Bacterial Adaptor Membrane Fusion Proteins and the Structurally Dissimilar Outer Membrane Auxiliary Proteins Have Exchanged Central Domains in α-Proteobacteria

Transport systems frequently include auxiliary proteins that perform subfunctions within the transporter protein complex. Two such proteins found in Gram-negative bacteria are the Membrane Fusion Proteins (MFPs) and the Outer Membrane Auxiliary (OMA) proteins. We here demonstrate that OMAs present in alpha-proteobacteria (but not in other bacterial types) contain a long alpha-helical region that is homologous to corresponding regions in the MFPs. The results suggest that during their evolution, OMAs, specifically from alpha-proteobacteria, exchanged their own alpha-helical domain for one derived from an MFP. The structural and functional implications of these findings are discussed.