The effects of SecA—lipid interactions on lipid mobility were studied by electron spin resonance (ESR) spectroscopy in bilayer systems containing phospholipids spin-labeled at different positions along the acyl chain. The SecA protein, which functions in protein translocation at the cytosolic side of the E. coli inner membrane, was found to decrease the mobility of the lipids upon its interaction with the membrane. The restriction of lipid motion, at all chain positions measured, reflects the ability of SecA to penetrate the membrane. At a 49:1 lipid/protein molar ratio, a second, motionally more restricted component is observed in ESR spectra of phospholipids spin-labeled close to the methyl ends of the chains (12th and 14th positions). Furthermore, SecA was found to eliminate the order-to-disorder phase transition of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol bilayers. A remarkably strong reduction in the ability of SecA to penetrate the membrane was found when the nucleotides ATP and ADP + Pi were present. The presence of the non-hydrolyzable analogue AMP-PNP had no effect. These results clearly demonstrate that SecA perturbs, in a nucleotide dependent manner, lipid mobility upon insertion into the bilayer. The implications of these findings for translocation of precursor proteins across the E. coli inner membrane are discussed.
