Phosphatidylinositol 4,5-Bisphosphate Sensing Lipid Raft via Inter-Leaflet Coupling Regulated by Acyl Chain Length of Sphingomyelin

Abstract

Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important molecule located at the inner leaflet of cell membrane, where it serves as anchoring sites for a cohort of membrane-associated molecules and as a broad-reaching signaling intermediate. The lipid raft is thought as the major platform recruiting proteins for signal transduction and also known to mediate PIP2 accumulation across the membrane. While the significance of this cross-membrane coupling is increasingly appreciated, it remains unclear whether and how PIP2 senses the dynamic change of the ordered lipid domains over the packed hydrophobic core of the bilayer. Herein, by means of molecular dynamic simulation, we reveal that inner PIP2 molecules can sense the outer lipid domain via inter-leaflet coupling, and the coupling manner is dictated by the acyl chain length of sphingomyelin (SM) partitioned to the lipid domain. Shorter SM promotes membrane domain registration, whereby PIP2 accumulates beneath the domain across the membrane. In contrast, the anti-registration is thermodynamically preferred if the lipid domain has longer SM due to the hydrophobic mismatch between the corresponding acyl chains in SM and PIP2. In this case, PIP2 is expelled by the domain with a higher diffusivity. These results provide molecular insights into the regulatory mechanism of correlation between the outer lipid domain and inner PIP2, both of which are critical components for cell signal transduction.