Abstract
The interaction of phosphatidylserine (PS) synthase from Escherichia coli with lipid membranes was studied with a recently developed variant of the surface plasmon resonance technique, referred to as coupled plasmon-waveguide resonance spectroscopy. The features of the new technique are increased sensitivity and spectral resolution, and a unique ability to directly measure the structural anisotropy of lipid and proteolipid films. Solid-supported lipid bilayers with the following compositions were used: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC); POPC-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (POPA) (80:20, mol/mol); POPC-POPA (60:40, mol/mol); and POPC-1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) (75:25, mol/mol). Addition of either POPA or POPG to a POPC bilayer causes a considerable increase of both the bilayer thickness and its optical anisotropy. PS synthase exhibits a biphasic interaction with the bilayers. The first phase, occurring at low protein concentrations, involves both electrostatic and hydrophobic interactions, although it is dominated by the latter, and the enzyme causes a local decrease of the ordering of the lipid molecules. The second phase, occurring at high protein concentrations, is predominantly controlled by electrostatic interactions, and results in a cooperative binding of the enzyme to the membrane surface. Addition of the anionic lipids to a POPC bilayer causes a 5- to 15-fold decrease in the protein concentration at which the first binding phase occurs. The results reported herein lend experimental support to a previously suggested mechanism for the regulation of the polar head group composition in E. coli membranes.