Intragenic suppressors of an OmpF assembly mutant and assessment of the roles of various OmpF residues in assembly through informational suppressors

Interactions between folding factors and bacterial outer membrane proteins

The outer membrane is the first line of contact between Gram-negative bacteria and their external environment. Embedded in the outer membrane are integral outer membrane proteins (OMPs) that perform a diverse range of tasks. OMPs are synthesized in the cytoplasm and are translocated across the inner membrane and probably diffuse through the periplasm before they are inserted into the outer membrane in a folded and biologically active form. Passage through the periplasm presents a number of challenges, due to the hydrophobic nature of the OMPs and the choice of membranes into which they can insert. Recently, a number of periplasmic proteins and one OMP have been shown to play a role in OMP biogenesis. In this review, we describe what is known about these folding factors and how they function in a biological context. In particular, we focus on how they interact with the OMPs at the molecular level and present a comprehensive overview of data relating to a possible effect on OMP folding yield and kinetics. Furthermore, we discuss the role of lipo-chaperones, i.e. lipopolysaccharide and phospholipids, in OMP folding. Important advances have clearly been made in the field, but much work remains to be done, particularly in terms of describing the biophysical basis for the chaperone-OMP interactions which so intricately regulate OMP biogenesis.

YaeT (Omp85) affects the assembly of lipid-dependent and lipid-independent outer membrane proteins ofEscherichia coli

The Omp85 family of proteins has been found in all Gram-negative bacteria and even several eukaryotic organisms. The previously uncharacterized Escherichia coli member of this family is YaeT. The results of this study, consistent with previous Omp85 studies, show that the yaeT gene encodes for an essential cellular function. Direct examinations of the outer membrane fraction and protein assembly revealed that cells depleted for YaeT are severely defective in the biogenesis of outer membrane proteins (OMPs). Interestingly, assemblies of the two distinct groups of OMPs that follow either SurA- and lipopolysaccharide-dependent (OmpF/C) or -independent (TolC) folding pathways were affected, suggesting that YaeT may act as a general OMP assembly factor. Depletion of cells for YaeT led to the accumulation of OMPs in the fraction enriched for periplasm, thus indicating that YaeT facilitates the insertion of soluble assembly intermediates from the periplasm to the outer membrane. Our data suggest that YaeT's role in the assembly of OMPs is not mediated through a role in lipid biogenesis, as debated for Omp85 in Neisseria, thus advocating a conserved OMP assembly function of Omp85 homologues.

Assembly of TolC, a structurally unique and multifunctional outer membrane protein of Escherichia coli K-12

TolC is a multifunctional outer membrane protein of Escherichia coli that folds into a novel alpha-beta-barrel conformation absent in the other model outer membrane proteins used in assembly studies. The data presented in this work show that the unique folded structure of TolC reflects a unique assembly pathway. During its assembly, the newly translocated nascent TolC monomers are released in the periplasm. Maturation of these nascent monomers, and possibly their oligomerization, in the periplasm precedes their insertion in the outer membrane. The completion of the assembly process is signaled by the development of a characteristic proteinase K-resistant fragment generated by cleavage at a single, periplasmically exposed, protease-sensitive site of the membrane-anchored trimer. None of the assembly steps of TolC is affected by known folding factors, such as SurA, Skp, and lipopolysaccharide, which have profound effects on the assembly of other model trimeric outer membrane proteins. Two assembly-defective TolC mutants were isolated and characterized. One of the mutants (TolC(I106N)) was defective in the folding of nascent monomers, while the other (TolC(S350F)) was impaired in steps involving trimerization and membrane insertion of folded monomers.