A putative ER-PM contact site complex relocalizes in response to Rare Earth Elements –induced endocytosis

ABSTRACT

In plant cells, environmental stressors induce changes in the cytosolic concentration of calcium ([Ca²⁺]_cyt) that are transduced by Ca²⁺-sensing proteins. To confer specificity to the stress signaling response, [Ca²⁺]_cyt sensing must be tightly regulated in space and time; the molecular mechanisms that restrict the localization and dynamics of Ca²⁺ sensors in plants, however, are largely unknown. In this report, we identify a putative Ca²⁺-sensitive complex containing the synaptotagmins 1 and 5 (SYT1 and SYT5) and the Ca²⁺-dependent lipid binding protein (CLB1), which is enriched at ER-PM contact sites (EPCS) and relocalizes in response to Rare Earth Elements (REEs)-induced endocytosis. Our results show that endocytosed REEs influence cytosolic Ca²⁺ signaling, as indicated by the activation of the Ca²⁺/Calmodulin-based ratiometric sensor GCaMP3, and promote the cytoskeleton-dependent accumulation of ER-PM contact sites at the cell cortex. Based on these results, we propose that the EPCS-localized SYT1/SYT5/CLB1 complex is part of an evolutionarily conserved and spatially regulated Ca²⁺-responsive mechanism that control cER-PM communication during stress episodes.