Detergent-induced folding of the outer-membrane protein PhoE, a pore protein induced by phosphate limitation

The folding of the in vitro synthesized outer-membrane protein PhoE, a protein induced by phosphate limitation, has previously been studied using immunoprecipitation experiments with monoclonal antibodies that recognize conformational epitopes [De Cock, H., Hendricks, R., de Vrije, T. & Tommassen, J. (1990) J. Biol. Chem. 265, 4646-4651]. A folded monomer of the protein was detected in this way, while the addition of outer membranes was required to induce trimerization. In this study, we demonstrate that the folding of the in vitro synthesized PhoE protein did not occur spontaneously, but was dependent on the detergent that was present in the immunoprecipitation buffer. A remarkable specificity of phenyl-containing detergents on the efficient in vitro folding of PhoE molecules was observed. Furthermore, trimerization was detected in the absence of outer membranes when such detergents were present. However, the rate of trimerization was increased by the addition of crude cell envelopes containing outer membranes. The outer membranes probably enhanced trimerization by concentrating the folded PhoE molecules.