Three-dimensional image reconstruction of helical aggregates of trypsin modified elongation factor EF-Tu from Escherichia coli
The three-dimensional structure of trypsin-modified EF-Tu polymers was analyzed to a resolution of 30 Å with electron image reconstruction techniques after negative staining. In a 70% saturated ammonium sulfate solution the modified protein forms cylindrical aggregates with a diameter of about 340 Å. The repeat distance of the structure along the cylindrical axis is 448 Å. The large number of subunits in one repeat hampers the assessment of the helical symmetry. The Fourier analysis and three-dimensional synthesis were therefore carried out with three different selection rules. The three reconstructed density distributions show marked differences. In all of them twofold axes perpendicular to the cylindrical axis are present. The half unit cell content of one of the reconstructions shows a striking similarity with the shape of intact EF-Tu.GDP previously proposed in a similar study. We suggest that in the assemblies investigated here dimers of trypsin-modified EF-Tu.GDP are arranged along a one-start basic helix with 15.4 subunits per turn and a pitch of 64 Å. The shape of the monomeric proteolyzed EF-Tu.GDP in this helical arrangement is very similar to that of the intact molecule in cylindrical assemblies studied at this resolution.