Interaction of the 2HF of SecA with SecYEG. (A) Structure of T. maritima SecA bound to SecYEG viewed from the side facing the lateral gate with the α-helices shown as cylinders (Zimmer et al., 2008); SecY (white), SecE (gray), SecG (dark gray) and SecA: 2HF (yellow), PPXD (green), HWD (red), NBD1 (pale blue), and NBD2 (pink). Key residues (corresponding to E. coli numbering) have also been highlighted: SecY_K268 (purple space fill), SecY_I183 (cyan space fill), SecA_Y794 (red space fill), and SecA_A795 (orange space fill). Residues shown to be in close proximity to the translocating preprotein are shown for SecA (blue sticks; Bauer and Rapoport, 2009) and SecY (magenta sticks; Cannon et al., 2005). (B) Color coding and numbering as in A. Close-up showing the cross-linking site between SecY_K268C (Y-268) and both SecA_Y794C (A-794) and SecA_A795C (A-795). The alternative cross-linking site in SecY is also shown (Y-183). The pathway for the preprotein is shown by a dashed line, on the basis of known cross-links to SecA and SecY. (C) Color coding and numbering as in A. Equivalent view as B of a molecular model based on the same SecA–SecYEG structure (Zimmer et al., 2008) constrained by a cross-link between the SecY_I183C (cyan space fill) and SecA_A795C (orange space fill). The blocked pathway for the preprotein is shown by the dashed line.