The Active Protein-conducting Channel of Escherichia coli Contains an Apolar Patch

Protein translocation across the cytoplasmic membrane of Escherichia coli is mediated by translocase, a complex of a protein-conducting channel, SecYEG, and a peripheral motor domain, SecA. SecYEG has been proposed to constitute an aqueous path for proteins to pass the membrane in an unfolded state. To probe the solvation state of the active channel, the polarity sensitive fluorophore N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole was introduced at specific positions in the C-terminal region of the secretory protein proOmpA. Fluorescence measurements with defined proOmpA-DHFR translocation intermediates indicate mostly a water-exposed environment with a hydrophobic region in the center of the channel.