Ionic selectivity of pores formed by the matrix protein (porin) of Escherichia coli.

Incorporation of the matrix protein (porin) from the outer membrane of Escherichia coli into black lipid films results in the formation of ion-permeable pores with a single-pore conductance of the order of 2 nS (in 1 M KCl). Information on the structure of this pore has been obtained by determining the selectivity for various species differing in charge and size. From the permeability of the pore for large organic ions (Tris+, glucosamine+, Hepes-) a minimum pore diameter of 0.8 nm is estimated. At neutral pH the pore is two to four times more permeable for alkali ions than for chloride. On the basis of the observed pH dependence of permeability, this cationic selectivity is explained by the assumption that the pore contains fixed negative charges.