Module swaps between related translocator proteins pIV(f1), pIV(IKe) and PulD: identification of a specificity domain.

In Gram-negative bacteria, type II and type III secretion and filamentous phage assembly systems use related outer membrane proteins for substrate-specific transport across the outer membrane. We show here that the specificity domain of the phage f1 outer membrane protein pIV is contained within the 149 N-terminal amino acid residues. When the pIV(f1) specificity domain is fused to the translocator domain of the related pIV of phage IKe, the chimeric construct supports f1 but not IKe assembly. Functional coupling between the two domains in this chimeric construct is poor and is improved by a single amino acid change in the translocator domain of the pIV(IKe). In native pIV(IKe), two amino acid changes within its specificity domain are both necessary and sufficient to change the specificity from IKe to f1 assembly. Analysis of 39 chimeric constructs between pIV(f1) and the outer membrane protein PulD of the pullulanase secretion system failed to identify a comparable exchangeable specificity domain. These results indicate that the two domains may not function autonomously, and suggest that tertiary and quarternary changes of the entire translocator component rather than of an autonomous functional domain are required for specific translocation across the outer membrane.