Recognition of Specific PIP2-subtype Composition Triggers the Allosteric Control Mechanism for Selective Membrane Targeting of Cargo Loading and Release Functions of the Intracellular Sterol Transporter StarD4

We present a comprehensive, quantitative model of the allosteric molecular mechanisms of selective cholesterol (CHL) uptake and delivery by the StarD4 protein - an intracellular cholesterol trafficking protein that facilitates the crucial non-vesicular sterol transport between the plasma membrane and the endoplasmic reticulum. This sterol-specific transfer protein is essential for maintaining the healthy life of human cells. In its physiological function, StarD4 targets both sterol donor and acceptor membranes via interactions with anionic lipids. Experiments have illuminated the kinetics of this sterol transfer and shown it to be modulated by specific phosphatidylinositol phosphates (PIPs) on the target membrane, but the molecular mechanism of the recognition of the PIP2 subtype by StarD4, and how this affects the direction and kinetics of cholesterol transport remained unclear. By revealing a heretofore unrecognized allosteric mechanism that connects the sterol binding site to the part of the protein embedded in the membrane, we show here how StarD4 can respond with different actions to diverse organelle membranes based on their PIP2-subtype composition, in agreement with physiological and experimental evidence. The trajectories of extensive (millisecond range) molecular dynamics (MD) simulation of the StarD4-membrane interactions we calculated, were analyzed with advanced machine learning and information theory methods. Our findings outline how the specific molecular mechanism for recognizing PIP2-subtypes in membranes by StarD4 couples to the defined allosteric pathway that induces the CHL binding pocket to propagate the signal for either uptake or release of the sterol. The central role determined for allostery in these significant advances in the understanding of intracellular cholesterol trafficking by StarD4, aligns with experimentally determined properties of StarD4 function, and interprets them in experimentally testable atomistic terms that explain function-altering results of mutations.