Kouji Makino, Toshiro Oda, Ichiro Yamashita, Keiichi Namba, Yuichiro Maeda
International Institute for Advanced Research, Matsushita Electric Industrial Co., Ltd. 3-4 Hikaridai, Seika 619-02, Japan
In order to understand the mechanism of calcium regulation of muscular contraction, it is important to know the structure of the whole thin filament (actin + tropomyosin + troponin). Previous works, however, either X-ray diffraction studies or 3D reconstructions of electron micrographs, suffer from the lack of information originating from the helical symmetry of tropomyosin and troponin.
In the present study, the native thin filaments have been prepared from porcine cardiac muscle and the filaments have been oriented in glass capillaries as sols composed of nematic liquid-crystalline phase. From the filaments, we have recorded X-ray diffraction patterns both in the presence of EGTA (the low calcium state) and calcium ions (the high calcium state). The X-ray diffraction patterns show not only the major reflexions which are associated with the helical symmetry of actin subunits, but also the on- and near-meridional reflexions which are indexed as orders of 77 nm, the helical pitch of the arrangement of tropomyosin and troponin. Especially, for the first time, the troponin-associated reflexions have been observed up to 2.7 nm actin reflexion. Moreover the differences between two images are seen in the intensities of second and third actin layer-lines, confirming the well-documented observations, as well as in the intensities of troponin-associated reflexions.
At the conference, two sets of intensity data will be presented and implications to structural changes of the thin filament will be discussed.