Structure of the F-actin–tropomyosin complex

• Published in: Nature (London) Vol. 519; no. 7541; pp. 114 - 117
• Main Authors: von der Ecken, Julian, Müller, Mirco, Lehman, William, Manstein, Dietmar J., Penczek, Pawel A., Raunser, Stefan
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 05.03.2015; Nature Publishing Group
• Subjects: 101/28; 631/535/1258/1259; 631/80/128/1276; Actin; Actins - chemistry; Actins - genetics; Actins - metabolism; Adenosine Diphosphate - metabolism; Analysis; Animals; Cryoelectron Microscopy; Cytoskeleton; Genetic aspects; Hearing loss; Humanities and Social Sciences; letter; Magnesium - metabolism; Mice; Models, Molecular; Molecular structure; multidisciplinary; Muscle proteins; Mutation; Mutation - genetics; Myosin; Polymerization; Protein Conformation; Proteins; Rabbits; Science; Static Electricity; Tropomyosin - chemistry; Tropomyosin - genetics; Tropomyosin - metabolism; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14033

Electron cryomicroscopy reveals the three-dimensional structure of F-actin at a resolution of 3.7 Å in complex with tropomyosin at a resolution of 6.5 Å; the stabilizing interactions and the effects of disease-causing mutants are also investigated. The interaction of F-actin and tropomyosin (Rausner SS) Filamentous actin (F-actin) — a main component of the cytoskeleton — is the major protein of thin filaments in the muscle. The binding of the motor protein myosin to F-actin is mediated by another protein called tropomyosin, which also binds to F-actin in smooth muscle and in non-muscle cells, stabilizing and regulating these filaments. Using electron cryomicroscopy, Stefan Raunser and colleagues have obtained the first high-resolution, three-dimensional structure of F-actin in complex with tropomyosin. The structure reveals the interactions that stabilize the F-actin and sheds light on the possible effect of prominent disease-causing mutations. Comparison of the F-actin structure with the crystal structure of monomeric (G)-actin reveals conformational changes associated with filament formation. Filamentous actin (F-actin) is the major protein of muscle thin filaments, and actin microfilaments are the main component of the eukaryotic cytoskeleton. Mutations in different actin isoforms lead to early-onset autosomal dominant non-syndromic hearing loss 1 , familial thoracic aortic aneurysms and dissections 2 , and multiple variations of myopathies 3 . In striated muscle fibres, the binding of myosin motors to actin filaments is mainly regulated by tropomyosin and troponin 4 , 5 . Tropomyosin also binds to F-actin in smooth muscle and in non-muscle cells and stabilizes and regulates the filaments there in the absence of troponin 6 . Although crystal structures for monomeric actin (G-actin) are available 7 , a high-resolution structure of F-actin is still missing, hampering our understanding of how disease-causing mutations affect the function of thin muscle filaments and microfilaments. Here we report the three-dimensional structure of F-actin at a resolution of 3.7 Å in complex with tropomyosin at a resolution of 6.5 Å, determined by electron cryomicroscopy. The structure reveals that the D-loop is ordered and acts as a central region for hydrophobic and electrostatic interactions that stabilize the F-actin filament. We clearly identify map density corresponding to ADP and Mg 2+ and explain the possible effect of prominent disease-causing mutants. A comparison of F-actin with G-actin reveals the conformational changes during filament formation and identifies the D-loop as their key mediator. We also confirm that negatively charged tropomyosin interacts with a positively charged groove on F-actin. Comparison of the position of tropomyosin in F-actin–tropomyosin with its position in our previously determined F-actin–tropomyosin–myosin structure 8 reveals a myosin-induced transition of tropomyosin. Our results allow us to understand the role of individual mutations in the genesis of actin- and tropomyosin-related diseases and will serve as a strong foundation for the targeted development of drugs.