Porins in prokaryotes and eukaryotes: common themes and variations

Abstract

Gram-negative bacteria and mitochondria are both covered by two distinct biological membranes. These membrane systems have been maintained during the course of evolution from an early evolutionary precursor. Both outer membranes accommodate channels of the porin family, which are designed for the uptake and exchange of metabolites, including ions and small molecules, such as nucleosides or sugars. In bacteria, the structure of the outer membrane porin protein family of β-barrels is generally characterized by an even number of β-strands; usually 14, 16 or 18 strands are observed forming the bacterial porin barrel wall. In contrast, the recent structures of the mitochondrial porin, also known as VDAC (voltage-dependent anion channel), show an uneven number of 19 β-strands, but a similar molecular architecture. Despite the lack of a clear evolutionary link between these protein families, their common principles and differences in assembly, architecture and function are summarized in the present review.

Outer membrane β-barrel protein folding is physically controlled by periplasmic lipid head groups and BamA

“…To obtain generally applicable insights into the biological folding of outer membrane β-barrels, we investigated the folding of eight distinct OMPs: full-length outer membrane protein A (OmpA 325 ), OmpT, OmpX, OmpW, PhoP/PhoQ-activated gene product (PagP), outer membrane phospholipase (OmpLA), the long-chain fatty acid transport protein (FadL), and the β-barrel assembly machinery subunit A (BamA) from E. coli. We chose these OMPs for several reasons: (i) These proteins all derive from the same biological membrane and have experienced similar evolutionary pressures with respect to their native lipid environments; (ii) they have distinct sequences with no significant pairwise sequence similarity; (iii) their folding has been extensively studied in vitro and in vivo (15); and (iv) their structures are solved at high atomic resolution, which shows that the set is structurally diverse within the β-barrel fold (20). All of these proteins will fold into synthetic lipid vesicles composed of phosphocholine head groups.…”

Structural basis for alginate secretion across the bacterial outer membrane

“…S8). The β-barrel fold observed for AlgE suggests an export mechanism that is more typical of the translocation of hydrophilic molecules across the bacterial outer membrane whereby transport is mediated by a single polypeptide chain folded in the membrane (35). In contrast, group 1 and 4 capsular polysaccharides are secreted through a Wza-like octameric transmembrane lipoprotein, which contains little in the way of conservation among the residues that line the interior of the translocation pathway and is much wider (17 Å at its narrowest point) than AlgE.…”

Mitochondria and the lectin pathway of complement

“…The major pathogenassociated molecular pattern of Gram-negative bacteria, encompassing ␣-proteobacteria and hence the protomitochondrion, is lipopolysaccharide of the outer membrane. However, in mitochondria, the lipid composition of the outer membrane resembles that of the eukaryotic plasma membrane and does not contain LPS (43). Although the outer mitochondrial membrane does contain porins with similar ultrastructure and function to that of Gram-negative bacteria, there is no apparent evolutionary link between these (43), suggesting that they may be perceived by the immune system as two different entities.…”