A Four-Step Cycle Driven by PI(4)P Hydrolysis Directs Sterol/PI(4)P Exchange by the ER-Golgi Tether OSBP

Abstract

Several proteins at endoplasmic reticulum (ER)-Golgi membrane contact sites contain a PH domain that interacts with the Golgi phosphoinositide PI(4)P, a FFAT motif that interacts with the ER protein VAP-A, and a lipid transfer domain. This architecture suggests the ability to both tether organelles and transport lipids between them. We show that in oxysterol binding protein (OSBP) these two activities are coupled by a four-step cycle. Membrane tethering by the PH domain and the FFAT motif enables sterol transfer by the lipid transfer domain (ORD), followed by back transfer of PI(4)P by the ORD. Finally, PI(4)P is hydrolyzed in cis by the ER protein Sac1. The energy provided by PI(4)P hydrolysis drives sterol transfer and allows negative feedback when PI(4)P becomes limiting. Other lipid transfer proteins are tethered by the same mechanism. Thus, OSBP-mediated back transfer of PI(4)P might coordinate the transfer of other lipid species at the ER-Golgi interface.

Phosphoregulation of the Ceramide Transport Protein CERT at Serine 315 in the Interaction with VAMP-associated Protein (VAP) for Inter-organelle Trafficking of Ceramide in Mammalian Cells

“…A novel model for the ER-to-Golgi transfer of oxysterol by oxysterol-binding protein (OSBP) has very recently been proposed (32). In this model, OSBP was shown to transfer oxysterol from the ER to the Golgi apparatus and, in turn, transfer PtdIns(4)P from the Golgi and ER at membrane contact sites by a tight coupling mechanism.…”

“…The overall module architecture of CERT is similar to that of OSBP with an N-terminal PtdIns(4)Pspecific PH domain, C-terminal lipid-transfer domain, and FFAT motif between them, although no similarities have been observed in the primary structures of the lipid-transfer domains. OSBP was shown to have the potential to connect the ER with the Golgi apparatus (32). Thus, various proteins having both a PtdIns(4)P-specific PH domain and FFAT motif may contribute to the formation of ER-Golgi membrane contact sites in cells, and the yet unidentified phosphorylation/dephosphorylation system responsible for the phosphorylation of CERT Ser-315 might play a role in regulatory formation of ERGolgi membrane contact sites.…”

Arfs at a Glance

“…In addition, they have a motif that mediates binding to the ER-localized protein VAP (Lev, 2010;Levine and Loewen, 2006). These two localization regions of OSBP are sufficient to mediate contact between the ER and Golgi, and moreover, OSBP activity regulates this association (Mesmin et al, 2013). FAPP2-mediated delivery of glucosylceramide to the trans-Golgi has been shown to be dependent on Arf1 and also to be involved in vesicular trafficking from the Golgi to the plasma membrane (D'Angelo et al, 2007;D'Angelo et al, 2013).…”

“…Arf proteins are positive activators of the insulin signaling pathway through their activation of PtdIns(4,5)P 2 (Fuss et al, 2006;Hafner et al, 2006;Lim et al, 2010). At the Golgi, Arf1 recruits and stimulates the activity of phosphatidylinositol 4-kinase, forming PtdIns4P, an important regulator of sterol trafficking in cells (Mesmin et al, 2013).…”

Golgi and plasma membrane pools of PI(4)P contribute to plasma membrane PI(4,5)P 2 and maintenance of KCNQ2/3 ion channel current

“…In addition, transient apposition between organelles can alter phosphoinositide levels by presenting membrane-bound phosphatases in trans. For example, the endoplasmic reticulum (ER) can make contacts with the Golgi, allowing 4-phosphatases of the ER to dephosphorylate Golgi phosphatidylinositol 4-phosphate [PI(4)P] (6)(7)(8).…”