title: Structural basis for tropomyosin overlap in thin (actin) filaments and the generation of a molecular swivel by troponin-T.
abstract: Head-to-tail polymerization of tropomyosin is crucial for its actin binding, function in actin filament assembly, and the regulation of actin-myosin contraction. Here, we describe the 2.1 A resolution structure of crystals containing overlapping tropomyosin N and C termini (TM-N and TM-C) and the 2.9 A resolution structure of crystals containing TM-N and TM-C together with a fragment of troponin-T (TnT). At each junction, the N-terminal helices of TM-N were splayed, with only one of them packing against TM-C. In the C-terminal region of TM-C, a crucial water in the coiled-coil core broke the local 2-fold symmetry and helps generate a kink on one helix. In the presence of a TnT fragment, the asymmetry in TM-C facilitates formation of a 4-helix bundle containing two TM-C chains and one chain each of TM-N and TnT. Mutating the residues that generate the asymmetry in TM-C caused a marked decrease in the affinity of troponin for actin-tropomyosin filaments. The highly conserved region of TnT, in which most cardiomyopathy mutations reside, is crucial for interacting with tropomyosin. The structure of the ternary complex also explains why the skeletal- and cardiac-muscle specific C-terminal region is required to bind TnT and why tropomyosin homodimers bind only a single TnT. On actin filaments, the head-to-tail junction can function as a molecular swivel to accommodate irregularities in the coiled-coil path between successive tropomyosins enabling each to interact equivalently with the actin helix.
authors.affiliation: Department of Biosciences, School of Science and Engineering, Teikyo University, Toyosatodai 1-1, Utsunomiya 320-8551, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Murakami
authors.forename: Murray
authors.initials: M
authors.lastname: Stewart
authors.forename: Kayo
authors.initials: K
authors.lastname: Nozawa
authors.forename: Kumiko
authors.initials: K
authors.lastname: Tomii
authors.forename: Norio
authors.initials: N
authors.lastname: Kudou
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Igarashi
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Shirakihara
authors.forename: Soichi
authors.initials: S
authors.lastname: Wakatsuki
authors.forename: Takuo
authors.initials: T
authors.lastname: Yasunaga
authors.forename: Takeyuki
authors.initials: T
authors.lastname: Wakabayashi
mesh_terms: D000199:Actins / Q000737:chemistry*; D000818:Animals; D009202:Cardiomyopathies / Q000378:metabolism*; D018360:Crystallography, X-Ray / Q000379:methods; D019281:Dimerization; D008954:Models, Biological; D008958:Models, Molecular; D008968:Molecular Conformation; D054792:Muscle, Striated / Q000473:pathology; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011817:Rabbits; D014335:Tropomyosin / Q000737:chemistry*; D020107:Troponin T / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D000199:Actins; D014335:Tropomyosin; D020107:Troponin T; D014867:Water