Cell-To-Cell Variability in Plasma Membrane Lipid Rafts

Abstract

In recent years compelling evidence pointed to an important role of cholesterol enriched microdomains, so-called lipid rafts in miscellaneous cellular functions. Supposably, this lateral sub-compartmentalization facilitates selective protein-protein interactions by a local enrichment of the components involved. To study lipid rafts in the context of virus assembly, we produced several variants of two viral proteins, the Influenza virus transmembrane protein hemagglutinin and the Human Immunodeficiency Virus glycoprotein gp41. Fluorescence lifetime imaging microscopy was used to report Forster Resonance Energy Transfer (FRET) between a raft marker labelled with a cyan fluorescent protein and viral chimeras in living cells. Since it is highly distance dependent, occurring FRET indicates a co-clustering of both fluorescent protein species in membrane microdomains. Both viral proteins were found to be associated with plasma membrane lipid rafts and interestingly we observed a significant cell-to-cell variability in our samples exhibiting two distinct cell-populations with clearly differing raft related properties. To verify this intercellular lipid raft heterogeneity we used fluorescence polarization microscopy, displaying local enrichment of GPI-anchored proteins as a complementary approach. Furthermore, we utilized the environment-sensitive fluorescence lifetime of NBD-PC to further study membrane order on a single-cell level. We surmise, that the cell-to-cell variability detected is representative of a context-dependence of cellular plasma membrane organization. Cell synchronization and cell cycle analysis were conducted to elucidate a potential correlation of plasma membrane condition and the position of the cells in the cell cycle.Considering the high spatial and temporal dynamics of plasma membrane lipid rafts and the sophisticated methods that are necessary to study them it is conceivable, that the additional complexity of the membrane organization described here has not been detected in previous investigations. Nevertheless, it might be representative of cellular mechanisms that enables adoption to environmental demands or proliferative progression.