Permeability properties of chemically modified porin trimers from Escherichia coli B

The pore-forming protein of the outer membrane of Escherichia coli, porin, was chemically modified with acetic anhydride, succinic anhydride, and glycinamide. Extensive modification of amino groups of the functional porin trimers caused reduced diffusion rates of the negatively charged solutes such as p-nitrophenyl phosphate and AMP, but did not reduce significantly the diffusion of positively charged molecules carbobenzoxy-glycyl-prolyl-arginine-p-nitranilide and tosyl-glycyl-prolyl-arginine-p-nitranilide. Modification of carboxyl groups of trimers caused decreased diffusion rates of the positively charged solutes more significantly than the diffusion rates of negatively charged solutes. The results suggest that the ionic interactions play an important role for the diffusion of charged solutes through the porin pore. The diffusion of p-nitrophenyl alpha-D-glucoside, an uncharged solute, ws not influenced significantly by modification of either amino or carboxyl groups. This observation suggests that modifications only occurred in areas outside of the narrowest portion of the pore or, alternatively, that amino and carboxyl groups are exclusively located at noncylindrical area of the pore. The structural integrity of the acetylated and the succinylated trimers seemed well preserved. On the other hand, modification of carboxyl groups decreased the thermal stability of trimers and extensive modifications caused the dissociation of trimers into monomers at 37 degrees C.