REFINEMENT OF THE F-ACTIN STRUCTURE AGAINST FIBER DIFFRACTION DATA. M. M. Tirion, K.C. Holmes, M. Lorenz.

The crystal structure of G-actin permits development of atomic models of F-actin based on X-ray fiber diffraction images from aligned filaments. The initial atomic model of F-actin, obtained using only 4 structural parameters, reproduced most features of the X-ray and electron-microscopic data. However, the 7A resolution X-ray fiber diffraction images contain sufficient data to refine more than 4 structural parameters. Refinement at atomic level is too fine-grained and would overfit the diffraction data. The challenge therefore, is to identify appropriate structural parameters which reduce the fiber diffraction residual while preserving the stereochemistry of actin.

We present two different methods: use of slow, normal modes of G-actin as structural refinement parameters, and a Monte-Carlo method to select appropriate subdomains in G-actin, for the refinement of F-actin. Both methods resulted in a significant reduction of the residual and showed similar trends in the structural changes from G- to F-actin, including the closure of the nucleotide binding pocket between the large and small domains. This finds strong support from biochemical experiments which show that polymerization of actin inhibits the nucleotide exchange rate.