Order, Disorder and Heterogeneity in Helical Polymers.

Edward Egelman.

University of Virginia, USA.

We have been studying for many years helical polymers formed by actin and the RecA/RadA/Rad51 family of recombination proteins. In both systems a picture emerges using genetics, spectroscopy, biochemistry, x-ray crystallography and electron microscopy that there is not one structure of the filament, but rather a large range of different structures reflecting a multiplicity of different subunit-subunit interfaces. As a result of the variability and polymorphism within these filaments, we have created a new method, Iterative Helical Real Space Reconstruction, for generating three-dimensional reconstructions from electron micrographs of helical polymers. Applications of this method to filamentous bacteriophage, bacterial pili, and bacterial Type Three Secretion System components is providing new insights into the structure of these polymers. In particular, the EspA needle extension of Enteropathogenic E. coli will be discussed, since it provides a picture of how structural polymorphism within a helical polymer is related to function.