Reversible - through calmodulin - electrostatic interactions between basic residues on proteins and acidic lipids in...

Area of Science:

• Cell biology
• Biophysics
• Molecular dynamics

Background:

• The plasma membrane's inner leaflet contains acidic lipids like phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)).
• Proteins with basic residue clusters bind to these acidic lipids, a process influenced by intracellular effectors.
• Calmodulin (CaM) is known to reverse this binding for various proteins, including MARCKS, GAP43, and the NMDA receptor.

Purpose of the Study:

• To elucidate the electrostatic mechanism of myristoylated alanine-rich C kinase substrate (MARCKS) binding to acidic lipids on the plasma membrane.
• To investigate the role of Ca2+/CaM in releasing PtdIns(4,5)P(2) from MARCKS.
• To establish MARCKS as a potential reversible buffer for PtdIns(4,5)P(2) in cellular signaling.

Main Methods:

• Utilized first principles of physics, atomic models, and the Poisson-Boltzmann equation for theoretical calculations.
• Employed four independent experimental measurements to validate theoretical predictions.
• Measured high-affinity binding of Ca2+/CaM to the MARCKS effector domain.

Main Results:

• Theoretical calculations demonstrated that the basic cluster of MARCKS electrostatically sequesters PtdIns(4,5)P(2), even in the presence of excess univalent lipids.
• Experimental data confirmed the predicted lateral sequestration of PtdIns(4,5)P(2).
• Ca2+/CaM binds with high affinity (Kd ≈ 10 nM) and releases PtdIns(4,5)P(2) from MARCKS.

Conclusions:

• MARCKS functions as a reversible PtdIns(4,5)P(2) buffer, binding the lipid in quiescent cells and releasing it upon increased intracellular Ca2+.
• This reversible sequestration is critical for regulating PtdIns(4,5)P(2)'s diverse roles in cell biology.
• The mechanism of CaM-mediated reversible membrane binding for other basic/hydrophobic cluster proteins requires further investigation.