OmpF

OmpF is a major outer membrane porin protein found in many Gram-negative bacteria, including Escherichia coli. It forms a trimer in the outer membrane and acts as a passive diffusion channel that allows small, hydrophilic molecules to cross the outer membrane.

Structurally, each OmpF monomer constructs a beta-barrel pore composed of multiple beta-strands. The trimeric assembly presents three individual channels. The constriction region, formed by the L3 loop of each subunit, creates a selective gate that influences which solutes can enter the pore. The overall pore is large enough to permit a range of small molecules, while excluding larger entities.

Functionally, OmpF facilitates the diffusion of nutrients and ions required by the cell, as well as various metabolites. It also plays a significant role in the uptake of certain antibiotics, such as beta-lactams and fluoroquinolones, making porin-mediated permeability a factor in antimicrobial sensitivity. Diffusion through OmpF is influenced by the size and charge of solutes, with the pore exhibiting selectivity beyond simple size exclusion.

Expression of ompF is tightly regulated in response to environmental conditions. The EnvZ-OmpR two-component system modulates transcription in response to osmolarity: ompF expression is typically higher at low osmolarity and reduced at high osmolarity, with the opposite pattern observed for the related porin ompC. Additional regulation involves small RNAs, such as MicF, which can downregulate OmpF translation.

OmpF is widely studied as a model outer membrane porin and contributes to the permeability properties of Gram-negative bacteria. It is also a factor in antibiotic susceptibility and resistance mechanisms when porin-mediated uptake is altered.