A FAMILY OF GRAM-NEGATIVE BACTERIAL OUTER-MEMBRANE FACTORS THAT FUNCTION IN THE EXPORT OF PROTEINS, CARBOHYDRATES, DRUGS AND HEAVY-METALS FROM GRAM-NEGATIVE BACTERIA

Gram-negative bacteria have evolved transport complexes that export ma cromolecules and toxic substances across the two membranes of the cell envelope in a single energy coupled step. The process requires (1) a cytoplasmic membrane export system, (2) a membrane fusion protein (MFP ), and (3) an outer membrane factor (OMF). Families comprising the for mer two constituents have been described previously. We here present a n analysis of the phylogenetic and structural characteristics of the O MF family. Twenty-one members of this family have been identified, and based on available evidence, they function in conjunction with ABC, R ND, MFS and/or other types of cytoplasmic transport systems. OMFs exhi bit fairly uniform sizes (398-495 residues with two exceptions), and b ased on computational analyses, they may form beta-barrel structures c onsisting of up to 16 beta-strands. Phylogenetic analyses reveal that while the MFPs cluster in accordance with the type of cytoplasmic memb rane transport systems with which they function, OMFs do not. We concl ude that OMFs probably comprise a family of outer membrane porin-type proteins of uniform structure which did not coevolve with their cognat e cytoplasmic membrane transport systems.