Proteins of the Omp85 family are considered as very ancient and can be found in almost all prokaryotic species with an outer membrane and in the outer membrane of the endosymbi-otic organelles mitochondrion and chloroplast. They are involved in the protein translocation across or the insertion of proteins into the membrane. In all species analyzed so far the protein signs essential for the biogenesis of the outer membrane or, in chloroplasts, of the organelle. Therefore, this protein family is an interesting indicator for both, the evolutionary development of organelles as well as proteins. Recent studies have emphasized differences between Omp85 proteins from cyanobacteria and proteobacteria. Here we challenge the observation by the comparison of the func-tionality in the native context of a bamA mutant of Escherichia coli. We demonstrate that Omp85 from Anabaena sp. PCC 7120 in-serted into the outer membrane of E. coli does not complement the mutant strain but in contrast appears to be lethal for the proteobacterial cells. Possible explanations for this are discussed.