Outer membrane assembly

There is a report that the signal for the outer membrane assembly resizes at the C termini of proteins (Struyve et al., 1991 de Cock et al., 1997). That is, most C-terminal residue must be phenylalanine in the outer membrane proteins. Experiments using mutant proteins showed that C-terminal phenylalanine is important for the efficient assembly of PhoE. 

Daptomycin localises to the cell division septum in B. subtilis 34 it causes rapid cell death without lysis,48 causes the formation of aberrant cell wall septa48 and treats biofilms effectively in S. aureus. 9 Daptomycin induces the cell wall stress stimulon in S. aureus and B. subtilis,30 34 but has very poor activity (MIC of 128) against an E. coli imp mutant defective in outer membrane assembly,5 whereas vancomycin, another bulky peptide that normally does not penetrate the outer membrane, has an MIC of 0.8 against E. coli imp.50... 

The production of adhesive P pili requires PapC, which is an 88-kDa outer membrane protein. Genetic lesions in PapC result in a block in the assembly pathway, leading to an accumulation of chaperone-pilus protein complexes in the periplasm (Norgren et al., 1987). Thus, in the absence of PapC it seems that the chaperone preassembly complexes are no longer targeted to outer membrane assembly sites. As a consequence, the periplasmic chaperone remains bound to the subunits, preventing their assembly into pili. 

While it has been known for many years that the N-terminal presequence is sufficient to promote mitochondrial targeting and assembly, the subsequent interaction of the precursor molecule with the outer mitochondrial membrane and the uptake of the protein is still an area of active research. There seems little doubt, however, that there are proteins on the outer mitochondrial membrane which are required for the import process. The function of these proteins is uncertain, but they may act as receptors with the subsequent transfer through the membrane at proteinous pores located at contact sites between the inner and outer membranes. Several proteins have been identified which seem to play an important role as either receptor proteins or part of the import channel (Pfanner et al., 1991). Again, not all proteins seem to depend on this mechanism. Cytochrome c, which is loosely associated with the outer aspect of the inner mitochondrial membrane, can cross... 

Outer membrane proteins face several problems. They have to cross the inner membrane thus implying that they are not allowed to contain hydrophobic membrane anchor or stop transfer sequences. Moreover, correct targeting to and stable insertion into the outer membrane is important for proper functioning. A number of OM proteins assemble into oligomers even prior to insertion. 

Typically, functional porins are homotrimers, which assemble from monomers and then integrate into the outer membrane. The general porins, water-filled diffusion pores, allow the passage of hydrophilic molecules up to a size of approximately 600 Daltons. They do not show particular substrate specificity, but display some selectivity for either anions or cations, and some discrimination with respect to the size of the solutes. The first published crystal structure of a bacterial porin was that of R. capsulatus [48]. Together with the atomic structures of two proteins from E. coli, the phosphate limitation-induced anion-selective PhoE porin and the osmotically regulated cation-selective OmpF porin, a common scheme was found [49]. Each monomer consists of 16 (3-strands spanning the outer membrane and forming a barrel-like structure. 

Figure 4 (Plate 5). Atomic structure of the sucrose specific selective porin ScrY isolated from the outer membrane. Longer loops are directed to the outside, shorter turns are facing the periplasm. Monomer and assembled homotrimer in side view (left and middle) top view of assembled trimer (right). (Reproduced by permission of W. Welte and A. Brosig)...

Danese, P., and Silhavy, T. (1998). Targeting and assembly of periplasmic and outer-membrane proteins in Escherichai coli. Annu. Rev. Genet. 12, 59-94. 

Struyve, M., Moons, M., and Tommassen, J. (1991). Carboxy-terminal phenylalanine is essential for the correct assembly of a bacterial outer membrane protein. J. Mol. Biol. 218, 141-148. 

Based on the intrinsic mitochondriotropism of dequalinium and its unique self-assembly behavior, we have developed a strategy for direct mitochondrial transfection (47-49), which involves the transport of a DNA-mitochondrial leader sequence (MLS) peptide conjugate to mitochondria using DQAsomes, the liberation of this conjugate from the cationic vector upon contact with the mitochondrial outer membrane followed by DNA uptake via the mitochondrial protein import machinery. We have demonstrated that DQAsomes fulfill all essential prerequisites for a mitochondria-specific DNA delivery system they bind and condense pDNA (24), protect it from... 

The insertion and assembly of -barrel outer membrane proteins, including Tom40, are assisted by the sorting and assembly machinery (SAM) complex (Fig. 1). The SAM complex consists of four subunits, the core translocase Sam50, which is itself a putative -barrel protein (Kozjak et al. 2003 Paschen et al. 2003 Gentle et al. 2004), and the additional proteins Sam35,... 

Ishikawa D, Yamamoto H, Tamura Y, Moritoh K, Endo T (2004) Two novel proteins in the mitochondrial outer membrane mediate beta-barrel protein assembly. J Cell Biol 166 621-627... 

Kozjak V et al. (2003) An essential role of Sam50 in the protein sorting and assembly machinery of the mitochondrial outer membrane. J Biol Chem 278 48520-48523... 

Wiedemann N, Truscott KN, Pfannschmidt S, Guiard B, Meisinger C, Pfanner N (2004) Biogenesis of the protein import channel Tom40 of the mitochondrial outer membrane intermembrane space components are involved in an early stage of the assembly pathway. J Biol Chem 279 18188-18194... 

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