Photon Factory Activity Report 2004 #22 Part B (2005)
Biological Science

15A/2003G323

Axial Profile of the Actin-based Meridional Reflections Associated with the Actin
Extensibility during Force Generation of Skeletal Muscles
Yasunori TAKEZAWA1, Noriko SATOI1, Kanji OSHIMA1,Takakazu KOBAYASHI2, and Katsuzo WAKABAYASHI*1
1
Division of Biophysical Engineering, Graduate School of Engineering Science, Osaka University,Toyonaka, Osaka 560-8531, Japan
2
Facluty of Engineering, Shibaura Institute of Technology, Shibaura, Minato-ku,
Tokyo 108-8548, Japan
Introduction
We confirmed that actin filaments extends by ~0.36%
under isometric contraction in the previous experiments[1][2]. In a half sarcomere actin filaments overlap withmyosin filaments in the overlap region, but do not in theI-band. The magnitude of force is proportional to thelength of the overlap in both filaments, that is, the numberof the myosin heads interacting with actin filaments. Thus,
it is thought that the extension of an actin filament is not
uniform along with the fiber axis. We developed the onedimensional model for the distribution of actin
periodicity: the elongation of an actin filamentaccumulates linearly from the pointed end to theboundary with the I-band in the overlap region and ismaximum and constant in the I-band. We investigated theeffect of the nonuniformity in the elongation of actin
filaments on the axial profiles of the actin-based
meridional reflections and compared with those axial
profiles observed in the X-ray diffraction pattern.

disposition of actin monomers estimated from theobserved widths of the meridional reflections in theresting state. As a result, the separation of double peaksof each reflection became invisible but asymmetricfeature of the profile in the third and fourth reflections(Fig. 1; dark blue line) still remained. On the other hand,the axial profiles of the meridional reflections observed in
the contracting state did not appear to have such an
asymmetric feature. To more precisely clarify whether theeffect of the nonuniformity of actin elongation supposedin the model occur or not, we need to measure the profilesof the third and fourth reflections using the beam of muchsharper width.
References[1] Wakabayashi et al., Biophys. J., 67, 2422 – 2435
(1994).
[2] Takezawa et al., Adv. Exp. Med. Biol., 453, 309 –317 (1998).

Experimantal
Live sartorius muscles of the bullfrog were used for thisstudy. We measured two-dimensional X-ray diffraction
patterns from muscles. The diffraction patterns in theresting state and during isometric contraction wererecorded on image plates. The specimen-to-detector
distance was set at ~0.5 m to measure the actin-based first-1-1-1(1/2.73 nm ), second (1/1.36 nm ), third (1/0.91 nm ) and-1fourth (1/0.68 nm ) order meridional reflections. The
fiber axis was tilted at a proper angle corresponding to themeridional reflection whose axial intensity profile wasbrought into the Ewald sphere.
Results and Discussion
The calculation from the model of ~0.4% elongationshows that the axial intensity profiles had double peaks
mainly coming from interference between the overlap
region and I-band and two peaks separated more
prominently, as the order of meridional reflection
increased (Fig. 1; light blue line). The amount of a shift in
the centroid in each reflection was same as the percentageof average elongation in the entire actin filament,
indicating that the centroids of the meridional reflections
were not affected by the nonuniformity in the elongationof actin filaments. In order to compare with the intensity
profiles observed in the X-ray diffraction pattern, thecalculated profiles were convolved with the spread of thedirect beam on a detector and the interference function
originating from the second kind-like disorder of the246

Users' Report

Figure 1 The axial intensity profiles of the actin-based
first (A) and the fourth (B) meridional reflections
normalized to their peak height. Green filled circles; theobserved data in contracting state, light blue line; thecalculated profile from 1D model, black line; the verticalprofile of the direct beam, red line; the profiles of theinterference function due to the second kind-like disorderof the actin dispositions, dark blue line; the resulting
profiles after the calculated profiles were convolved withthose of the beam and the interference function.
* waka@bpe.es.osaka-u.ac.jp