The structure and flexibility analysis of the Arabidopsis synaptotagmin 1 reveal the basis of its regulation at membrane contact sites.

Juan L Benavente,Miguel A Botella,Dritan Siliqi,Armando Albert,Federico Gago,Alberto Mills,Lourdes Infantes,María J Sánchez-Barrena,Laura Lagartera,Noemí Ruiz-López

doi:10.26508/lsa.202101152

Abstract

Non-vesicular lipid transfer at ER and plasma membrane (PM) contact sites (CS) is crucial for the maintenance of membrane lipid homeostasis. Extended synaptotagmins (E-Syts) play a central role in this process as they act as molecular tethers of ER and PM and as lipid transfer proteins between these organelles. E-Syts are proteins constitutively anchored to the ER through an N-terminal hydrophobic segment and bind the PM via a variable number of C-terminal C2 domains. Synaptotagmins (SYTs) are the plant orthologous of E-Syts and regulate the ER-PM communication in response to abiotic stress. Combining different structural and biochemical techniques, we demonstrate that the binding of SYT1 to lipids occurs through a Ca2+-dependent lipid-binding site and by a site for phosphorylated forms of phosphatidylinositol, thus integrating two different molecular signals in response to stress. In addition, we show that SYT1 displays three highly flexible hinge points that provide conformational freedom to facilitate lipid extraction, protein loading, and subsequent transfer between PM and ER.

Highlights

In plants, abiotic stress induces changes in lipid homeostasis that lead to plasma membrane (PM) instability; plants tend to readjust lipid composition by the action of enzymes involved in lipid modification or by promoting lipid transfer between the membranes of different organelles (Hou et al, 2016; Zhu, 2016)
Main differences are found in the loop connecting β6 and β7, which is folded as an α-helix in E-Syt2C2A and it is not present in SYT1C2A; in the loop connecting β1 and β2, which is unusually long in E-Syt1C2A (Xu et al, 2014); and in a long insertion of 12 amino acids connecting β7 and β8, which is characteristic of Arabidopsis SYT1, SYT2, and SYT3 (Fig 1C)
The core structure of SYT1C2A is found in E-Syt2C2B and E-Syt2C2C domains; whereas SYT1C2A, E-Syt2C2A, and E-Syt2C2B display the topology of those C2 domains with the N and C termini opposite to the Ca2+-binding site, E-Syt2C2C has both ends at the same side of the Ca2+-binding site (Fig S1)

Summary

Introduction

Abiotic stress induces changes in lipid homeostasis that lead to plasma membrane (PM) instability; plants tend to readjust lipid composition by the action of enzymes involved in lipid modification or by promoting lipid transfer between the membranes of different organelles (Hou et al, 2016; Zhu, 2016). CS consist of narrow appositions of plasma and ER membranes, ranging from 10 to 30 nm, which rely on the regulated localization of different molecular tethers and lipid transfer proteins (Fernandez-Busnadiego et al, 2015; Krauβ & Haucke, 2016; Wong et al, 2018; Collado et al, 2019). The extended synaptotagmins (E-Syts) represent a family of conserved ER-localized proteins that display a dual function as molecular tethers between ER and PM and as lipid transfer proteins at CS (Saheki & De Camilli, 2017b; Wong et al, 2018). Plant synaptotagmins (SYTs) are orthologs of the mammalian E-Syts and trigger an increase in the size and connectivity of CS in response to salt (Lee et al, 2019) and rare earth elements (Lee et al, 2020) or different forms of mechanical stress (Perez-Sancho et al, 2015)

Methods

Results

Conclusion