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Identification of a novel protein tether enriched at endoplasmic reticulum-plasma membrane contact sites and in volved in salt stress tolerance in Arabidopsis

University of British Columbia

Plants are sessile and are exposed to environmental changes that can compromise their survival and yield. Salt stress is one of the most common environmental stresses. Arabidopsis SYT1 is a key player in plant response to salt stress mediated by Ca²⁺, and acts as protein bridge, tethering the endoplasmic reticulum to the plasma membrane. These regions of close contact between organelles, called membrane contact sites, are conserved in eukaryotes, and are involved in functions such as Ca²⁺ signaling and lipid homeostasis. However, their relationship with salt stress in plants is unknown. In this project, I aimed to discover new contact site tethers in the model organism Arabidopsis thaliana and shed light on the relation of endoplasmic reticulum-plasma membrane contact sites (EPCS) and salt stress tolerance. I used bioinformatic, phylogenetic and cellular biological approaches and I related an Arabidopsis protein family, the N-terminal transmembrane C2 domain (NTMC2) family, to contact sites. A putative tether called Arabidopsis thaliana Ca²⁺-dependent Lipid Binding (AtCLB) belongs to the NTMC2 family. AtCLB has a subcellular localization pattern of beads and strings, which resembles the pattern found for the plant EPCS tether SYT1. AtCLB pattern becomes more punctate under depletion of cytosolic Ca²⁺, suggesting that intracellular Ca²⁺ is important for AtCLB contact with the plasma membrane. Mutant plants lacking functional AtCLB did not show any visible phenotype in salt stress conditions. Under salt stress treatments, however, the normal subcellular pattern of AtCLB was altered, EPCS formation was increased and the intermembrane distance was decreased. This result suggests a role for AtCLB in salt stress tolerance that might be phenotypically masked by functional redundancy. This subcellular alteration and the reduction of intermembrane distance was mimicked when the amount of phosphatidylinositol-(4,5)-bisphosphate was artificially increased at the plasma membrane. These results point towards a model of EPCS tethering involving cytosolic Ca²⁺, salt stress and negatively charged phosphoinositides. Further study will be required to fully understand EPCS regulation. In summary, these discoveries clarify some mechanistic aspects of EPCS tethering in plants and open a door to the discovery of new contact site protein tethers from the plant protein family NTMC2

Text

2018-06-05

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/66218

Botany

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/