In this study, we were concerned with the structural role of the surface-exposed extracellular loops of the N-terminal transmembrane (TM) domain of OmpA. A variant of the TM domain of outer membrane protein A (OmpA) with all four such loops shortened, which we call the β-barrel platform (BBP), was successfully refolded. This indicates that the removed parts of the surface-exposed loops indeed do not contain amino acid sequences critical for this membrane proteinŐs refolding in vitro. BBP has the potential to be used as a template β-barrel membrane protein structure for the development of novel functions, although our results also highlight the potential difficulties that can arise when functionality is being engineered into the loop regions of membrane proteins. We have used solution nuclear magnetic resonance spectroscopy to determine the global fold of BBP+EF, BBP with a metal ion-binding EF-hand inserted in one of the shortened loops. BBP and BBP+EF in dihexanoylphosphatidylcholine micelles are eight-stranded antiparallel β-barrels, and BBP represents the smallest β-structured integral membrane protein known to date.
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