Type I secretion systems (TISS) are associated with the virulence of Gram-negative bacteria and the secretion of pathogenic molecular determinants. The Shewanella oneidensis MR-1 outer-membrane protein AggA is part of a TISS. Recombinant AggA expressed in Escherichia coli as inclusion bodies can be efficiently refolded in vitro, and can form active channel-tunnels as shown by proteoliposome swelling assays and electrophysiological measurements in lipid bilayers. Structure-based sequence alignments identify AggA as a TolC-like protein, and point to a conserved structural framework among such proteins despite their marginal sequence similarity. Phylogenetic analysis reveals that clustering of TolC-like proteins can be correlated with their involvement in TISS, Resistance/Nodulation/Division (RND) or Major Facilitator Superfamily (MFS) complexes. Taken together. our results establish a first set of structure-function relationships for a bacterial outer-membrane protein likely to be exclusively involved in TISS. and may contribute towards a more accurate classification of Outer-Membrane Factor (OMF) family proteins. (C) 2009 Elsevier Inc. All rights reserved.