Paracrystalline arrays of protein-synthesis elongation factor Tu. Comparison with polymerized actin

Homogeneous protein synthesis elongation factor Tu from Escherichia coli forms aggregates at high concentrations of ammonium sulfate which have a filamentous appearance in the light microscope. Electron microscopy of negatively stained preparations shows that these aggregates are paracrystalline, including three different forms. On the basis of analyses by optical diffraction, this polymorphism can be explained in terms of three different tubular foldings of the same basic two-dimensional surface lattice. This can be compared with that underlying the structure of actin filaments, thus providing a crucial test of the putative relationship between the elongation factor and actin [Rosenbusch, J. P. et al. (1976) J. Supramol. Struct. 5, 391-396]. The differences between the surface lattices, in conjunction with the negative results of sensitive immunochemical tests for possible cross-reactivities between the two proteins, suggest that any such relationship is very remote.