Translocation of phospholipids is facilitated by a subset of membrane-spanning proteins of the bacterial cytoplasmic membrane.
The mechanism by which phospholipids are transported across biogenic membranes, such as the bacterial cytoplasmic membrane, is unknown. We hypothesized that this process is mediated by the presence of the membrane-spanning segments of inner membrane proteins, rather than by dedicated flippases. In support of the hypothesis, it was demonstrated that transmembrane alpha-helical peptides, mimicking the membrane-spanning segments, mediate flop of 2-6-(7-nitro-2,1,3-benzoxadiazol-4-yl) aminocaproyl (C6-NBD)-phospholipids (Kol, M. A., de Kroon, A. I., Rijkers, D. T., Killian, J. A., and de Kruijff, B. (2001) Biochemistry 40, 10500-10506). Here the dithionite reduction assay was used to measure transbilayer equilibration of C6-NBD-phospholipids in proteoliposomes, composed of Escherichia coli phospholipids and a subset of bacterial membrane proteins. It is shown that two well characterized integral proteins of the bacterial cytoplasmic membrane, leader peptidase and the potassium channel KcsA, induce phospholipid translocation, most likely by their transmembrane domains. In contrast, the ATP-binding cassette transporter from the E. coli inner membrane MsbA, a putative lipid flippase, did not mediate phospholipid translocation, irrespective of the presence of ATP. OmpT, an outer membrane protein from E. coli, did not facilitate flop either, demonstrating specificity of protein-mediated phospholipid translocation. The results are discussed in the light of phospholipid transport across the E. coli inner membrane.