Characterization of Escherichia coli expressing an Lpp'OmpA(46-159)-PhoA fusion protein localized in the outer membrane

The Lpp'OmpA(46-159) hybrid protein can serve as an efficient targeting vehicle for localizing a variety of procaryotic and eucaryotic soluble proteins onto the E. coli surface, thus providing a system for several possible biotechnology applications. Here we show that fusions between Lpp'OmpA(46-159) and bacterial alkaline phosphatase (PhoA), a normally periplasmic dimeric enzyme, are also targeted to the outer membrane. However, protease accessibility experiments and immunoelectron microscopy revealed that, unlike other periplasmic proteins, the PhoA domain of these fusions is not exposed on the cell surface in cells having an intact outer membrane. Conditions that affect the formation of disulfide bonds and the folding of the PhoA domain in the periplasm not only did not facilitate targeting to the cell surface but led to lethality when the fusion was expressed from a high-copy-number plasmid. Furthermore, E. coli expressing the Lpp'OmpA(46-159)-PhoA fusion exhibited strain- and temperature-dependent alterations in outer-membrane permeability. Our results are consistent with previous studies with other vehicles indicating that PhoA is not displayed on the surface when fused to cell-surface expression vectors. Presumably, the enzyme rapidly assumes a tightly folded dimeric conformation that cannot be transported across the outer membrane. The large size and quaternary structure of PhoA may define a limitation of the Lpp'OmpA(46-159) fusion system for the display of periplasmic proteins on the cell surface. alkaline phosphatase is a unique protein among a group of five periplasmic proteins (beta-lactamase, alkaline phosphatase, Cex cellulase, Cex cellulose-binding domain, and a single-chain Fv antibody fragment), which have been tested as passengers for the Lpp'OmpA(46-159) expression system to date, since it was the only protein not displayed on the surface.