Role of porins in outer membrane permeability.

It is now well recognized that the outer membranes of gram-negative bacteria serve as molecular sieves which permit the passage of small hydrophilic molecules of sizes below a given cutoff, i.e., the exclusion limit (21, 27). This sieving property, which permits uptake of small substrate molecules but excludes potentially harmful enzymes and other large hydrophilic molecules, is due to a class of proteins called porins (21). Porins form trans-outermembrane, water-filled channels, the dimensions of which determine the exclusion limit. An indication of their importance in bacteria is the observaton that Spirochaeta aurantia, an organism which apparently lacks lipopolysaccharide in its outer membrane (also called its outer sheath) (A. Kropinski and E. P. Greenberg, personal communication), nevertheless contains a major porin protein (19) whose properties are closely related to those of, e.g., Escherichia coli (7). Porins have now been identified and characterized for 32 species of bacteria (R. E. W. Hancock, in M. Inouye, ed., Bacterial Outer Membranes as Model Systems, in