The Porin proteins in Gram-negative bacteria facilitate:
In Gram-negative organisms, outer membrane asymmetry places lipopolysaccharide in outer leaflet and phospholipid inside, rendering it intrinsically impermeable to many hydrophilic compounds, including bile salts and antibiotics like vancomycin. Nutrient acquisition therefore depends on abundant trimeric beta-barrel proteins known as porins, exemplified by general porins OmpF, OmpC, and PhoE in Escherichia coli. Each monomer folds as a 16-stranded antiparallel barrel creating a water-filled channel lined with charged residues forming eyelet constriction that determines size exclusion near 600 daltons and charge selectivity. Sugars, amino acids, small ions, and certain beta-lactams traverse down their concentration gradients by passive diffusion without direct energy coupling, achieving facilitated entry into periplasm where high-affinity binding proteins capture them for active transport across inner membrane via ABC transporters. Porin expression is environmentally regulated: high osmolarity favors narrower OmpC via EnvZ-OmpR two-component signaling, while low osmolarity induces broader OmpF. Porins do not synthesize ATP, assemble ribosomes, or mediate large protein export; those tasks belong to respiratory chain and secretion systems.
Ref: Alberts et al., Molecular Biology Cell, 7th ed., Chapter 10: Porins; Delcour, Biochim Biophys Acta 2009, Outer Membrane Permeability.