Mobility of proteins associated with the plasma membrane by interaction with inositol lipids

Translocation of a protein to the plasma membrane in response to the generation of polyphosphoinositol lipids is believed to be an important component of cellular regulation, in part because it increases the effective concentration of that protein relative to other proteins in the same membrane by restricting it to a two-dimensional space. However, such a concept assumes that, once translocated, a protein retains the free mobility it had in the cytoplasm, and also that the possible existence of partitioned pools of inositol lipids does not restrict its sphere of influence. We have explored by fluorescence recovery after photobleaching (FRAP) the mobility of four green-fluorescent-protein-tagged proteins, GAP1^IP4BP and GAP1^m, when they are either cytoplasmic or attached to the plasma membrane, and the PH domain of PI-PLC₁ and ICAM as representative of, respectively, another inositol-lipid-anchored protein and a single-transmembrane-span-domain protein. The data from GAP1^m and the PI-PLC₁ PH domain show that, when proteins associate with inositol lipids in the plasma membrane, they retain a mobility similar to that in the cytoplasm, and probably also similar to the inositol lipid to which they are attached, suggesting a free diffusion within the plane of the membrane. Moreover, this free diffusion is similar whether they are bound to PtdIns(3,4,5)P₃ or to PtdIns(4,5)P₂, and no evidence was found by these criteria for restricted pools of PtdIns(4,5)P₂. The mobility of GAP1^IP4BP, which has been reported to associate with PtdIns(4,5)P₂ in the plasma membrane, is much lower, suggesting that it might interact with other cellular components. Moreover, the mobility of GAP1^IP4BP is not detectably altered by the generation of either of its two potential regulators, Ins(1,3,4,5)P₄ or PtdIns(3,4,5)P₃.