#outer membrane

Protein Sorting and Trafficking

Which complex is responsible for mitochondrial protein import into the outer membrane?

Mitochondrial proteome exceeds 1000 proteins but mitochondrial genome encodes only 13 hydrophobic subunits humans requiring massive import nuclear encoded precursors cytosolic ribosomes. Entry gateway TOM complex translocase outer membrane central hub 400-500 kDa. Core beta-barrel channel Tom40 19 beta strands hydrophilic pore about 20 angstrom unfolded polypeptide passage. Associated receptors provide specificity: Tom20 tetratricopeptide clamps amphipathic presequence hydrophobic face, Tom70 clamp binds internal hydrophobic signals carriers via Hsp70/Hsp90 chaperone docking, Tom22 central organizer acidic cytosolic intermembrane space domains. Small subunits Tom5 Tom6 Tom7 regulate assembly dynamics. Precursors engage receptor transfer Tom22 then traverse Tom40 aqueous channel intermembrane space where small Tim chaperones Tim9-Tim10 guide downstream translocases: TIM23 for presequence matrix proteins TIM22 for polytopic carriers SAM for outer beta-barrels MIA for oxidative folding IMS proteins. TIM23 TIM22 OXA reside inner membrane performing inner steps not outer; OXA inserts mitochondrially encoded proteins. TOM unique outer entry gate conserved yeast human essential viability and biogenesis and organelle maintenance.

Ref: Wiedemann & Pfanner, Annu Rev Biochem 2017: TOM complex outer membrane import machinery.

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Microbial Physiology

The permeability of the Gram-negative outer membrane is controlled by:

Gram-negative outer membrane serves as molecular sieve preventing entry of large hydrophilic and hydrophobic antibiotics like vancomycin, daptomycin, and bile salts. Permeability is governed primarily by porins, abundant trimeric beta-barrel proteins forming water-filled diffusion channels with constrictions determined by internal loop L3. General porins OmpF and OmpC allow passive diffusion of molecules below about 600 daltons, including nutrients and some beta-lactams, dependent on charge and size. Specific porins like LamB and ScrY facilitate uptake of maltodextrins and sucrose via binding site within channel, increasing efficiency. Regulation of porin abundance through two-component system EnvZ-OmpR and small RNAs like MicF enables adaptation to osmolarity, pH, and antibiotic pressure, with porin loss conferring resistance to carbapenems. Lipoteichoic acids are Gram-positive polymers, peptidoglycan thickness controls lysozyme sensitivity in Gram-positives, and ether bonds characterize archaeal lipids not controlling Gram-negative permeability. Hence porins act as major determinants of outer membrane exclusion limit, linking envelope permeability to nutrient acquisition and intrinsic drug resistance mechanisms studied by Nikaido pioneering work.

Ref: Nikaido, Microbiol Mol Biol Rev 2003, Outer Membrane Porins and Resistance; Delcour, BBA 2009, Porin Regulation.

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Microbial Physiology

The Gram-negative bacterial cell wall consists of:

Gram-negative cell wall is thin but complex, optimized for protection while maintaining permeability for nutrients. It comprises inner plasma membrane, narrow periplasmic space containing single or few layers of peptidoglycan only about 2 to 6 nanometers thick, and outer membrane distinguished by asymmetry. Outer membrane outer leaflet consists of lipopolysaccharide anchored by lipid A, while inner leaflet contains phospholipids; integral outer membrane proteins such as porins, TonB-dependent receptors, and OmpA form beta-barrels. Lipopolysaccharide confers negative charge, stabilizes outer membrane via bridging divalent cations, and provides endotoxin activity and serotype specificity. Thick peptidoglycan, characteristic of Gram-positives, is absent; instead, thin layer provides shape but contributes less mechanical strength than in Gram-positives. Monolayer lipid and teichoic acid-only models do not reflect Gram-negative dual membrane architecture. Assembly of outer membrane requires Lpt pathway transporting LPS from inner membrane across periplasm to outer leaflet and Bam complex folding beta-barrel proteins. This organization explains staining behavior, antibiotic susceptibility, and resistance to detergents and bile in enteric organisms.

Ref: Silhavy et al., The Bacterial Cell Envelope, 2nd ed.; Vollmer & Seligman, Trends Microbiol 2010, Gram-Negative Wall.

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Microbial Physiology

Which structure is unique to Gram-negative bacteria?

Gram-negative and Gram-positive cell envelope architectures diverge sharply. Gram-positive envelope comprises thick peptidoglycan multilayer with interspersed wall teichoic acids and lipoteichoic acids anchored to underlying plasma membrane, without outer membrane. Gram-negative envelope contains thin peptidoglycan layer sandwiched in periplasm plus additional outer membrane asymmetric membrane whose outer leaflet is almost exclusively lipopolysaccharide and inner leaflet phospholipid, harboring porins, Bam complex, and Lpt transport machinery. Peptidoglycan itself, plasma membrane composed of ester-linked phospholipids, and lipoteichoic acids are not exclusive; peptidoglycan occurs in both but differs in thickness, plasma membrane is universal among cellular life, lipoteichoic acids are Gram-positive signature. Outer membrane is absent from Gram-positives, archaea, and eukaryotes, representing evolutionary innovation of Gram-negative lineage that confers intrinsic resistance to lysozyme, detergents, and many antibiotics, while also providing additional barrier requiring specialized protein folding chaperones like SurA and Skp in periplasm for outer membrane protein biogenesis and transport. Cryo-electron tomography reveals outer membrane is densely packed with lipopolysaccharide stabilized by ionic crosslinks, and its biogenesis demands coordinated transport of proteins and lipids from inner membrane, making outer membrane assembly a target for novel Gram-negative specific antibiotics like darobactin that inhibits Bam complex folding.

Ref: Silhavy et al., The Bacterial Cell Envelope; Alberts et al., Molecular Biology Cell, Membrane Architecture.

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Microbial Physiology

What happens to Gram-negative bacteria when the outer membrane is removed?

Gram-negative bacteria possess a thin peptidoglycan layer confined to the periplasm between the inner cytoplasmic membrane and a distinctive outer membrane containing lipopolysaccharide and trimeric porins. The outer membrane provides substantial mechanical support, contributes to shape maintenance, and functions as a selective barrier against hydrophobic toxins, bile salts, and many antibiotics. When this membrane is disrupted by EDTA, which chelates magnesium and calcium ions that stabilize LPS contacts, or by mild detergents and lysozyme treatment, barrier and load-bearing capacity are lost. If peptidoglycan is simultaneously degraded, the rod-shaped cell collapses into an osmotically fragile, spherical structure that retains some membrane fragments and residual wall polymers. This intermediate is termed a spheroplast, in contrast to a protoplast produced from Gram-positive bacteria where enzymatic wall removal leaves only plasma membrane. Spheroplasts require isotonic media supplemented with sucrose or salts to prevent lysis but remain transcriptionally active. Their laboratory formation, studied extensively in Escherichia coli, illustrates recent evidence that outer membrane contributes to stiffness and growth integrity.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 3: Bacterial Envelope; Nature 2018, Outer membrane as load-bearing element.

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