Identification of SYT1 interactors connects calcium signaling, endoplasmic reticulum bending, plasmodesmata and membrane contact sites.

Abstract

Synaptotagmin1 (SYT1) is an Arabidopsis thaliana protein essential for tolerance to several abiotic stresses (Schapire et al., 2008; Pérez-Sancho et al., 2015; Ruiz-Lopez et al., 2020). SYT1 forms endoplasmic reticulum-plasma membrane contact sites (ER-PM CS), microdomains conserved across eukaryotes where protein tethers maintain the membranes of the ER and the PM in close apposition (∼30 nm) without fusing. The short distance between membranes facilitates processes such as ion and lipid transport (Pérez-Sancho et al., 2016). For example, SYT1 transports diacylglycerol (DAG) from the PM to the ER during abiotic stress to assure PM integrity (Ruiz-Lopez et al., 2020). Usually, protein complexes form the core of contact sites. In particular, SYT1 forms dimers with SYT1, SYT3, SYT5 and CLB1; all members of the Arabidopsis SYT family and EPCS tethers (Lee et al., 2020; Ruiz-Lopez et al., 2020). We found that SYT1 interacts with proteins involved in different cellular processes by non-targeted proteomic approaches (IP-MS and TAP-tag). Thus, SYT1 interacts with reticulons (RTN), ER-resident proteins responsible for ER curvature, which is crucial for ER morphology and lipid transport at ER-PM CS (Collado et al., 2019). Additionally, SYT1 interacts with ECAs, Ca2+-ATPases located at the ER membrane. SYT1 also interacts with sterol methyltransferases (SMTs), key enzymes in the route of sitosterol and stigmasterol biosynthesis. The homeostasis of the sitosterol and stigmasterol is crucial for the tolerance to wound, heat and bacterial stress presumably by affecting PM fluidity. MCTPs (multiple C2 domains and transmembrane region proteins), plasmodesmata-exclusive proteins are also SYT1 interactors (Brault et al., 2019). We are now investigating the role of SYT1 in these processes using biochemical, genetic and cellular biology approaches.