Figure 6. SecA interconverts between an open and closed conformation in solution.

(A) SecA shown in the so-called open (left) and closed (right) conformations. Interconversion between the two conformations requires that PBD undergo a ~60° rigid-body rotation (Osborne et al., 2004). PBD is displayed as semi-transparent surface. The green sphere indicates residue 830, where a paramagnetic spin label was introduced. Residues Ile304 and Ile789 are shown as yellow and red spheres, respectively. Characteristic distances in the two conformations are indicated. A strong NOE between Ile304 and Ile789 was observed demonstrating that SecA adopts predominantly the open conformation in solution.

(B) Overlaid ¹H-¹³C HMQC spectra of SecA bearing a spin label in position 830 in the reduced (blue) and oxidized (red) state. Residues that approach the spin label, even transiently, experience a broadening effect, which is suppressed in the reduced state.