Journal of Molecular Biology
Volume 99, Issue 3, 15 December 1975, Pages 461-464, IN19-IN26, 465-475
Journal home page for Journal of Molecular Biology

Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments

https://doi.org/10.1016/S0022-2836(75)80138-0Get rights and content

Paracrystals of thin filaments prepared from actin, tropomyosin and troponin (actin + TM:TN have different filament structure and packing at high Ca2+ concentrations (>10−5 m) than at low (<10−6 m). In the former condition, the paracrystals have periodic transverse stripes, adjacent filaments have alternate polarity, and the presence of tropomyosin in the long-pitch helical grooves of actin can be discerned. In the latter, there are no transverse stripes, all filaments have the same polarity and the paracrystals appear identical to those of pure actin. These features are revealed by electron microscopy, optical diffraction and filtering and, for the demonstration of polarity, by the “decoration” of the filaments with heavy meromyosin.

The contribution of tropomyosin to the structure of actin + TM:TN paracrystals at high Ca2+ has been separated from that of troponin by an analysis of paracrystals containing only actin and tropomyosin. Apart from lacking the transverse stripes, the latter paracrystals have the same filament structure and polarity as the former. The helical symmetry of the actin filaments in these two types of paracrystal is 28 subunits in 13 turns and the filaments are arranged with pgg plane group symmetry.

In the actin + TM:TN, high Ca2+ paracrystals, the troponin repeat matches that of actin. There is one troponin molecule for every seven actin subunits, and the troponins in adjacent filaments are related by the plane group symmetry to give a stripe width comparable with that seen in the electron micrographs. The helical symmetry in paracrystals of actin + TM:TN formed at low Ca2+ is 13 actin subunits in six turns, and, with the one to seven troponin to actin ratio maintained, the actin pitch and troponin repeat do not match. The paracrystals accommodate the precise interfilament register of the actin helices but the troponins in adjacent filaments have no particular spatial relationship to each other and there are therefore no transverse stripes.

References (28)

  • EbashiS. et al.
  • GillisJ.M. et al.

    J. Mol. Biol.

    (1975)
  • GosslingT.H.

    J. Mol. Biol.

    (1969)
  • HaselgroveJ.C.

    J. Mol. Biol.

    (1975)
  • MillerA. et al.

    J. Mol. Biol.

    (1972)
  • MorimotoK. et al.

    J. Mol. Biol.

    (1974)
  • O'BrienE.J. et al.

    J. Mol. Biol.

    (1972)
  • ParryD.A.D. et al.

    J. Mol. Biol.

    (1973)
  • PotterJ.D.

    Arch. Biochem. Biophys.

    (1974)
  • SpudichJ.A. et al.

    J. Mol. Biol.

    (1972)
  • VibertP.J. et al.

    J. Mol. Biol.

    (1972)
  • BaileyK.

    Biochem. J.

    (1948)
  • EbashiS. et al.

    Quart. Rev. Biophys.

    (1969)
  • HansonJ.
  • Cited by (0)

    Present address: Department of Physiology, U.C.L. 5540, 55 Avenue Hippocrate, B-1200, Brussels, Belgium.

    View full text