Lipid-Protein Interactions by Advanced EPR Methods

Abstract

Biological membranes contain proteins that are responsible for many vital biological functions, and understanding the structure-function relationships between the two are fundamentally important. In this work we have utilized Electron Paramagnetic Resonance (EPR) spectroscopy to investigate the chemistry of the phospholipid biding protein Sec14p, and the insertion profiles of synthetic peptides corresponding to the E2 transmembrane domain of selected mutants of the Sindbis Virus. Sec14p is associated with the secretory pathway in the Gogli apparatus, and is thought to be involved in regulating membrane composition. It has been shown to bind phosphatidylcholine (PC) and phosphatidylinositol (PI) in vitro. We demonstrate that Sec14p binds spin labeled homologous of the PC lipid, and the corresponding EPR spectra provide information about the mobility of the bound lipid and the polarity within the binding pocket. Accessibility measurements also ascertain the orientation of the PC molecule within the binding pocket. The results show that the PC molecule is highly restricted inside the biding pocket; polarity and procticity decrease with distance from the polar head region, with increase polarity and procticity at the distal end of the sn-2 acyl chain. EPR data indicate that the molecule adopts a headgroup-out orientation and the polarity profile provides a hydrophobic matching necessary for Sec14p to extract a lipid from a membrane in a energy-independent mechanism. Sinbis virus infects both insects and mammal, and replication requires the incorporation of the host cell’s membrane into its structure. The physical properties of these membranes differ, and the transmembrane domains of the virus’s structural proteins must be able to assemble into the same structure in both membrane types. Virus mutants having truncated transmembrane domains exhibit differential ability to reproduce, where certain mutants favor the production of new virus in mammalian cells, while other mutants favor insect cells. We have investigated this differential infectivity of the Sindbis virus mutants by examining the insertion profiles of synthetic peptides STM16 and STM18 in insect and mammalian membrane mimics. These peptides correspond to the transmembrane segments of Sindbis virus mutants TM-16 and TM-18. The results indicate that while both peptides assume transmembrane orientation in the insect membrane mimic, addition of cholesterol affects peptide-membrane interactions in a cholesterol-concentration dependant manner.