MembraneTopology andMutational Analysis oftheTolQ Protein ofEscherichia coli Required fortheUptakeof Macromolecules andCell Envelope Integrity

regions. Theamino-terminal 19residues as well as a smallloopinthe155to170residue region appear exposed intheperiplasm, whilethecarboxy terminus anda large loopafter thefirst transmembrane region arecytoplasmic. Amino-terminal sequence analysis ofTolQpurified fromthemembrane revealed thepresenceoftheinitiating formyl methionine group, suggesting a rapid translocation oftheamino-terminal region acrossthecytoplasmic membrane. Analysis of various tolQmutantstrains suggests thatthethird transmembrane region as well as partsofthelarge cytoplasmic loop arenecessary foractivity.