Endoplasmic reticulum-plasma membrane contact sites integrate sterol and phospholipid regulation

Date

Sep 21, 2017

Time

11:00 AM - 12:00 PM

Speaker

Anant K. Menon

Affiliation

Weill Cornell Medical College, New York, USA

Series

MPI-CBG Thursday Seminar

Language

en

Main Topic

Biologie

Other Topics

Biologie

Host

André Nadler

Description

Tether proteins attach the endoplasmic reticulum (ER) to other cellular membranes thereby creating contact sites that are proposed to form platforms for regulating lipid homeostasis and facilitating non-vesicular lipid exchange. Cholesterol and its yeast counterpart ergosterol, are synthesized in the ER and transported by non-vesicular mechanisms to the plasma membrane (PM) where they represent almost half of all PM lipids and contribute to the barrier function of the PM. To determine whether contact sites are important for sterol exchange between the ER and PM, we generated ∆-super-tether (∆-s-tether) yeast cells that lack six previously identified tethering proteins (yeast extended synatotagmins (E-Syts), VAPs, and a TMEM16-anoctamin homolog) as well as the presumptive tether Ice2. We now show that ∆-s-tether cells lack ER-PM contacts, yet sustain robust ER-PM sterol exchange, indicating that the sterol transport machinery is either absent from or not uniquely located at contact sites. Unexpectedly, we found that the transport of exogenously supplied sterol to the ER occurs more slowly in ∆-s-tether cells than in wild-type cells. We pinpointed this defect to changes in sterol organization and transbilayer movement within the PM bilayer caused by lipid dysregulation, evinced by changes in the abundance and organization of PM lipids. Indeed, deletion of either OSH4, which encodes a sterol/phosphatidylinositol-4-phosphate (PI4P) exchange protein, or the PI4P phosphatase encoded by SAC1, caused synthetic lethality in ∆-s-tether cells due to disruptions in redundant pathways regulating both PI4P and phospholipid synthesis. The growth defect of ∆-s-tether cells was rescued with an artificial tether assembled from unrelated non-yeast proteins, indicating that endogenous tether proteins play non-specific bridging functions. Finally, we discovered that sterols play a role in regulating ER-PM contact site formation. In sterol-depleted cells, levels of the yeast E-Syt tether Tcb3 were induced and ER-PM contact increased dramatically. These results support a model in which ER-PM contact sites act as a nexus for co-ordinating the complex inter-relationship between sterols, sphingolipids, and phospholipids that maintain PM composition and integrity.

Last modified: Sep 22, 2017, 10:02:48 AM