GRP1 pleckstrin homology domain: Activation parameters and novel search mechanism for rare target lipid

Pleckstrin homology (PH) domains play a central role in a wide array of signaling pathways by binding second messenger lipids of the phosphatidylinositol phosphate (PIP) lipid family. A given type of PIP lipid is formed in a specific cellular membrane where it is generally a minor component of the bulk lipid mixture. For example, the signaling lipid PI(3,4,5)P-3 (or PIP3) is generated primarily in the inner leaflet of the plasma membrane where it is believed to never exceed 0.02% of the bulk lipid. The present study focuses on the PH domain of the general receptor for phosphoinositides, isoform 1 (GRP1), which regulates the actin cytoskeleton in response to PIP3 signals at the plasma membrane surface. The study systematically analyzes both the equilibrium and kinetic features of GRP1-PH domain binding to its PIP lipid target on a bilayer surface. Equilibrium binding measurements utilizing protein-to-membrane fluorescence resonance energy transfer (FRET) to detect GRP1-PH domain docking to membrane-bound PIP lipids confirm specific binding to PIP3. A novel FRET competitive binding measurement developed to quantitate docking affinity yields a K-D of 50 +/- 10 nM for GRP1-PH domain binding to membrane-bound PIP3 in a physiological lipid mixture approximating the composition of the plasma membrane inner leaflet. This observed KD lies in a suitable range for regulation by physiological PIP3 signals. Interestingly, the affinity of the interaction decreases at least 12-fold when the background anionic lipids phosphatidylserine (PS) and phosphatidylinositol (PI) are removed from the lipid mixture. Stopped-flow kinetic studies using protein-to-membrane FRET to monitor association and dissociation time courses reveal that this affinity decrease arises from a corresponding decrease in the on-rate for GRP1-PH domain docking with little or no change in the off-rate for domain dissociation from membrane-bound PIP3. Overall, these findings indicate that the PH domain interacts not only with its target lipid, but also with other features of the membrane surface. The results are consistent with a previously undescribed type of two-step search mechanism for lipid binding domains in which weak, nonspecific electrostatic interactions between the PH domain and background anionic lipids facilitate searching of the membrane surface for PIP3 headgroups, thereby speeding the high-affinity, specific docking of the domain to its rare target lipid.