X-Ray Diffraction Pattern of Non-Uniformly Stretched Actin Filament

Abstract

Helical structures within living cells are exposed to mechanical forces and are consequently deformed. However, interpretations of X-ray fiber diffraction patterns of such structures assume Fourier transform of helices with fixed periodicities. Huxley et al. and Wakabayashi et al. (Biophys. J. 67: 2422-2435 and 2411-2421, 1994) calculated the extensibility of actin filaments in contracting muscle fibers from the X-ray diffraction patterns using mean values of strain. Single value of strain corresponds to symmetric, narrow meridional peak profile, while, non-uniform deformation of actin filament provides profile similar to the observations, noticeably changing elastic constant for actin filament. Due to discrete, stochastic attachments of myosin heads to actin filaments the strain along the filament increases stepwise from the free end of the actin filament to the Z-line. We have developed a methodology for predicting X-ray diffraction patterns with stepwise increases of strain along actin filament. Using PDB data for the crystal structures of G-actin and rules for constructing actin fibers we reconstructed the geometry of actual deformed fibers. Fiber deformations are determined by Monte Carlo calculations using the computational platform, MUSICO. Predicted X-ray diffraction patterns show smeared layer lines, caused by different pitch of helices, in contrast to distinct layer lines originating from constant pitch of undeformed helices. Calculated meridional X-ray diffraction peak profiles from deformed helices are skewed and closely resemble the observed profiles by Huxley et al. and Wakabayashi et al. The proposed methodology for analyzing deformed helices provides realistic intensity profiles and spacing values of molecular structures in inverse space. This enables the extraction of much more information concerning molecular structure from the X-ray diffraction data from any helical structures in living cells.Supported by: NIH 9 P41 GM103622, R01 AR048776 and R01 DC 011528.