porin

Porins are a class of pore-forming proteins located in the outer membranes of Gram-negative bacteria and in the outer membranes of mitochondria and chloroplasts. They create channels that allow the passive diffusion of small hydrophilic molecules, such as ions, sugars, amino acids, and nucleotides, down their concentration gradients. Porins help set the size limit for substances crossing the membrane and contribute to the overall permeability of the membrane.

Most bacterial porins are beta-barrel proteins that form beta-sheet structures spanning the outer membrane. They often assemble as trimers and feature extracellular loops that influence selectivity. Classic examples in Escherichia coli include OmpF, OmpC, and PhoE, which differ in pore size and regulatory properties. Expression of these porins is modulated by environmental conditions, such as osmolarity.

In mitochondria and chloroplasts, porins are represented by voltage-dependent channels in the outer membrane, commonly known as voltage-dependent anion channels (VDACs). These channels regulate exchange of metabolites, including ATP, ADP, phosphate, and other small metabolites, between the cytosol and the organellar interior. Plant and fungal mitochondria possess porin homologs with similar roles.

Porins are central to nutrient uptake, metabolite exchange, and antibiotic susceptibility. Because they determine membrane permeability, changes in porin expression or pore properties can influence bacterial fitness and contribute to antibiotic resistance by limiting drug entry. Porins remain a key topic in studies of membrane transport, bacterial physiology, and pathogenesis.