Exploring the Interplay of Lipids and Membrane proteins

Abstract: The interplay between lipids and membrane proteins is known to affectmembrane protein topology and thus have significant effect (control) ontheir functions. In this PhD thesis, the influence of lipids on the membraneprotein function was studied using three different membrane protein models.A monotopic membrane protein, monoglucosyldiacylglyecerol synthase(MGS) from Acholeplasma laidlawii is known to induce intracellular vesicleswhen expressed in Escherichia coli. The mechanism leading to this unusualphenomenon was investigated by various biochemical and biophysicaltechniques. The results indicated a doubling of lipid synthesis in the cell,which was triggered by the selective binding of MGS to anionic lipids. Multivariatedata analysis revealed a good correlation with MGS production.Furthermore, preferential anionic lipid sequestering by MGS was shown toinduce a different fatty acid modeling of E. coli membranes. The roles ofspecific lipid binding and the probable mechanism leading to intracellularvesicle formation were also investigated.As a second model, a MGS homolog from Synechocystis sp. PCC6803 wasselected. MgdA is an integral membrane protein with multiple transmembranehelices and a unique membrane topology. The influence of differenttype of lipids on MgdA activity was tested with different membrane fractionsof Synechocystis. Results indicated a very distinct profile compared toAcholeplasma laidlawii MGS. SQDG, an anionic lipid was found to be thespecies of the membrane that increased the MgdA activity 7-fold whereastwo other lipids (PG and PE) had only minor effects on MgdA. Additionally,a working model of MgdA for the biosynthesis and flow of sugar lipids betweenSynechocystis membranes was proposed.The last model system was another integral membrane protein with a distinctstructure but also a different function. The envelope stress sensor, CpxA andits interaction with E. coli membranes were studied. CpxA autophosphorylationactivity was found to be positively regulated by phosphatidylethanolamineand negatively by anionic lipids. In contrast, phosphorylation of CpxRby CpxA revealed to be increased with PG but inhibited by CL. Non-bilayerlipids had a negative impact on CpxA phosphotransfer activity.Taken together, these studies provide a better understanding of the significanceof the interplay of lipids and model membrane proteins discussed here.