Bacterial Adaptor Membrane Fusion Proteins and the Structurally Dissimilar Outer Membrane Auxiliary Proteins Have Exchanged Central Domains in -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 -proteobacteria (but not in other bacterial types) contain a long -helical region that is homologous to corresponding regions in the MFPs. The results suggest that during their evolution, OMAs, specifically from -proteobacteria, exchanged their own -helical domain for one derived from an MFP. The structural and functional implications of these findings are discussed. 1. Introduction Transmembrane transport proteins often function in conjunction with auxiliary proteins that facilitate their vectorial activities [1, 2]. Two such auxiliary protein types are found only in the prokaryotic world. These proteins are the Membrane Fusion Proteins (MFPs; Transporter Classification Number (TC#) 8.A.1 in the Transporter Classification Database (TCDB; http://www.tcdb.org/ [3]), which function in conjunction with a variety of transport system types in Gram-negative and Gram-positive bacteria [4] and the Outer Membrane Auxiliary (OMA; 1.B.18) proteins which function in conjunction with a different set of transporter types, exclusively in Gram-negative bacteria [4, 5]. MFPs function as “adaptors,” connecting a primary porter in the cytoplasmic membrane, belonging to one of four families of exporters (MFS (TC# 2.A.1), RND (2.A.6), ABC (3.A.1), or AAE (2.A.85)) with an outer membrane factor (OMF; 1.B.17) that provides a porin or channel function in the outer membrane [6]. Thus, in conjunction with an MFP and an OMF, the primary porter in the cytoplasmic membrane pumps molecules out of the cytoplasm, across both membranes of the cell envelope, and into the external milieu without equilibration of solutes in the periplasm, all in a single energy coupled step. Crosslinking studies of the MFP, AcrA of E. coli (8.A.1.6.1), with its cognate transporter, AcrB (2.A.6.2.2), and its OMF, TolC (1.B.17.1.1), revealed that AcrA could be crosslinked to both AcrB (via the C-terminal portion of AcrA) and TolC (via the central coiled-coil region of AcrA) [6]. Most MFPs are about 350–500 residues in length and either span the cytoplasmic membrane once at their N-termini or are anchored to the cytoplasmic membrane via a lipoyl moiety. These proteins cluster phylogenetically into subfamilies in accordance with the type of cytoplasmic membrane transporter with which they interact, although the
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