Concatemers of Outer Membrane Protein A Take Detours in the Folding Landscape

Outer membrane protein A (OmpA) is the most abundant protein in the outer membrane of Escherichia coli. The N-terminal domain forms an eight-stranded membrane-embedded β-barrel that is widely used as a model protein for in vitro folding into the membrane and into surfactant micelles. Under conditions that include a low surfactant concentration, OmpA can form stable higher-order structures by intermolecular association. Other β-barrel membrane proteins also associate to form noncovalently linked trimers in vivo. This inspired us to test how topological constraints imposed by intramolecular links between individual OmpA molecules affect this process. Here we report on the properties of concatemers consisting of two and three copies of the transmembrane part of OmpA. Both concatemers could be folded to a native state in surfactant micelles according to spectroscopy and electrophoretic band shifts. This native state had the same thermodynamic stability against chemical denaturation as the original OmpA. Above 1.5 M GdmCl, concatemerization increased both refolding and unfolding rates, which we attribute to entropic effects. However, below 1.5 M GdmCl, folding kinetics were 2-3 orders of magnitude slower and more complex, involving a greater degree of parallel folding steps and species that could be classified as off-pathway. Only OmpA2 could quantitatively be folded into vesicles (though to an extent lower than that of OmpA), while OmpA3 formed three species with different levels of folding. Thus, close spatial and sequential proximity of OmpA domains on the same polypeptide chain have a strong tendency to trap the protein in different misfolded states.