Packing Effects of N-Ras Binding to a DOPC Membrane – a Neutron Reflectivity and TIRF Spectroscopy High-Pressure Study

Abstract

Abstract We have analyzed the structural changes of a 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) membrane upon insertion of the lipidated N-Ras protein as a function of pressure. N-Ras is a small GTPase that plays an important role for signal transduction and cell growth. After lipidation with hexadecyl (HD) and farnesyl (Far) residues, N-Ras HD-Far can insert into lipid membranes. In this study, we describe the concomitant changes of the DOPC membrane structure using neutron reflectometry (NR) and total internal reflection fluorescence (TIRF) spectroscopy. The data have been recorded as a function of pressure up to 2000 bar, because pressure can be used to explore low-volume states and high packing densities. The analysis of the NR data reveals that the DOPC membrane adopts a more ordered state with sharpened polar-apolar interfaces upon pressurization. After binding of N-Ras HD-Far via insertion of the two lipid anchors, this effect becomes even more pronounced. Complementary TIRF experiments corroborate these findings. Whereas pressure increases the generalized polarization (GP) value of the DOPC membrane indicating a pressure-induced dense packing of the lipid chains, the incorporation of the N-Ras HD-Far into the DOPC membrane is also found to enhance the GP value. Moreover, application of high pressure does not decrease the degree of N-Ras HD-Far binding to the DOPC membrane confirming the bound state to be a low-volume state. The results of this study shed some light on the effect of pressure on membrane-associated Ras-controlled signaling events in extreme environments, such as the deep sea, where pressures up to 1 kbar are reached.