Arabidopsis SYT1 maintains stability of cortical endoplasmic reticulum networks and VAP27-1-enriched endoplasmic reticulum-plasma membrane contact sites.

Wei Siao,Frantisek Baluska,Patrick J Hussey,Boris Voigt,Pengwei Wang

doi:10.1093/jxb/erw381

Wei Siao, Frantisek Baluska + Show 3 more

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https://doi.org/10.1093/jxb/erw381

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Abstract

Arabidopsis synaptotagmin 1 (SYT1) is localized on the endoplasmic reticulum-plasma membrane (ER-PM) contact sites in leaf and root cells. The ER-PM localization of Arabidopsis SYT1 resembles that of the extended synaptotagmins (E-SYTs) in animal cells. In mammals, E-SYTs have been shown to regulate calcium signaling, lipid transfer, and endocytosis. Arabidopsis SYT1 was reported to be essential for maintaining cell integrity and virus movement. This study provides detailed insight into the subcellular localization of SYT1 and VAP27-1, another ER-PM-tethering protein. SYT1 and VAP27-1 were shown to be localized on distinct ER-PM contact sites. The VAP27-1-enriched ER-PM contact sites (V-EPCSs) were always in contact with the SYT1-enriched ER-PM contact sites (S-EPCSs). The V-EPCSs still existed in the leaf epidermal cells of the SYT1 null mutant; however, they were less stable than those in the wild type. The polygonal networks of cortical ER disassembled and the mobility of VAP27-1 protein on the ER-PM contact sites increased in leaf cells of the SYT1 null mutant. These results suggest that SYT1 is responsible for stabilizing the ER network and V-EPCSs.

Highlights

In eukaryotic cells, proteins with multiple C2 domains are often found to participate in membrane trafficking or membrane-tethering processes, which can probably be Ca2+ regulated (Nalefski and Falke, 1996; Min et al, 2007)
The co-expression of cyan fluorescent protein (CFP)–HDEL followed by FM4-64 staining showed that synaptotagmin 1 (SYT1)–green fluorescent protein (GFP) was localized on the endoplasmic reticulum (ER) and attached to the plasma membrane (PM) at specific stationary regions, namely the endoplasmic reticulum–plasma membrane (ER–PM) contact sites (Fig. 1B, C)
The results showed that neither SYT1–GFP nor VAMP/synaptobrevin-associated protein 27 (VAP27)-1–GFP was co-localized with the brefeldin A (BFA) compartments (Supplementary Fig. S1C, D), indicating that these two proteins were not incorporated into the endosomes

Summary

Introduction

Proteins with multiple C2 domains are often found to participate in membrane trafficking or membrane-tethering processes, which can probably be Ca2+ regulated (Nalefski and Falke, 1996; Min et al, 2007). The protein structure of Arabidopsis SYT1 is similar to that of both synaptotagmins (SYTs) and extended synaptotagmins (E-SYTs) in metazoans (Craxton, 2010). At least 17 SYT isoforms have been found in mammals; most of them are expressed in neurons or neuroendocrine cells, and play essential roles in Ca2+-regulated neurotransmission and hormone secretion (Moghadam and Jackson, 2013). Mammalian E-SYTs are endoplasmic reticulum (ER) membrane proteins that have similar functional domains to plant homologs (Min et al, 2007).

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