OMPS

Outer membrane proteins (OMPs) are a broad class of proteins located in the outer membrane of Gram-negative bacteria, and they include homologous proteins found in the outer membranes of mitochondria and chloroplasts. In bacteria, most OMPs are beta-barrel proteins that function as channels, receptors, transporters, or adhesion factors. They are typically synthesized with an N-terminal signal peptide that directs them through the Sec pathway into the periplasm, where chaperones guide them to the outer membrane and the beta-barrel assembly machinery (Bam) inserts them into the membrane. Lipoprotein-anchored OMPs use the Lol system to reach the outer membrane.

Most OMPs share a common structural theme: a beta-barrel spanning the outer membrane, formed by 8 to 26 beta-strands with extracellular loops exposed to the environment. Porins are a prominent subtype that allow passive diffusion of small solutes, while TonB-dependent transporters are larger, substrate-specific uptake systems that require energy transduction from the TonB–ExbB–ExbD complex.

Functions of OMPs include nutrient uptake, waste efflux, and maintenance of membrane integrity, as well as roles in virulence and immune interactions. Expression of OMPs is tightly regulated and responsive to environmental conditions such as iron availability, osmolarity, and stress. Some OMPs are major antigens and vaccine targets, while others influence antibiotic sensitivity by altering membrane permeability.

In eukaryotes, mitochondrial and chloroplast outer membranes contain homologous beta-barrel proteins derived from bacterial ancestors, but their targeting and assembly differ from bacterial OMPs. Studying OMPs informs understanding of membrane biology, pathogenesis, and the development of vaccines and antimicrobial strategies.