Pore Characteristics of Nontypeable Haemophilus influenzae Outer Membrane Protein P5 in Planar Lipid Bilayers

The structure of outer membrane protein P5 of NTHi, a homolog of Escherichia coli OmpA, was investigated by observing its pore characteristics in planar lipid bilayers. Recombinant NTHi P5 was overexpressed in E. coli and purified using ionic detergent, LDS-P5, or nonionic detergent, OG-P5. LDS-P5 and OG-P5 could not be distinguished by their migration on SDS-PAGE gels; however, when incorporated into planar bilayers of DPhPC between symmetric aqueous solutions of 1 M KCl at 22°C, LDS-P5 formed narrow pores (58 ± 6 pS) with low open probability, whereas OG-P5 formed large pores (1.1 ± 0.1 nS) with high open probability (0.99). LDS-P5 narrow pores were gradually and irreversibly transformed into large pores, indistinguishable from those formed by OG-P5, at temperatures ≥40°C; the process took 4–6 h at 40°C or 35–45 min at 42°C. Large pores were stable to changes in temperatures; however, large pores were rapidly converted to narrow pores when exposed to LDS at room temperatures, indicating acute sensitivity of this conformer to ionic detergent. These studies suggest that narrow pores are partially denatured forms and support the premise that the native conformation of NTHi P5 is that of a large monomeric pore.