Fig. 4.

Inactive ExbB mutants assemble correctly with ExbD. Equivalent numbers of mid-exponential-phase bacteria were cross-linked with paraformaldehyde and evaluated by immunoblotting with anti-ExbD antibody. Wild-type W3110 (WT), the parent strain for all of the plasmid variants RA1017 (ΔexbBD ΔtolQRA), the parent plasmid encoding both ExbB and ExbD (pExbBD), and the plasmids expressing the ExbB variants are indicated above each lane of the immunoblot. The lower panel is a shorter exposure of the upper panel to allow evaluation of monomer ExbD levels. The positions of the ExbD monomer, the ExbD-ExbD dimer, the ExbD-ExbB heterodimer, and the ExbD-TonB heterodimer are indicated on the right. The positions of mass standards are indicated on the left.