Reversible binding and rapid diffusion of proteins in complex with inositol lipids serves to coordinate free movement with spatial information

Polyphosphoinositol lipids convey spatial information partly by their interactions with cellular proteins within defined domains. However, these interactions are prevented when the lipids' head groups are masked by the recruitment of cytosolic effector proteins, whereas these effectors must also have sufficient mobility to maximize functional interactions. To investigate quantitatively how these conflicting functional needs are optimized, we used different fluorescence recovery after photobleaching techniques to investigate inositol lipid-effector protein kinetics in terms of the real-time dissociation from, and diffusion within, the plasma membrane. We find that the protein-lipid complexes retain a relatively rapid (~0.1-1 μm²/s) diffusion coefficient in the membrane, likely dominated by protein-protein interactions, but the limited time scale (seconds) of these complexes, dictated principally by lipid-protein interactions, limits their range of action to a few microns. Moreover, our data reveal that GAP1IP⁴BP, a protein that binds PtdIns(4,5)P₂ and PtdIns(3,4,5)P₃ in vitro with similar affinity, is able to "read" PtdIns(3,4,5)P₃ signals in terms of an elongated residence time at the membrane.