Oligo-(R)-3-hydroxybutyrate modification of sorting signal enables pore formation by Escherichia coli OmpA

The outer membrane protein A (OmpA) of Escherichia coli is a well-known model for protein targeting and protein folding. Wild-type OmpA, isolated either from cytoplasmic inclusion bodies or from outer membranes, forms narrow pores of approximately 80 pS in planar lipid bilayers at room temperature. The pores are well structured with narrow conductance range when OmpA is isolated using lithium dodecyl sulfate (LDS) or RapiGest surfactant but display irregular conductance when OmpA is isolated with urea or guanidine hydrochloride. Previous studies have shown that serine residues S163 and S167 of the sorting signal of OmpA (residues 163-169), i.e., the essential sequence for outer membrane incorporation, are covalently modified by oligomers of (R)-3-hydroxybutyrate (cOHB). Here we find that single-mutants S163 and S167 of OmpA, which still contain cOHB on one serine of the sorting signal, form narrow pores in planar lipid bilayers at room temperature with lower and more irregular conductance than wild-type OmpA, whereas double mutants S163:S167 and S163:V166 of OmpA, with no cOHB on the sorting signal, are unable to form stable pores in planar lipid bilayers. Our results indicate that modification of serines in the sorting signal of OmpA by cOHB in the cytoplasm enables OmpA to incorporate into lipid bilayers at room temperature as a narrow pore. They further suggest that cOHB modification may be an important factor in protein targeting and protein folding.