#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 org

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

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 me

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

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

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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