Structure of the lac carrier protein of Escherichia coli

Circular dichroic measurements on the lac carrier protein purified from the cytoplasmic membrane of Escherichia coli indicate that 85 +/- 5% of the amino acid residues comprising this integral membrane protein are arranged in helical secondary structures. Analysis of the sequential hydropathic character of this protein by the method of Kyte and Doolittle (J. Mol. Biol. (1982) 157, 105-132) indicates that the protein is composed of at least 12 hydrophobic segments with a mean length of 24 +/- 4 residues/segment. Approximately 70% of the 417 amino acids in the lac carrier are found in these domains. The hydropathic profile, together with the circular dichroic measurements, suggest that the 12 hydrophobic segments are largely in a helical conformation. If the segments are assumed to be alpha-helical, the mean length of each domain approximates the thickness of the most hydrophobic portion of the lipid bilayer. Based on these considerations, it is proposed that the lac carrier protein consists of at least 12 alpha-helical segments that traverse the membrane in a perpendicular sense, i.e. in a fashion similar to bacteriorhodopsin.