Membrane specificity of the human cholesterol transfer protein STARD4

Abstract

STARD4 regulates cholesterol homeostasis by transferring cholesterol between plasma membrane and endoplasmic reticulum. The STARD4 structure features a helix-grip fold surrounding a large hydrophobic cavity holding the sterol. Its access is controlled by a gate formed by two flexible loops - {Omega}1 and {Omega}4- and the C-terminal -helix. Besides this, little is known about the mechanisms by which STARD4 binds to membranes and extract/releases cholesterol. All available structures of STARD4 are without a bound sterol and display the same closed conformation of the gate. The cholesterol transfer activity of the mouse STARD4 is enhanced in the presence of anionic lipids, and in particular of phosphatidylinositol biphosphates (PIP2) for which two binding sites were proposed on the mouse STARD4 surface. Yet only one of these sites is conserved in human STARD4. We here report the results of a liposome microarray-based assay and microseconds-long molecular dynamics simulations of apo-and holo forms of human STARD4 with complex lipid bilayers mimicking the composition of the donor and acceptor membranes. We show that the binding of apo form of human STARD4 is sensitive to the presence of PIP2 through two specific binding sites, one of which was not identified on mouse STARD4. We report two novel conformations of the gate in holo-STARD4: a yet-unobserved close conformation and an open conformation of {Omega}4 shedding light on the opening/closure mechanism needed for cholesterol uptake/release. Overall, the modulation of human STARD4 membrane-binding by lipid composition, and by the presence of the cargo supports the capacity of human STARD4 to achieve directed transfer between specific organelle membranes.