Figure 1

Structure of SecA-N68 (A) Domain structure of SecA and deletion constructs. Full length SecA consists of nucleotide binding domain 1 (NBD1); the preprotein cross-linking domain (PPXD), which is connected to NBD1 through a β-hairpin linker; NBD2; the C-terminal domains (CTD), which can be further sub-divided into the helical scaffold domain (HSD), residues 611 to 670 and 755 to 835 and helical wing domain (HWD), residues 671 to 754; finally there is the zinc binding domain, which consists of a 22 residue zinc-binding motif at the extreme C-terminus, connected to residue 835 by an unstructured linker. The unstructured N-terminus of SecA is illustrated by the short green line, and has the sequence MLIKLLTKVFGSRN; in some constructs, the N-terminal sequence included a hexahistidine tag with sequence MHHHHHHLTKVFGSRN. An unstructured C-terminal sequence (short orange line) was also present in SecA-N68, which, starting from residue 597, had the sequence RIFASDRVSGMMRK. (B) Structure of SecA-N68∆NC, with domains colour-coded as in Panel A. The structure also contained Mg²⁺ (green sphere) and ADP (sticks), bound between NBD1 and NBD2. (C) Structure-based sequence alignment of the PPXDs from the SecA molecules of 5 different organisms: M. tuberculosis (1NKT)⁶³, T. maritima (3JUX)⁴⁸, T. thermophilus (2IPC)⁶⁴, B. subtilis (1TF5)¹¹, and E. coli. The numbering and secondary structure (H for helix, B for strand) is for E. coli SecA. Residues that have a relatively small CA RMSD between the 5 structures are indicated with black dots. (D) Superposition of PPXD structures. The E. coli PPXD from the SecA-N68∆NC structure is shown as a grey ribbon and the other four structures (as in Panel C) are shown as CA-traces. The 5 superposed PPXD structures have been mapped onto the SecYEG-bound PPXD in the T. maritima SecA-SecYEG complex (3DIN)⁶.