OmpA protein of XXXX outer membrane occurs in open and closed channel forms.

@entity969 @entity4583 @entity526 @entity1133
OmpA protein of @entity526 outer membrane can produce diffusion channels when reconstituted into proteoliposomes (Sugawara, E., and Nikaido, H. (1992) J. Biol. Chem. 267, 2507-2511). The pore size is similar to that of the classical @entity526 porins OmpF and OmpC, but the penetration rates of small solutes through the OmpA channel are about 50 times slower than that through the OmpF channel. Here we examined the possibility that only a small fraction of the OmpA molecules produces open channels. Unilamellar proteoliposomes were made so that each vesicle contained only a small number of OmpA molecules. These vesicles, containing 0.3 M @entity1133 within, were fractionated on a linear iso-osmolar density gradient made of @entity1133 and @entity969 . This resulted in the clear separation of vesicles not containing any open channel, staying on top of the gradient, from those containing at least one open channel, sedimenting close to the bottom. Calculation using Poisson distribution indicated that only between 2 and 3% of the OmpA molecules contained open channel. The open form is estimated to allow the diffusion of @entity4583 at a rate comparable with that through the OmpF porin channel. The open and closed states were relatively stable properties of the protein. Denaturation of open form OmpA and its subsequent renaturation converted it into a nonfunctional or closed form, suggesting that the open and closed forms represent two alternative conformers of this protein.
