The inner leaflet of a typical mammalian plasma membrane contains 20-30% univalent PS (phosphatidylserine) and 1% multivalent PtdIns(4,5)P2. Numerous proteins have clusters of basic (or basic/hydrophobic) residues that bind to these acidic lipids. The intracellular effector CaM (calmodulin) can reverse this binding on a wide variety of proteins, including MARCKS (myristoylated alanine-rich C kinase substrate), GAP43 (growth-associated protein 43, also known as neuromodulin), gravin, GRK5 (G-protein-coupled receptor kinase 5), the NMDA (N-methyl-d-aspartate) receptor and the ErbB family. We used the first principles of physics, incorporating atomic models and the Poisson-Boltzmann equation, to describe how the basic effector domain of MARCKS binds electrostatically to acidic lipids on the plasma membrane. The theoretical calculations show the basic cluster produces a local positive electrostatic potential that should laterally sequester PtdInsP2, even when univalent acidic lipids are present at a physiologically relevant 100-fold excess; four independent experimental measurements confirm this prediction. Ca2+/CaM binds with high affinity (Kd approximately 10nM) to this domain and releases the PtdIns(4,5)P2. MARCKS, a major PKC (protein kinase C) substrate, is present at concentrations comparable with those of PtdIns(4,5)P2 (approx. 10 μM) in many cell types. Thus MARCKS can act as a reversible PtdIns(4,5)P2 buffer, binding PtdIns(4,5)P2 in a quiescent cell, and releasing it locally when the intracellular Ca2+ concentration increases. This reversible sequestration is important because PtdIns(4,5)P2 plays many roles in cell biology. Less is known about the role of CaM-mediated reversible membrane binding of basic/hydrophobic clusters for the other proteins.

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