Katsuzo Wakabayashi, Yutaka Ueno*, Yasunori Takezawa and Yasunobu Sugimoto
Department of Biophysical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka Osaka 560, Japan (*Present address, Electrotechnical Laboratory, Tsukuba, Ibaraki 305, Japan)
Since actin participates of molecular interactions that appear to be functionally important for muscle contraction, it is necessary to have knowledge of the actin structure and its structural changes that occur in the contractile process. In a skeletal muscle, actin together with regulatory proteins tropomyosin and troponin forms the thin filament. X-ray diffraction studies have shown that when skeletal muscle contracts, the myosin crossbridge interaction with actin occurs in the incommensulate periodicities of the thick and thin filaments, and in this framework characteristic intensity changes take place in the thin filament-associated layer lines. Thus the structural changes of the thin actin filament was investigated by measuring the intensities of their layer lines. Following a procedure of Holmes et al. (1990), the model of F-actin in a frog muscle was constructed by fitting the atomic structure of the actin monomer into a filament using the observed resting layer-line intensities up to 13Å. The difference Fourier calculated from the resting and activated data revealed that the intensity changes of the thin filament layer lines during contraction could be related to the structural changes within the thin filament itself in the presence of myosin interaction. Reasonable modelling of the X-ray patterns from the thin filaments of the activated muscle could be done with subdomain movements within the actin monomer together with a tropomyosin shift from the resting structure. The resulting thin filament structure in the active state has an increased nature of the four-fold rotational symmetry due to changes in domain structure of the actin monomer and a tropomyosin shift. The most substantial movement within the actin monomer appears to be in subdomain 2. Difference between two relative subdomain movements, 2-3 and 1-4 could explain the differential intensity changes of the layer lines from the two genetic helices. These changes in domain structure of the actin monomer accompanied the elastic extension of the actin filament in the active state. By considering the initial shortening in the activation process, the filament extension amounted to about 0.4% under the maximum force generated by a muscle, estimated from the spacing increase of the first and second actin meridional reflections. The extensibility might be related approximately to the movement of actin monomer along the left-handed genetic track as evidenced from the differential changes in the axial spacing of the two genetic helices, causing the untwisting and twisting of the helical filament. The relationship between the conformational changes of actin monomers and the filament extensibility will be discussed.