Evolution of the Translocation and Assembly Module (TAM).
Bottom Line: We report that TamA and a closely related protein TamL are confined almost exclusively to Proteobacteria and Bacteroidetes/Chlorobi respectively, whereas TamB is widely distributed across the majority of Gram-negative bacterial lineages.Several sequence characteristics were discovered to define the TamB protein family: A signal-anchor linkage to the inner membrane, beta-helical structure, conserved domain architecture and a C-terminal region that mimics outer membrane protein beta-strands.Taken together, the structural and phylogenetic analyses suggest that the TAM likely evolved from an original combination of BamA and TamB, with a later gene duplication event of BamA, giving rise to an additional Omp85 sequence that evolved to be TamA in Proteobacteria and TamL in Bacteroidetes/Chlorobi.
Affiliation: Department of Microbiology, Monash University, Melbourne, Victoria, Australia.Show MeSH
Mentions: Bacterial outer membrane proteins show a range of complexity in their domain arrangements, ranging from comparatively simple porins to large and complex molecules such as autotransporters, intimins, and invasins. The canonical structure for the membrane-embedded domain of these proteins is a beta-barrel, composed of antiparallel beta-strands. After synthesis in the cytoplasm, beta-barrel proteins reach the periplasm in an unfolded state, and are then folded and inserted into the outer membrane by the beta-barrel assembly machinery (BAM, fig. 1; Knowles et al. 2009; Hagan et al. 2011; Selkrig et al. 2013). The BAM complex in Escherichia coli consists of five subunits, BamA–E (fig. 1). BamA is essential (Genevrois et al. 2003; Voulhoux et al. 2003) and is found in all Gram-negative bacteria (Heinz and Lithgow 2014). As a member of the Omp85 protein family, BamA is itself a membrane-embedded beta-barrel protein (Noinaj et al. 2013) with several periplasmic polypeptide transport-associated (POTRA) domains (fig. 1). BamD, the other component essential in Proteobacteria, has also been identified in other groups of bacteria (Anwari et al. 2012; Webb et al. 2012), whereas other components of the BAM complex in E. coli, BamB, BamC, and BamE are outer membrane lipoproteins found only in subsets of the Proteobacteria (Anwari et al. 2012; Webb et al. 2012).Fig. 1.—
Affiliation: Department of Microbiology, Monash University, Melbourne, Victoria, Australia.