Increased Phospholipid Flux Bypasses Overlapping Essential Requirements for the Yeast Sac1p Phosphoinositide

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Updated: Jul 21, 2025

Fluorescence-Based Measurements of Phosphatidylserine/Phosphatidylinositol 4-Phosphate Exchange Between Membranes
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Fluorescence-Based Measurements of Phosphatidylserine/Phosphatidylinositol 4-Phosphate Exchange Between Membranes

Published on: March 14, 2021

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Aleksa Nenadic1, Mohammad F Zaman1, Jesper Johansen1

  • 1Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada.

The Journal of Biological Chemistry
|July 28, 2023
PubMed

Insights

Sac1p and endoplasmic reticulum-plasma membrane tethering proteins coordinate lipid distribution at membrane contact sites. Disrupting this process impacts phospholipid biosynthesis and triggers cellular stress responses.

Area of Science:

  • Cell Biology
  • Membrane Biology
  • Biochemistry

Background:

  • The plasma membrane (PM) in budding yeast is closely associated with the endoplasmic reticulum (ER) at membrane contact sites (MCSs).
  • This association is maintained by seven ER membrane proteins, known as ER-PM tethers, some of which interact with the phosphoinositide phosphatase Sac1p.
  • The functional significance of these interactions for tethering and cellular processes remains largely unexplored.

Purpose of the Study:

  • To investigate the functional relationship between Sac1p and ER-PM tethering proteins.
  • To elucidate the role of Sac1p in regulating phospholipid biosynthesis and phosphoinositide distribution at ER-PM MCSs.
  • To identify mechanisms that compensate for the loss of Sac1p function in the context of ER-PM tethering.

Main Methods:

  • Genetic analysis of Sac1p and ER-PM tether mutants in budding yeast (Saccharomyces cerevisiae).
  • Transcriptomic profiling to assess cellular responses to genetic perturbations.
  • Lipidomic analysis and determination of intracellular phospholipid distributions.
  • Identification and characterization of high-copy suppressors for genetic lethality.

Main Results:

  • Sac1p inactivation exacerbates the increase in ER-PM MCSs in cells lacking specific synaptojanin-like genes (INP52, INP53).
  • Loss of Sac1p function in cells lacking all ER-PM tethers leads to disrupted phospholipid biosynthesis, altered phosphoinositide distribution, and lethality.
  • SAC1 deletion induces an ER membrane stress response and affects autophagy gene expression, dependent on phosphoinositides.
  • Overexpression of phospholipid biosynthesis genes or the phosphatidylserine/phosphatidylinositol-4-phosphate transfer protein Osh6 can suppress the lethality associated with Sac1p loss.

Conclusions:

  • Sac1p and ER-PM tethers share overlapping functions, crucial for maintaining cellular homeostasis.
  • Sac1p and ER phospholipid flux are critical for regulating lipid distribution at ER-PM MCSs.
  • Osh6p-dependent counter-exchange of phosphatidylserine and phosphatidylinositol-4-phosphate at ER-PM MCSs is a key process regulated by Sac1p and ER phospholipid flux.
Keywords:
ER-PM membrane contact sitesSaccharomyces cerevisiaeautophagyendoplasmic reticulum (ER)phosphatidylinositol phosphate phosphatasephosphatidylserinephosphoinositidesphospholipid metabolismplasma membrane (PM)

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