X-ray studies of thin filaments in a tonically contracting molluscan smooth muscle.

In the X-ray pattern of the ABRM, layer lines from the thin filaments can be clearly observed not only at the small angles but also at the medium angles up to 0.145 A-1, and the layer lines are changed in intensity in the tonically contracting state in both the small and medium angle regions. Because tropomyosin does not contribute significantly to the layer lines except the three near the equator, it is suggested that structural change of actin occurs in the tonically contracting state. The intensity profiles of the layer lines with the 0th Bessel order corresponding to the axial periods 27.9, 13.8, 9.3, and 6.9 A indicate axial staggering of the inner and outer domains of actin by about 1/3 of the axial period of actin, and the presence of a small mass on the shoulder of the outer domain at a distance of 50 A from the filament axis. The difference Patterson functions calculated from the intensities of the four layer lines with the 0th Bessel order suggest that peptide chains are oriented perpendicularly to the filament axis in the actin molecules. The peptide chain direction is presumably tilted against the filament axis in the outer part of actin as a result of structural change by tonic contraction.