An electrostatic switching mechanism to control the lipid transfer activity of Osh6p

Nicolas-Frédéric Lipp,Alenka Čopič,Maud Magdeleine,Véronique Albanèse,Romain Gautier,Guillaume Drin,Maxime Renard

doi:10.1038/s41467-019-11780-y

Abstract

A central assumption is that lipid transfer proteins (LTPs) bind transiently to organelle membranes to distribute lipids in the eukaryotic cell. Osh6p and Osh7p are yeast LTPs that transfer phosphatidylserine (PS) from the endoplasmic reticulum (ER) to the plasma membrane (PM) via PS/phosphatidylinositol-4-phosphate (PI4P) exchange cycles. It is unknown how, at each cycle, they escape from the electrostatic attraction of the PM, highly anionic, to return to the ER. Using cellular and in vitro approaches, we show that Osh6p reduces its avidity for anionic membranes once it captures PS or PI4P, due to a molecular lid closing its lipid-binding pocket. Thus, Osh6p maintains its transport activity between ER- and PM-like membranes. Further investigations reveal that the lid governs the membrane docking and activity of Osh6p because it is anionic. Our study unveils how an LTP self-limits its residency time on membranes, via an electrostatic switching mechanism, to transfer lipids efficiently.

Highlights

A central assumption is that lipid transfer proteins (LTPs) bind transiently to organelle membranes to distribute lipids in the eukaryotic cell
When Osh6p is chromosomally tagged with GFP, it can be observed in patches at the cell cortex, confirming its enrichment at endoplasmic reticulum (ER)-plasma membrane (PM) contact sites[15,40]
Many models implicitly assume that LTPs bind transiently to organelles to quickly transfer lipids[30,32,33], yet almost nothing is known about these association/dissociation steps

Summary

Introduction

A central assumption is that lipid transfer proteins (LTPs) bind transiently to organelle membranes to distribute lipids in the eukaryotic cell. Osh6p and Osh7p are yeast LTPs that transfer phosphatidylserine (PS) from the endoplasmic reticulum (ER) to the plasma membrane (PM) via PS/phosphatidylinositol-4-phosphate (PI4P) exchange cycles. It is unknown how, at each cycle, they escape from the electrostatic attraction of the PM, highly anionic, to return to the ER. Due to the abundance of PS, the cytosolic side of the PM is highly negatively-charged compared to that of other organelles[9,22,23] This generates high electrostatic forces that are exploited by signaling proteins targeting this region: MARCKS24, Src[25], K-Ras[26] or Rac[127] bind to the PM via a stretch of positively-charged amino-acids and a lipidic tail; conventional PKC28 recognizes PS using a C2 domain.

Methods

Results

Conclusion