Abstract
Human ORP3 belongs to the oxysterol-binding protein (OSBP) family of lipid transfer proteins and is involved in lipid trafficking and cell signaling. ORP3 localizes to the ER-PM interfaces and is implicated in lipid transport and focal adhesion dynamics. Here, we report the 2.6-2.7 Å structures of the ORD (OSBP-related domain) of human ORP3 in apo-form and in complex with phosphatidylinositol 4-phosphate. The ORP3 ORD displays a helix grip β-barrel fold with a deep hydrophobic pocket which is conserved in the OSBP gene family. ORP3 binds PI(4)P by the residues around tunnel entrance and in the hydrophobic pocket, whereas it lacks sterol binding due to the narrow hydrophobic tunnel. The heterologous expression of the ORDs of human ORP3 or OSBP1 rescued the lethality of seven ORP (yeast OSH1-OSH7) knockout in yeast. In contrast, the PI(4)P-binding site mutant of ORP3 did not complement the OSH knockout cells. The N-terminal PH domain and FFAT motif of ORP3 are involved in protein targeting but are not essential in yeast complementation. This observation suggests that the essential function conserved in the ORPs of yeast and human is mediated by PI(4)P-binding of the ORD domain. This study suggests that the non-vesicular PI(4)P transport is a conserved function of all ORPs in eukaryotes.
Highlights
The lipid compositions of membrane bilayers in eukaryotes are unique for each intracellular organelles and the proper distribution of lipids is crucial for cell function [1]
Full length ORP3 resides in puncta at the PM, which seems the part of the cortical ER-PM contact sites
The crystal structures of the human ORP3 OSBP-related ligandbinding domain (ORD), together with biochemical and yeast complementation experiments provide a clear picture of the conserved PI(4)P-binding by ORDs and a shared function of the ORP family proteins
Summary
The lipid compositions of membrane bilayers in eukaryotes are unique for each intracellular organelles and the proper distribution of lipids is crucial for cell function [1]. Various phosphoinositide lipids are localized in the membranes of the subcellular organelles and serve as major regulators in membrane trafficking, and lipid signaling by re recruiting and/or activating effector proteins [2]. Non-vesicular lipid transport by soluble protein carriers plays a significant role in lipid distribution between the organelles that are not connected by membrane trafficking pathways [3]. Oxysterol-binding proteins (OSBP)-related proteins (ORPs) compose a large conserved family of lipid-binding proteins in eukaryotes and are implicated in many cellular processes including cell signaling, vesicular trafficking, lipid metabolism, and transport [4].
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