Structural studies of the shortest extended synaptotagmin with only two C2 domains from Trypanosoma brucei

Emma Stepinac,Nicolas Landrein,Daria Skwarzyńska,Patrycja Wójcik,Johannes Lesigang,Iva Lučić,Cynthia Y He,Mélanie Bonhivers,Derrick R Robinson,Gang Dong

doi:10.1016/j.isci.2021.102422

Abstract

SummaryExtended synaptotagmins (E-Syts) localize at membrane contact sites between the endoplasmic reticulum (ER) and the plasma membrane to mediate inter-membrane lipid transfer and control plasma membrane lipid homeostasis. All known E-Syts contain an N-terminal transmembrane (TM) hairpin, a central synaptotagmin-like mitochondrial lipid-binding protein (SMP) domain, and three or five C2 domains at their C termini. Here we report an uncharacterized E-Syt from the protist parasite Trypanosoma brucei, namely, TbE-Syt. TbE-Syt contains only two C2 domains (C2A and C2B), making it the shortest E-Syt known by now. We determined a 1.5-Å-resolution crystal structure of TbE-Syt-C2B and revealed that it binds lipids via both Ca2+- and PI(4,5)P2-dependent means. In contrast, TbE-Syt-C2A lacks the Ca2+-binding site but may still interact with lipids via a basic surface patch. Our studies suggest a mechanism for how TbE-Syt tethers the ER membrane tightly to the plasma membrane to transfer lipids between the two organelles.

Highlights

Eukaryotic cells are compartmentalized into membrane-bound organelles, which are defined by their unique membrane lipid composition and specific functions
We report an uncharacterized Extended synaptotagmins (E-Syts) from the protist parasite Trypanosoma brucei, namely, T. brucei E-Syt (TbE-Syt)
Our studies suggest a mechanism for how TbE-Syt tethers the endoplasmic reticulum (ER) membrane tightly to the plasma membrane to transfer lipids between the two organelles

Summary

Introduction

Eukaryotic cells are compartmentalized into membrane-bound organelles, which are defined by their unique membrane lipid composition and specific functions. Extended synaptotagmins (ESyts) are a family of evolutionarily conserved proteins localizing at the MCS between the endoplasmic reticulum (ER) and the plasma membrane and mediate lipid transfer between them (Fernandez-Busnadiego et al, 2015; Giordano et al, 2013). E-Syts belong to the tubular lipid binding (TULIP) superfamily. Proteins from this group contain hydrophobic tunnels in their TULIP domains and often function in lipid traffic and signaling by carrying lipids through the aqueous phase of the cell (Beamer et al, 1997; Kopec et al, 2011; Qiu et al, 2007; Wong and Levine, 2017). E-Stys differ from Syts in both their localization and function

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