The formation and function of ER-endosome membrane contact sites

AbstractGradients of PtdIns4P between organelle membranes and the endoplasmic reticulum (ER) are thought to drive counter-transport of other lipids via non-vesicular traffic. This novel pathway requires the SAC1 phosphatase to degrade PtdIns4P in a “cis” configuration at the ER to maintain the gradient. However, SAC1 has also been proposed to act in “trans” at membrane contact sites, which could oppose lipid traffic. It is therefore crucial to determine which mode SAC1 uses in living cells. We report that acute inhibition of SAC1 causes accumulation of PtdIns4P in the ER, that SAC1 does not enrich at membrane contact sites, and that SAC1 has little activity in “trans”, unless a linker is added between its ER-anchored and catalytic domains. The data reveal an obligate “cis” activity of SAC1, supporting its role in non-vesicular lipid traffic and implicating lipid traffic more broadly in inositol lipid homeostasis and function.

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IP3 Receptor Properties and Function at Membrane Contact Sites

Advances in Experimental Medicine and Biology - Membrane Dynamics and Calcium Signaling ◽

10.1007/978-3-319-55858-5_7 ◽

2017 ◽

pp. 149-178 ◽

Cited By ~ 10

Author(s):

Gemma Roest ◽

Rita M. La Rovere ◽

Geert Bultynck ◽

Jan B. Parys

Keyword(s):

Ip3 Receptor ◽

Membrane Contact Sites ◽

Contact Sites ◽

Membrane Contact ◽

And Function

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GRAM domain proteins specialize functionally distinct ER-PM contact sites in human cells

eLife ◽

10.7554/elife.31019 ◽

2018 ◽

Vol 7 ◽

Cited By ~ 45

Author(s):

Marina Besprozvannaya ◽

Eamonn Dickson ◽

Hao Li ◽

Kenneth S Ginburg ◽

Donald M Bers ◽

...

Keyword(s):

Human Cells ◽

Lipid Homeostasis ◽

Dependent Manner ◽

Membrane Contact Sites ◽

Contact Sites ◽

Membrane Contact ◽

And Function ◽

Pleiotropic Functions ◽

Human Family

Endoplasmic reticulum (ER) membrane contact sites (MCSs) are crucial regulatory hubs in cells, playing roles in signaling, organelle dynamics, and ion and lipid homeostasis. Previous work demonstrated that the highly conserved yeast Ltc/Lam sterol transporters localize and function at ER MCSs. Our analysis of the human family members, GRAMD1a and GRAMD2a, demonstrates that they are ER-PM MCS proteins, which mark separate regions of the plasma membrane (PM) and perform distinct functions in vivo. GRAMD2a, but not GRAMD1a, co-localizes with the E-Syt2/3 tethers at ER-PM contacts in a PIP lipid-dependent manner and pre-marks the subset of PI(4,5)P2-enriched ER-PM MCSs utilized for STIM1 recruitment. Data from an analysis of cells lacking GRAMD2a suggest that it is an organizer of ER-PM MCSs with pleiotropic functions including calcium homeostasis. Thus, our data demonstrate the existence of multiple ER-PM domains in human cells that are functionally specialized by GRAM-domain containing proteins.

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Rod-shaped tricalbins contribute to PM asymmetry at curved ER-PM contact sites

10.1101/579128 ◽

2019 ◽

Cited By ~ 2

Author(s):

Patrick C. Hoffmann ◽

Tanmay A. M. Bharat ◽

Michael R. Wozny ◽

Elizabeth A. Miller ◽

Wanda Kukulski

Keyword(s):

Protein Composition ◽

Lipid Transfer ◽

Cellular Lipid ◽

Membrane Contact Sites ◽

Contact Sites ◽

Membrane Morphology ◽

Structural Framework ◽

Structural Functions ◽

Membrane Contact ◽

And Function

AbstractLipid flow between cellular organelles occurs via membrane contact sites that form dynamic conduits. Extended-synaptotagmins, known as tricalbins in yeast, mediate lipid transfer between the endoplasmic reticulum (ER) and plasma membrane (PM). How these proteins regulate the membrane architecture to transport lipids across the aqueous space between bilayers remains unknown. Using correlative microscopy, electron cryo-tomography and high-throughput genetics we address this interplay of architecture and function in budding yeast. We find that ER-PM contacts are diverse in protein composition and membrane morphology, not in intermembrane distance. In situ cryo-EM of tricalbins reveals their molecular organisation that suggests an unexpected structural framework for lipid transfer. Genetic analysis identifies functional redundancies, both for tricalbin domains and cellular lipid routes, and points to tricalbin function in maintenance of PM asymmetry. These results uncover a modularity of molecular and structural functions of tricalbins, and of their roles within the network of cellular lipid fluxes.

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