Gram-negative bacteria are defined by an accessory membrane, the outer membrane, covering the bacterial cell. This membrane has many unique features, such as it being an asymmetric bilayer where the outer leaflet is composed of lipopolysaccharide, a type of glycolipid exclusively found in Gram-negative bacteria. In addition, the integral membrane proteins of the outer membrane differ from those found in the cytoplasmic membrane of both prokaryotes and eukaryotes. The “standard” transmembrane proteins of the cytoplasmic membrane contain hydrophobic (fat-soluble) alpha-helices that anchor the proteins in the membrane. By contrast, the transmembrane proteins of the outer membrane have a different structure: they contain a beta-barrel domain, which could be considered to be a sheet-like structure where one face is hydrophobic and the other hydrophilic (or water-loving). The sheet closes in on itself to form a cylinder with a hydrophobic exterior and a hydrophilic interior. AROM’s Jack C. Leo and colleagues from Oslo have written a review focussing on the transmembrane beta-barrel proteins of Gram-negative bacteria. The review gives a general overview of these proteins, discussing their structure, folding, function, evolution as well as their medical relevance and role in antibiotic resistance.
Hermansen S, Linke D, Leo JC. Transmembrane beta-barrel proteins of bacteria: from structure to function. Adv Protein Chem Struct Biol. https://doi.org/10.1016/bs.apcsb.2021.07.002.