Monitoring the Diffusion of Single Heterotrimeric G Proteins in Supported Cell-membrane Sheets Reveals their Partitioning into Microdomains

Supported cell-membrane sheets are promising in vitro systems to investigate the properties of membranes with native protein/lipid composition, in particular their sub-compartmentalization and the differential localization of proteins associated to them. While such studies are usually performed using static microscopy techniques, we demonstrate here the potential offered by dynamic diffusion measurements. Whereas the overall fluidity of the lipid bilayer was preserved, the preparation of the membrane sheets led to the selective immobilization of extracellular and transmembrane (TM) glycosylated proteins and the anchored proteins/lipids associated with them. Taking advantage of this, we investigated the association of the G protein Gq with TM proteins, in particular G-protein coupled receptors (GPCRs), by monitoring the changes in diffusion occurring after preparation of the supported membranes. Two fluorescently tagged Galphaq proteins were constructed, which remained either mostly monomeric in the plasma membrane or associated with Gbetagamma in heterotrimers. While both constructs diffused similarly in living cells, the preparation of the supported membranes led to the selective immobilization of the heterotrimers with minimal changes of the diffusion of the monomeric Galphaq. The diverse mobility of monomeric and heterotrimeric Galphaq was a result of their different lipid anchors as demonstrated by monitoring the diffusion of the corresponding anchors alone. We propose that the immobilization of the heterotrimer was caused by its partitioning inside membrane microdomains surrounding GPCRs.