P014
Osh4p is a carrier designed to use PI(4)P metabolism for
transferring sterol against its gradient in cells
Joachim Moser von Filseck, Stefano Vanni,
Bruno Mesmin, Bruno Antonny and Guilllaume Drin
Institute of Pharmacology and Molecular Cell - CNRS,
Valbonne, France
In eukaryotic cells, sterols are transported to late membranes
at the expense of the ER, where sterol is synthesized, along
non-vesicular routes whose nature remains elusive. Recently,
we revealed that Osh4p, a member of the oxysterol-binding
protein (Osh/ORP) family can exchange sterol for PI(4)P between
membranes. Therefore, we posit that Osh4p ensures an anterograde transport of sterol at the ER/Golgi interface by exploiting
the PI(4)P gradient established by two spatially distant enzymes:
the Golgi PI4-kinase Pik1p and the ER-resident PI(4)P phosphatase Sac1p. This might explain how a carrier can create an
intracellular sterol gradient. To further support this hypothesis, we
measured with an unprecedented accuracy, using novel real-time
assays, the sterol/PI(4)P exchange activity of Osh4p between
ER- and Golgi-mimicking membranes.
Osh4p quickly exchanges sterol for PI(4)P between membranes
until full equilibration of PI(4)P. The hydrolysis of PI(4)P by Sac1p
on ER-like liposomes maintains the PI(4)P gradient, sustaining
the sterol transfer by Osh4p. Strikingly, Osh4p can efficiently
release sterol into Golgi-like membranes even against a preexisting sterol gradient. Sterol sequestration in these membranes
by saturated lipids and sphingolipids amplifies this activity. The
equal affinity of Osh4p for sterol and PI(4)P and its ability to specifically control the release of each lipid are key for its activity.
Thus, we provide the first demonstration that Osh4p is likely to
be a genuine sterol carrier able to transport sterol against its concentration gradient under the control of PI(4)P to maintain sterol
homeostasis in cells.