Membrane specificity of the human cholesterol transfer protein STARD4

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• STARD4 protein is crucial for regulating cholesterol homeostasis.
• It facilitates cholesterol transfer between cellular membranes.
• The precise mechanisms of membrane binding and cholesterol extraction by STARD4 remain largely unknown.

Purpose of the Study:

• To investigate the mechanisms of human STARD4 binding to membranes.
• To elucidate the role of lipids, particularly PIP2, in modulating STARD4 activity.
• To characterize novel conformations of STARD4 involved in cholesterol transfer.

Main Methods:

• Liposome microarray-based assays were employed to study STARD4-membrane interactions.
• Microseconds-long molecular dynamics simulations were performed using human STARD4 and complex lipid bilayers.
• Structural analysis focused on STARD4 conformations and lipid-binding sites.

Main Results:

• Human STARD4 binding to membranes is sensitive to phosphatidylinositol biphosphates (PIP2) via two distinct binding sites.
• One PIP2 binding site identified in human STARD4 was not previously observed in mouse STARD4.
• Two novel conformations of the STARD4 gate (closed and open Ω4) were revealed, providing insights into the cholesterol uptake and release mechanism.

Conclusions:

• The lipid composition of membranes, especially PIP2, significantly modulates human STARD4 binding and activity.
• The identified binding sites and novel conformations support STARD4's role in directed cholesterol transfer between specific organelle membranes.