The outer membranes of mitochondria and chloroplasts are distinguished by the presence of beta-barrel membrane proteins. The outer membrane of Gram-negative bacteria also harbours beta-barrel proteins. In mitochondria these proteins fulfil a variety of functions such as transport of small molecules (porin/VDAC), translocation of proteins (Tom40) and regulation of mitochondrial morphology (Mdm10). These proteins are encoded by the nucleus, synthesized in the cytosol, targeted to mitochondria as chaperone-bound species, recognized by the translocase of the outer membrane, and then inserted into the outer membrane where they assemble into functional oligomers. Whereas some knowledge has been accumulated on the pathways of insertion of proteins that span cellular membranes with alpha-helical segments, very little is known about how beta-barrel proteins are integrated into lipid bilayers and assembled into oligomeric structures. Here we describe a protein complex that is essential for the topogenesis of mitochondrial outer membrane beta-barrel proteins (TOB). We present evidence that important elements of the topogenesis of beta-barrel membrane proteins have been conserved during the evolution of mitochondria from endosymbiotic bacterial ancestors.
"These proteins are then translocated into the IMS through the import channel Tom40. In the IMS, the chaperone complexes Tim9-Tim10 and Tim8-Tim13 bind to β-barrel proteins (Hoppins and Nargang, 2004; Wiedemann et al., 2004) and participate in the transport of these proteins to the sorting and assembly machinery (SAM complex) on the mitochondrial outer membrane (Paschen et al., 2003; Wiedemann et al., 2003). The β-barrel proteins are inserted into a hydrophilic environment within the SAM complex. "
[Show abstract][Hide abstract] ABSTRACT: Chloroplasts and mitochondria are endosymbiotic organelles thought to be derived from endosymbiotic bacteria. In present-day eukaryotic cells, these two organelles play pivotal roles in photosynthesis and ATP production. In addition to these major activities, numerous reactions, and cellular processes that are crucial for normal cellular functions occur in chloroplasts and mitochondria. To function properly, these organelles constantly communicate with the surrounding cellular compartments. This communication includes the import of proteins, the exchange of metabolites and ions, and interactions with other organelles, all of which heavily depend on membrane proteins localized to the outer envelope membranes. Therefore, correct and efficient targeting of these membrane proteins, which are encoded by the nuclear genome and translated in the cytosol, is critically important for organellar function. In this review, we summarize the current knowledge of the mechanisms of protein targeting to the outer membranes of mitochondria and chloroplasts in two different directions, as well as targeting signals and cytosolic factors.
"Later, a similar function was shown for the Omp85 homologs in Escherichia coli [5-7], Pseudomonas aeruginosa  and Borrelia burgdorferi . Omp85 homologs have been renamed BamA, for β-barrel assembly machine component A. Interestingly, a BamA homolog is also present and required for the assembly of β-barrel OMPs in mitochondria [10-12]. In E. coli, four lipoproteins associated with BamA (EcBamA) have been identified: BamB, BamC, BamD, and BamE [6,13]. "
[Show abstract][Hide abstract] ABSTRACT: The BamA protein is the key component of the Bam complex, the assembly machinery for outer membrane proteins (OMP) in gram-negative bacteria. We previously demonstrated that BamA recognizes its OMP substrates in a species-specific manner in vitro. In this work, we further studied species specificity in vivo by testing the functioning of BamA homologs of the proteobacteria Neisseria meningitidis, Neisseria gonorrhoeae, Bordetella pertussis, Burkholderia mallei, and Escherichia coli in E. coli and in N. meningitidis. We found that no BamA functioned in another species than the authentic one, except for N. gonorrhoeae BamA, which fully complemented a N. meningitidis bamA mutant. E. coli BamA was not assembled into the N. meningitidis outer membrane. In contrast, the N. meningitidis BamA protein was assembled into the outer membrane of E. coli to a significant extent and also associated with BamD, an essential accessory lipoprotein of the Bam complex.Various chimeras comprising swapped N-terminal periplasmic and C-terminal membrane-embedded domains of N. meningitidis and E. coli BamA proteins were also not functional in either host, although some of them were inserted in the OM suggesting that the two domains of BamA need to be compatible in order to function. Furthermore, conformational analysis of chimeric proteins provided evidence for a 16-stranded β-barrel conformation of the membrane-embedded domain of BamA.
PLoS ONE 12/2013; 8(12):e85799. DOI:10.1371/journal.pone.0085799 · 3.23 Impact Factor
"The superfamily also includes transporters in membranes of various organelles of endosymbiotic origin. Thus, Sam50/Tob55 is the central piece of a complex that mediates protein insertion into the outer membrane of mitochondria, and it is likely that Toc75-V (Oep80) has a similar function in chloroplasts (Hsu and Inoue, 2009; Paschen et al., 2003; Patel et al., 2008; Schleiff and Soll, 2005). Toc75-III is the core of a hetero-oligomeric complex involved in translocating proteins from the cytosol across the outer membrane of chloroplasts (Schleiff et al., 2003; Sommer et al., 2011). "
[Show abstract][Hide abstract] ABSTRACT: The two-partner secretion (TPS) pathway is a branch of type V secretion. TPS systems are dedicated to the secretion across the outer membrane of long proteins that form extended β-helices. They are composed of a 'TpsA' cargo protein and a 'TpsB' transporter, which belongs to the Omp85 superfamily. This basic design can be supplemented by additional components in some TPS systems. X-ray structures are available for the conserved TPS domain of several TpsA proteins and for one TpsB transporter. However, the molecular mechanisms of two-partner secretion remain to be deciphered, and in particular, the specific role(s) of the TPS domain and the conformational dynamics of the TpsB transporter. Deciphering the TPS pathway may reveal functional features of other transporters of the Omp85 superfamily.
Research in Microbiology 03/2013; 164(6). DOI:10.1016/j.resmic.2013.03.009 · 2.71 Impact Factor
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