Modulating Motion in the Membrane

Science Signaling  16 Feb 2010:
Vol. 3, Issue 109, pp. ec51
DOI: 10.1126/scisignal.3109ec51

Cells use a limited number of signaling components to control an astounding array of biological processes. This is accomplished, at least in part, through subtle variations in signaling mechanisms that allow specific signals to be conveyed. Gutman et al. provide insight into the molecular details of how such specificity might be accomplished for the enzyme phospholipase C–β (PLCβ). PLCβ is the enzyme that produces the key second messenger molecules inositol 1,4,5-trisphosphate and diacylglycerol by cleaving phosphatidylinositol 4,5-bisphosphate molecules at the plasma membrane. The PLCβ2 isoform can be activated by alpha subunits (αq) of the heterotrimeric guanine nucleotide–binding protein (G protein) Gq, by G protein βγ subunits, or by the small guanosine triphosphatase Rac. These activators use some overlapping and some distinct mechanisms to target PLCβ2 to the plasma membrane and activate signaling. Gutman et al. used a refinement of the fluorescence recovery after photobleaching (FRAP) method called beam-size analysis that allowed them to track the movement of PLCβ2 molecules activated by each mechanism. The method is useful for such a situation where recovery of fluorescence of tagged PLCβ2 molecules in an area that has been photobleached can occur not only by diffusion of PLCβ2 or the molecules with which it associates within the membrane but also by exchange of unbleached PLCβ2 molecules from the cytoplasm. Expression of a constitutively active form of Rac2 in cultured COS-7 cells (derived from monkey kidney) caused membrane interaction dynamics of PLCβ2, indicating that it diffused slowly and that FRAP was dominated by exchange. Thus, Rac2 appears to enhance interaction of PLCβ2 with slowly diffusing molecules in the membrane, such as transmembrane proteins or protein-lipid clusters. Stimulation of PLCβ2 through αq resulted in less-pronounced PLCβ2 movement to the membrane and movement within the membrane similar to that of lipids, indicating that it may be associated with lipids (perhaps through its PH or C2 domains) or with lipid-associated proteins. Both PLCβ2 diffusion and exchange appeared to contribute to recovery of fluorescence. Activation of PLCβ2 by βγ subunits caused a transient association with the membrane, and FRAP occurred primarily by rapid lateral diffusion in the membrane. The authors propose that such interactions would allow rapid “surfing-like” diffusion of βγ-activated PLCβ2 that would allow it to interact with substrate molecules dispersed throughout large areas of the plasma membrane. Activation by Rac2, on the other hand, seems to create spatially restricted signaling by PLCβ2, with activation by αq falling somewhere in between. These properties may allow PLCβ2 signaling to control distinct biological processes.

O. Gutman, C. Walliser, T. Piechulek, P. Gierschik, Y. I. Henis, Differential regulation of phospholipase C-β2 activity and membrane interaction by Gαq, Gβ1γ2, and Rac2. J. Biol. Chem. 285, 3905–3915 (2010). [Abstract] [Full Text]