Figure 3
A SAXS-Based Model for the SecA-N68 Tetramer (A) The SecA-N68∆NC crystal structure was used with the program GLOBSYMM39 to find a tetramer with D2 symmetry that matches the solution SAXS data, which were recorded with SecA-N68 at 6.7 mg/mL33. The experimental SAXS data are indicated by the grey curve, with the vertical bars showing the standard deviation of the replicate measurements. The red curve is the scattering from the SecA-N68 tetramer model (Panels B, C, and D), calculated using Crysol65. The experimental radius of gyration was 48.2 Å, while the value for the hydrated tetramer model was 48.9 Å; the overall χ2 value for the fit of the model to the data was 2.24. The tetramer model is shown from three perspectives related by 90° rotations about a vertical axis (B to C) and horizontal axis (C to D). The N- and C-termini of SecA-N68∆NC are shown with red spheres. The additional unstructured residues at the N- and C-termini that mediate tetramer formation in SecA-N68 have been built into the structure to illustrate a potential mode of interaction. The extreme N-terminus is binding in the cleft between NBD1 and NBD2 to mediate dimer formation between the green and magenta protomers, and the yellow and cyan protomers. The extreme C-terminus is binding in a groove next to the hairpin connecting NBD1 to the PPXD, and mediates dimer formation between the green and cyan protomers, and yellow and magenta protomers. Together, the extreme N- and C-termini, illustrated with spheres at CA positions, mediate a dimer-of-dimers tetramer.
