Abstract
Vacuoles are one of the most prominent organelles in plant cells, and they play various important roles, such as degradation of waste materials, storage of ions and metabolites, and maintaining turgor. During the past two decades, numerous advances have been made in understanding how proteins are specifically delivered to the vacuole. One of the most crucial steps in this process is specific sorting of soluble vacuolar proteins. Vacuolar sorting receptors (VSRs), which are type I membrane proteins, are involved in the sorting and packaging of soluble vacuolar proteins into transport vesicles with the help of various accessory proteins. To date, large amounts of data have led to the development of two different models describing VSR-mediated vacuolar trafficking that are radically different in multiple ways, particularly regarding the location of cargo binding to, and release from, the VSR and the types of carriers utilized. In this review, we summarize current literature aimed at elucidating VSR-mediated vacuolar trafficking and compare the two models with respect to the sorting signals of vacuolar proteins, as well as the molecular machinery involved in VSR-mediated vacuolar trafficking and its action mechanisms.
Highlights
Plant cells contain a variety of endomembrane compartments
These studies have resulted in the proposal of two different models describing Vacuolar sorting receptors (VSRs)-mediated vacuolar trafficking: VSR-mediated trafficking from the trans-Golgi network (TGN) to the prevacuolar compartment (PVC) (Model I) and VSR-mediated trafficking from the endoplasmic reticulum (ER) to the TGN (Model II)
Pimpl [95] suggested that Ca2+-based vacuolar cargo sorting occurs at the TGN; according to this model, soluble vacuolar proteins interact with VSRs at the ER and the cargo-VSR complexes are transported to the cis-Golgi via COPII vesicles together with secretory cargo proteins
Summary
Plant cells contain a variety of endomembrane compartments. Of these, the vacuole is the most prominent organelle, occupying up to 90% of the cellular volume in mesophyll cells. Transient expression of dominant negative mutant forms of AtVSR1 and AtVSR2 causes secretion and/or inhibition of various coexpressed vacuole-destined proteins, such as Spo:GFP (a fusion protein consisting of a sorting signal of sporamin protein from sweet potato (Ipomoea batatas) and green fluorescent protein (GFP)), AALP:GFP (a fusion of Arabidopsis aleurain-like protein and GFP), and Spo:amylase (a fusion of sporamin and amylase, destined for the lytic vacuole) [18,19,34,35] These studies have provided strong evidence that VSRs are sorting receptors of soluble lytic vacuolar proteins, which was confirmed by genetic studies; in atvsr1atvsr double-mutant plants, a small amount of AALP is secreted into the apoplast in leaf tissues [13]. In contrast to other atvsr mutants, atvsr5atvsr double-mutant plants did not exhibit defective trafficking of protein to the two vacuoles (lytic vacuole and PSV) when two lytic vacuolar cargoes, sporamin:GFP and AALP:GFP, and two PSV proteins, 12S globulins and These results indicate that the luminal domain is involved in the specificity determination of AtVSR isoforms. Further studies are necessary to determine the exact role of these two isoforms in plant cells
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