Cell Biology and Signal Transduction VSRs in lytic vacuolar trafficking

In eukaryotic cells, protein trafficking plays an essential role in biogenesis of proteins that belong to the endomembrane compartments. In this process, an important step is the sorting of organellar proteins depending on their final destinations. For vacuolar proteins, vacuolar sorting receptors (VSRs) and receptor homology-transmembrane-RING H2 domain proteins (RMRs) are thought to be responsible. Arabidopsis contains seven VSRs. Among them, VSR1, VSR3 and VSR4 are involved in sorting storage proteins targeted to the protein storage vacuole (PSV) in seeds. However, it remains controversial about the identity of VSRs for soluble proteins of the lytic vacuole in vegetative cells. Here, we provide evidence that VSR1, VSR3 and VSR4 are involved in sorting soluble lytic vacuolar and PSV proteins in vegetative cells. In protoplasts from leaf tissues of vsr1vsr3 and vsr1vsr4 but not vsr5vsr6, rmr1rmr2 and rmr3rmr4 double mutants, soluble lytic vacuolar (AALP:GFP and CPY:GFP) and PSV (phaseolin) proteins, but not a vacuolar membrane protein AtβFruct4:GFP, exhibited defects in their trafficking; they accumulated to the endoplasmic reticulum (ER) with an increased secretion into medium. The trafficking defects in vsr1vsr4 protoplasts were rescued by VSR1 or VSR4 but not VSR5 or AtRMR1. Furthermore, of the luminal domain swapping mutants between VSR1 and VSR5, the mutant with the luminal domain of VSR1, but not that of VSR5, rescued the trafficking defects of AALP:GFP and phaseolin in vsr1vsr4 protoplasts. Based on these results, we propose that VSR1, VSR3 and VSR4, but not other VSRs, are involved in sorting soluble lytic vacuolar and PSV proteins for their trafficking to the vacuoles in vegetative cells. https://plantphysiol.org Downloaded on May 3, 2021. Published by Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.