5.8 Structure and Folding of Outer Membrane Proteins

When the first outer membrane protein (OMP) structure was solved in 1991, it was believed that all OMPs would look similar to the general diffusion porin from Rhodobacter capsulatus . This protein folds as a series of 16 amphipathic β strands that associate to form a β barrel with a hydrophobic exterior and a hydrophilic pore that allows diffusion-driven transport of small molecules based primarily on size. The prediction was that all OMPs would be β-barrel proteins of limited complexity and that all OMPs would use β strands to span the outer membrane. Both predictions turned out to be wrong. The majority of OMPs are indeed β-barrel proteins, but they range in complexity from simple cylindrical barrels to complex, multidomain structures that perform a variety of functions. Similarly, the folding of OMPs is a complex process both in vitro and in vivo . Both genetic and biophysical investigations are elucidating these pathways. Even with similar topologies, OMPs show a wide diversity in folding kinetic behavior in vitro , although there appear to be common chaperones in vivo ,