Quantitative immunoprecipitation of the lactose transporter from Escherichia coli confirms stoichiometry of substrate binding.

A procedure is presented for the immunoprecipitation of the E. coli lactose transporter which may be applicable to other membrane protein antigens. Antibodies to its chemically synthesized C-terminal decapeptide specifically precipitate 6.0 +/- 0.2% of the radioactivity from solubilized [14C]amino-acid-labeled total cell envelopes of the transporter-overproducing strain T206 corresponding to 1.0 +/- 0.1 nmol transporter/mg total membrane protein. Comparison with galactoside binding yields a stoichiometry of 1.1 +/- 0.2 mol galactoside bound/mol transporter.