interaction of the myosin heads (yellow, orange, red) with actin (green). Third bubble shows details of actin, tropomyosin and troponin in the A-band. Last bubble depicts irregular mesh of α-Actinin (blue) cross-linking actin filaments (green, pink) in the Z-disc.
Nanobodies combined with DNA-PAINT super-resolution reveal a staggered titin nano-architecture in flight muscles
Schueder F, Mangeol P, Chan EH, Rees R, Schünemann, J, Jungmann R*, Görlich D*, Schnorrer F*. (04/2022). bioRxiv
Insect flight muscles have a particular sarcomere architecture to power wing oscillations at 200 Hz. In this collaboration between the Jungmann, Görlich and Schnorrer groups we have revealed the precise titin nanoarchitecture in these special muscles. We combined DNA-PAINT super-resolution microscopy with our newly development titin nanobody toolbox and showed that two titin homologs display a staggered organisation at the I-band/A-band interface that may be critical for efficient force transduction of flight muscles.
- DNA-PAINT enables super-resolution for sarcomeric protein domains at 5 nm precision in intact muscles
- Sallimus bridges the short I-band of flight muscle sarcomeres to reach the myosin filament
- Projectin is located only at the beginning of the myosin filament
- C-terminus of Sallimus overlaps with the N-terminus of Projectin to possibly enable effective force transmission
Molecular model of a flight muscle sarcomere. In flight muscles the short isoform of the titin homolog Sallimus bridges over the short I-band from the Z-disc to the beginning the myosin filament. The Projectin protein is located at the beginning of the myosin filament only. Note the overlap of the C-terminus of Sallimus with the N-terminus of Projectin.
A nanobody toolbox to investigate localisation and dynamics of Drosophila titins
Loreau V, Rees R, Chan EH, Taxer W, Gregor K, Mußil B, Pitaval C, Luis NM, Mageol P, Schnorrer F*, Görlich D (04/2022). bioRxiv
Titin is the largest protein in mammalian muscle and determines the length of the mammalian sarcomere. This new collaborative effort between the Görlich and Schnorrer groups has generated a toolbox of small and highly specific nanobodies to study the precise location of the titin homologs in the different muscles of the fruit fly Drosophila melanogaster. These tools revealed a precise organisation of the two Drosophila titin homologs in the sarcomeres of the different muscle types, with the surprising finding that a fly titin can be longer than the mammalian titin.
- establishment of an efficient pipeline to generate nanobodies against Drosophila titins
- superior labelling qualities of nanobodies in intact muscle tissue demonstrated
- Drosophila Sallimus being larger than 2 µm long in relaxed larval muscles
- We find a polar organisation of Drosophila Projectin on the myosin filaments.
- Nanobodies can be used in living muscle to measure the dynamics of sarcomeric proteins
Molecular model of the larval sarcomere. Novel nanobodies revealed a highly organised architecture of the two Drosophila titin homologs Sallimus and Projectin in larval muscles. Sallimus is stretched across the more than 2 µm long I-band reaching the myosin filament. Projectin decorates the myosin filament in a polar orientation with its C-terminus facing towards the central M-band of the sarcomere.