Fig. 2.

SspB·ssrA and ClpX·ssrA contacts. Substrate delivery complex clipped in plane to highlight contacts between the proximal subunit of SspB (green surface representation), the ssrA tag (red stick or ball-and-stick representation), and ClpX (light purple; surface representation). The pore-2 loop (dark purple backbone representation) of chain A of ClpX blocks the axial channel and contacts the C terminus of the ssrA tag. Inset I highlights SspB contacts with the ssrA tag. The conformation of an ssrA tag from a crystal structure (pdb 1OX9) bound to SspB (cyan stick or ball-and-stick representation) is also shown. Inset II depicts contacts between ClpX and the C-terminal residues of the ssrA tag overlaid with models of the same tag residues and pore-2 loop (orange) from the recognition complex (pdb 6WRF).