OPTIMAL MODELING OF ELECTRON MICROSCOPIC 3D RECONSTRUCTIONS USING COMPONENTS OF KNOWN ATOMIC STRUCTURE. Tang, J., Blanc, E., & Chapman, M.S.; Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306-3015, USA

Several large complexes beyond the reach of x-ray crystallography have recently been analyzed using a combination of electron microscopy and interpretation with crystallographically derived atomic models of their smaller components. Analyses include complexes of actin and myosin, relevant to muscle function; viruses with antibodies and cellular receptor fragments. The focus of our work is the development of intuitive computational methods that optimize the agreement between model and EM based 3D reconstructions by adjusting the positions and orientations of domains, and experimental parameters defining phase contrast and magnification.

A function has been derived through which it is possible to calculate from an atomic model, the appearance of an electron density map at any arbitrary experimental resolution [Chapman (1995) Acta Crystallogr. A51: 69-80]. This is the basis of a stereochemically restrained least squares refinement protocol that has been applied to 3 virus x-ray crystallographic structures. The method has been adapted to lower resolution EM through the approximation of electron scattering factors by a 4-term exponential series. Following attenuation by an isotropic approximation to the contrast transfer function, the electron density contribution of each atom is calculated by analytic Fourier transformation. The sum of these contributions is compared to the EM reconstruction at each point. The method is being tested on the cryo-EM images of a virus-Fab complex, but will also be applicable to other EM techniques such as tomography.