Regulation of the lateral association of phospholipase Cbeta2 and G protein subunits by lipid rafts

One function of membrane domains of liquid-ordered lipids or "rafts" may be to stabilize complexes of signaling proteins, thereby playing a role in the transduction of cellular signals. Here, we have used fluorescence methods to directly test this idea by assessing the ability of phospholipase Cbeta2 (PLCbeta2) to associate with G protein subunits on model membranes in the fluid phase and on membranes that contain domains of lipids in the liquid-ordered phase (rafts). We find that the apparent dissociation constant for the equilibrium between PLCbeta2 and Galpha(q)(GTPgammaS) was identical on both types of membrane surfaces. However, the degree of association between PLCbeta2 and Gbetagamma subunits was significantly reduced on the surfaces containing rafts. Time studies indicate that this phenomenon is a dynamic process. Incorporating the lipid substrate of PLCbeta2 into membranes that forms rafts, we find that its basal activity is unaffected. However, its activation by Gbetagamma subunits is inhibited, supporting a reduced degree of interaction between these two proteins when rafts are present. Since lipid rafts affected PLCbeta2-Gbetagamma association and not PLCbeta2-Galpha(q)(GTPgammaS) association, we explored the possibility that the membrane interaction of Gbetagamma differed when rafts are present. We find that although the membrane partition coefficient of Gbetagamma is not significantly changed in the presence of rafts, proteolysis of Gbetagamma by trypsin increases and the ability of Gbetagamma Tyr/Trp fluorescence to be quenched by iodide ions decreases when rafts are present. These results suggest a model in which lipid rafts occlude the PLCbeta2 interaction site on Gbetagamma subunits by localizing these subunits at the domain interface.