Assembly of outer-membrane proteins in bacteria and mitochondria

Abstract

The cell envelope of Gram-negative bacteria consists of two membranes separated by the periplasm. In contrast with most integral membrane proteins, which span the membrane in the form of hydrophobic a-helices, integral outer-membrane proteins (OMPs) form b-barrels. Similar b-barrel proteins are found in the outer membranes of mitochondria and chloroplasts, probably reflecting the endosymbiont origin of these eukaryotic cell organelles. How these b-barrel proteins are assembled into the outer membrane has remained enigmatic for a long time. In recent years, much progress has been reached in this field by the identification of the components of the OMP assembly machinery. The central component of this machinery, called Omp85 or BamA, is an essential and highly conserved bacterial protein that recognizes a signature sequence at the C terminus of its substrate OMPs. A homologue of this protein is also found in mitochondria, where it is required for the assembly of b-barrel proteins into the outer membrane as well. Although accessory components of the machineries are different between bacteria and mitochondria, a mitochondrial b-barrel OMP can be assembled into the bacterial outer membrane and, vice versa, bacterial OMPs expressed in yeast are assembled into the mitochondrial outer membrane. These observations indicate that the basic mechanism of OMP assembly is evolutionarily highly conserved.

Neisserial Omp85 Protein Is Selectively Recognized and Assembled into Functional Complexes in the Outer Membrane of Human Mitochondria

“…The full complexity of the assembly of ␤-barrel proteins in bacteria and mitochondria has been elucidated only recently (1,24). New machineries necessary for membrane integration of ␤-barrel proteins have been identified (3,4,15) as well as the signals required for the proper sorting and assembly of ␤-barrel proteins (12,23).…”

“…The OMM contains ␤-barrel proteins, a class of pore-forming proteins additionally found only in chloroplasts and Gram-negative bacteria (1,2). Similar to the majority of other mitochondrial proteins, ␤-barrel proteins are synthesized in the cytosol and have to be imported into mitochondria with the help of the translocase of the outer mitochondrial membrane (TOM) complex.…”

“…Many parallels exist between integration and assembly of ␤-barrel proteins in bacterial and mitochondrial outer membranes (1). In bacteria, proteins are first transported into the periplasmic space, where the signal peptide is cleaved off.…”

Unusual N-terminal ααβαββα Fold of PilQ from Thermus thermophilus Mediates Ring Formation and Is Essential for Piliation

“…Thus, a specific protein quality control is supposed to lead to a controlled degradation of residues 65-136 and 126 -136, whereas the remaining part is protected against degradation. Quality control of outer membrane proteins was described as a complex network of periplasmic proteins acting as chaperones and/or proteases and thereby preventing aggregation of unfolded proteins and facilitating correct folding, finally preventing the accumulation of unfolded proteins in the periplasm (43,44). Because the two truncated PilQ derivatives were uniformly processed to identical PilQ ⌬136 proteins, a quality control resulting in defined degradation of N-terminal residues is FIGURE 7.…”

Involvement of Neisseria meningitidis Lipoprotein GNA2091 in the Assembly of a Subset of Outer Membrane Proteins

“…The central component of the Bam, known as Omp85/ NmBamA in Neisseria meningitidis or EcBamA in Escherichia coli, is conserved in all Gram-negative bacteria and even in eukaryotic cell organelles of endosymbiont origin, mitochondria, and chloroplasts (7,8). However, the accessory proteins also present in the complex are less well conserved (9).…”