Arabidopsis SYT 1 Maintains Stability of Cortical ER Networks and 1 VAP 27-1-Enriched ERPM Contact Sites

39 Arabidopsis synaptotagmin 1 (SYT1) is localized on the endoplasmic reticulum40 plasma membrane (ER-PM) contact sites in leaf and root cells. The ER-PM 41 localization of Arabidopsis SYT1 resembles that of the extended synaptotagmins (E42 SYTs) in animal cells. In mammals, E-SYTs have been shown to regulate calcium 43 signaling, lipid transfer, and endocytosis. Arabidopsis SYT1 was reported to be 44 essential for maintaining cell integrity and virus movement. This study provides 45 detailed insight into the subcellular localization of SYT1 and VAP27-1, another ER46 PM tethering protein. SYT1 and VAP27-1 were shown to be localized on distinct ER47 PM contact sites. The VAP27-1-enriched ER-PM contact sites (V-EPCSs) were 48 always in contact with the SYT1-enriched ER-PM contact sites (S-EPCSs). The V49 EPCSs still existed in the leaf epidermal cells of SYT1 null mutant; however, they 50 were less stable than that in the wild type. The polygonal networks of cortical ER 51 disassembled and the mobility of VAP27-1 protein on the ER-PM contact sites 52 increased in leaf cells of SYT1 null mutant. These results suggest that SYT1 is 53 responsible for stabilizing ER network and V-EPCSs. 54 55