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

15A/2003G323

Atomic Models of a Myosin Crossbridge Derived from Intensity Analysis ofMyosin-based X-ray Meridional Reflections
Kanji OSHIMA, Yasunori TAKEZAWA, Yasunobu SUGIMOTO, and Katsuzo WAKABAYASHI*
Division of Biophysical Engineering, School of Engineering Science, Osaka University,Toyonaka, Osaka 560-8531, Japan
Introduction
The modeling analysis of the myosin-based meridionalreflections in the X-ray diffraction patterns of live frog
skeletal muscles at the full-overlap length of actin andmyosin filaments in the relaxed and contracting states
was reported in the previous report [1,2]. The derived
density maps of the crossbridges projected onto the fiber
axis had asymmetric profiles and became sharper in thecontracting state than that in the relaxed state. On thisbasis, we progressed the atomic models of myosincrossbridges to reproduce the density maps ofcrossbridges in the relaxed and contracting states by
calculating the mass projection of pairs of thecrystallographically-determined structure of a myosin
sub-fragment 1 (S1).
Modeling calculation
The atomic coordinates of the α-carbons in thepolypeptide chain of myosin heads were used for thesimulation. Total number of the α-carbon was 2144 andeach α-carbon was translated so that the origin of aCartesian coordinate system was placed at the α-carbon(Pro840), which was thought to be close to the joint of
two myosin heads.
The simulation started from theatomic structure of a two headed myosin obtained byenergy minimization, reported by Offer et al. (1996). Thez-axis was assumed to be the fiber axis, onto which themass projection was calculated. The myosin heads wererotated around the x-, y-, z-axes by α, β, γ, respectively,which variable ranges were limited: 0 ≤α≤ 90, 0 ≤ β ≤ 90,and 0 ≤ γ ≤ 360. Thereafter, the mass projection onto thez-axis was calculated by summing the projected densityof the α-carbon which is approximated with the Gaussianfunction with 4σ (0.4 nm).
The calculated mass
projection was plotted in a 0.1 nm unit and the variationof the rotation was 15º.
Results and Discussion
The mass projection onto the z-axis was then comparedwith the previously determined ones [1,2]. The mostprobable atomic model was determined by searching thebest fit of the calculated density map to that previously
reported [1,2]. The results of this simulation are shown in
Fig.1, 2. The atomic structure in the regular region in therelaxed state was slightly different from that in thecontracting state, although the mass projections were very
similar. Each myosin head in the pair had a distinctly
different axial orientation in the perturbed region both in

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the relaxed and contracting states as expected previously[1,2]. But the orientation of a myosin head around thefiber axis couldn’t be determined because the rotation of ahead around the fiber axis unaltered the projected density.
The intensity analysis of the myosin-based layer line isnecessary to determine the orientation of each headaround the fiber axis.
References[1] Oshima et al., PF Activity Rep #21 (2004)[2] Oshima et al., Fibre. Dif. Rev. 13 (2005)

Figure 1 The axial disposition (upper) and density map(lower) of the two-headed myosin crossbridges. A;
regular region, B; perturbed region.

Figure 2 The axial disposition (upper) and density map(lower) of the two-headed myosin crossbridges. A;
regular region, B; perturbed region.
*waka@bpe.es.osaka-u.ac.jp