How vacuolar sorting receptor proteins interact with their cargo proteins: crystal structures of apo and cargo-bound forms of the protease-associated domain from an Arabidopsis vacuolar sorting receptor

A Sugarcane Smut Fungus Effector Hijacks Plant Vacuolar Sorting Receptor-Mediated Trafficking to Evade Host Immune Detection

The smut fungus Sporisorium scitamineum is a major pathogen in sugarcane, causing significant agricultural losses worldwide. However, the molecular mechanisms by which its effectors facilitate infection and evade host immunity remain largely unclear. In this study, we identified the sugarcane vacuolar sorting receptor 1 gene (ScVSR1), whose expression negatively correlate with several putative S. scitamineum effector genes in a co-expression network. Overexpression of ScVSR1 in Arabidopsis thaliana reduced resistance to a fungal powdery mildew pathogen, indicating the negative role of ScVSR1 in plant defence. Among the co-expressed S. scitamineum effectors, SsPE15, a secreted cerato-platanin-like protein (CPP), physically interacts with ScVSR1 and is sorted into the prevacuolar compartment (PVC) by interacting with ScVSR1 in plant cells. Deletion of SsPE15 in S. scitamineum enhanced fungal virulence, suggesting that SsPE15 acts as an immune elicitor. Furthermore, the C-terminal domain of the SsPE15, containing the VSR sorting signal, was found to facilitate vesicular location. Notably, fusing this C-terminal domain to the bacterial effector AvrRpt2 significantly reduced AvrRpt2-triggered programmed cell death in Arabidopsis, a process partially dependent on AtVSR1 and AtVSR2. These findings reveal an immune evasion strategy by which S. scitamineum effector SsPE15 hijacks the host's vesicular trafficking system to avoid immune detection.

ARCIMBOLDO at low resolution: Verification for coiled coils and globular proteins

Crystallography at low resolution must determine the atomic model from less experimental observations, which is challenging in the absence of a model. In addition, model bias is more severe when independent experimental data are scarce. Our methods solve the phase problem by combining the location of accurate model fragments using Phaser with density modification and interpretation of the resulting maps using SHELXE. From a partial, correct structure, the density modification process and the stereochemical constraints draw the rest of the structure, validating the result. This same principle is now exploited at low resolution. Coiled coils are important, ubiquitous structures but notoriously difficult to phase and to predict. Both correct solutions and incorrect ones are poorly discriminated by the crystallographic figures of merit as long as helices are correctly oriented. We incorporate coiled-coil verification, designed to set up competing, incompatible structural hypotheses to probe both the results and establish the power of the data to discriminate them. Efficiency of coiled-coil phasing and validation in test cases from 3 to 4 Å is demonstrated in ARCIMBOLDO_LITE, placing single helices, and in ARCIMBOLDO_SHREDDER, with fragments derived from AlphaFold models. SHELXE tracing at low resolution has been enhanced, maintaining its local character but extending the environment assessment. For non-helical structures, verification is demonstrated in the fragment location process. Its use is exemplified with the solution of the VSR1 structure at 3.5 Å, depending on LLG optimization and the emergence of new features in the electron density. Relying on verification, we have extended the use of the ARCIMBOLDO software to low resolution.

Functional implication of the homotrimeric multidomain vacuolar sorting receptor 1 (VSR1) from Arabidopsis thaliana

The vacuolar sorting receptors (VSRs) are specific to plants and are responsible for sorting and transporting particular proteins from the trans-Golgi network to the vacuole. This process is critically important for various cellular functions, including storing nutrients during seed development. Despite many years of intense studies on VSRs, a complete relation between function and structure has not yet been revealed. Here, we present the crystal structure of the entire luminal region of glycosylated VSR1 from Arabidopsis thaliana (AtVSR1) for the first time. The structure provides insights into the tertiary and quaternary structures of VSR1, which are composed of an N-terminal protease-associated (PA) domain, a unique central region, and one epidermal growth factor (EGF)-like domain followed by two disordered EGF-like domains. The structure of VSR1 exhibits unique characteristics, the significance of which is yet to be fully understood.

Recent advances in plant endomembrane research and new microscopical techniques

The endomembrane system consists of various membrane-bound organelles including the endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN), endosomes, and the lysosome/vacuole. Membrane trafficking between distinct compartments is mainly achieved by vesicular transport. As the endomembrane compartments and the machineries regulating the membrane trafficking are largely conserved across all eukaryotes, our current knowledge on organelle biogenesis and endomembrane trafficking in plants has mainly been shaped by corresponding studies in mammals and yeast. However, unique perspectives have emerged from plant cell biology research through the characterization of plant-specific regulators as well as the development and application of the state-of-the-art microscopical techniques. In this review, we summarize our current knowledge on the plant endomembrane system, with a focus on several distinct pathways: ER-to-Golgi transport, protein sorting at the TGN, endosomal sorting on multivesicular bodies, vacuolar trafficking/vacuole biogenesis, and the autophagy pathway. We also give an update on advanced imaging techniques for the plant cell biology research.

Retrograde transport in plants: Circular economy in the endomembrane system

The study of endomembrane trafficking is crucial for understanding how cells and whole organisms function. Moreover, there is a special interest in investigating endomembrane trafficking in plants, given its role in transport and accumulation of seed storage proteins and in secretion of cell wall material, arguably the two most essential commodities obtained from crops. The mechanisms of anterograde transport in the biosynthetic and endocytic pathways of plants have been thoroughly discussed in recent reviews, but, comparatively, retrograde trafficking pathways have received less attention. Retrograde trafficking is essential to recover membranes, retrieve proteins that have escaped from their intended localization, maintain homeostasis in maturing compartments, and recycle trafficking machinery for its reuse in anterograde transport reactions. Here, we review the current understanding on retrograde trafficking pathways in the endomembrane system of plants, discussing their integration with anterograde transport routes, describing conserved and plant-specific retrieval mechanisms at play, highlighting contentious issues and identifying open questions for future research.

Endomembrane mediated-trafficking of seed storage proteins: from Arabidopsis to cereal crops

Seed storage proteins (SSPs) are of great importance in plant science and agriculture, particularly in cereal crops, due to their nutritional value and their impact on food properties. During seed maturation, massive amounts of SSPs are synthesized and deposited either within protein bodies derived from the endoplasmic reticulum, or into specialized protein storage vacuoles (PSVs). The processing and trafficking of SSPs vary among plant species, tissues, and even developmental stages, as well as being influenced by SSP composition. The different trafficking routes, which affect the amount of SSPs that seeds accumulate and their composition and modifications, rely on a highly dynamic and functionally specialized endomembrane system. Although the general steps in SSP trafficking have been studied in various plants, including cereals, the detailed underlying molecular and regulatory mechanisms are still elusive. In this review, we discuss the main endomembrane routes involved in SSP trafficking to the PSV in Arabidopsis and other eudicots, and compare and contrast the SSP trafficking pathways in major cereal crops, particularly in rice and maize. In addition, we explore the challenges and strategies for analyzing the endomembrane system in cereal crops.

Wheat α-gliadin and high-molecular-weight glutenin subunit accumulate in different storage compartments of transgenic soybean seed

Wheat seed storage proteins (prolamins) are important for the grain quality because they provide a characteristic texture to wheat flour products. In wheat endosperm cells, prolamins are transported from the Endoplasmic reticulum to Protein storage vacuoles through two distinct pathways-a conventional pathway passing through the Golgi apparatus and an unconventional Golgi-bypassing pathway during which prolamins accumulate in the ER lumen, forming Protein bodies. Unfortunately, transport studies conducted previously achieved limited success because of the seed-specificity of the latter pathway and the multigene architecture of prolamins. To overcome this difficulty, we expressed either of the two families of wheat prolamins, namely α-gliadin or High-molecular-weight subunit of glutenin, in soybean seed, which naturally lacks prolamin-like proteins. SDS-PAGE analysis indicated the successful expression of recombinant wheat prolamins in transgenic soybean seeds. Their accumulation states were quite different-α-gliadin accumulated with partial fragmentation whereas the HMW-glutenin subunit formed disulfide-crosslinked polymers without fragmentation. Immunoelectron microscopy of seed sections revealed that α-gliadin was transported to PSVs whereas HMW-glutenin was deposited in novel ER-derived compartments distinct from PSVs. Observation of a developmental stage of seed cells showed the involvement of post-Golgi Prevacuolar compartments in the transport of α-gliadin. In a similar stage of cells, deposits of HMW-glutenin surrounded by membranes studded with ribosomes were observed confirming the accumulation of this prolamin as ER-derived PBs. Subcellular fractionation analysis supported the electron microscopy observations. Our results should help in better understanding of molecular events during the transport of prolamins in wheat.

Structural insights into how vacuolar sorting receptors recognize the sorting determinants of seed storage proteins

Seeds such as rice and soybean are major food staples in the human diet. During seed development, storage proteins are deposited in a specialized organelle called the protein storage vacuole and are mobilized to provide nutrients during germination. Storage proteins are transported as cargoes via specific protein–protein interactions with the vacuolar sorting receptors. Supported by structural and mutagenesis studies, our work provides insights into how the sequence-specific information, or the vacuolar sorting determinant, on the storage proteins is recognized by the vacuolar sorting receptors for their targeting to the vacuoles. Insights gained into the rules of receptor–cargo recognition will be useful in engineering recombinant proteins for biotechnological applications of the protein storage vacuoles in seeds.

In Arabidopsis, vacuolar sorting receptor isoform 1 (VSR1) sorts 12S globulins to the protein storage vacuoles during seed development. Vacuolar sorting is mediated by specific protein–protein interactions between VSR1 and the vacuolar sorting determinant located at the C terminus (ctVSD) on the cargo proteins. Here, we determined the crystal structure of the protease-associated domain of VSR1 (VSR1-PA) in complex with the C-terminal pentapeptide (₄₆₈RVAAA₄₇₂) of cruciferin 1, an isoform of 12S globulins. The ₄₆₈RVA₄₇₀ motif forms a parallel β-sheet with the switch III residues (₁₂₇TMD₁₂₉) of VSR1-PA, and the ₄₇₁AA₄₇₂ motif docks to a cradle formed by the cargo-binding loop (₉₅RGDCYF₁₀₀), making a hydrophobic interaction with Tyr99. The C-terminal carboxyl group of the ctVSD is recognized by forming salt bridges with Arg95. The C-terminal sequences of cruciferin 1 and vicilin-like storage protein 22 were sufficient to redirect the secretory red fluorescent protein (spRFP) to the vacuoles in Arabidopsis protoplasts. Adding a proline residue to the C terminus of the ctVSD and R95M substitution of VSR1 disrupted receptor–cargo interactions in vitro and led to increased secretion of spRFP in Arabidopsis protoplasts. How VSR1-PA recognizes ctVSDs of other storage proteins was modeled. The last three residues of ctVSD prefer hydrophobic residues because they form a hydrophobic cluster with Tyr99 of VSR1-PA. Due to charge–charge interactions, conserved acidic residues, Asp129 and Glu132, around the cargo-binding site should prefer basic residues over acidic ones in the ctVSD. The structural insights gained may be useful in targeting recombinant proteins to the protein storage vacuoles in seeds.

Soybean proteins/peptides: A review on their importance, biosynthesis, vacuolar sorting, and accumulation in seeds

Soybean is one of the most important sources of plant protein and is known for its wide range of agricultural, food, and industrial applications as well as health benefits. Interest in soybean proteins has been steadily growing as progressively more applications and benefits are discovered. This review article is focused on the major seed storage proteins of soybean, their three-dimensional structures, their nutritional importance and bioactive peptides, cellular synthesis, and accumulation in seeds. This will also summarize past efforts in the recombinant production of foreign proteins or bioactive peptides in soybean seed.

Structure, dynamics and immunogenicity of a catalytically inactive CXC chemokine-degrading protease SpyCEP from Streptococcus pyogenes

Over 18 million disease cases and half a million deaths worldwide are estimated to be caused annually by Group A Streptococcus. A vaccine to prevent GAS disease is urgently needed. SpyCEP (Streptococcus pyogenes Cell-Envelope Proteinase) is a surface-exposed serine protease that inactivates chemokines, impairing neutrophil recruitment and bacterial clearance, and has shown promising immunogenicity in preclinical models. Although SpyCEP structure has been partially characterized, a more complete and higher resolution understanding of its antigenic features would be desirable prior to large scale manufacturing. To address these gaps and facilitate development of this globally important vaccine, we performed immunogenicity studies with a safety-engineered SpyCEP mutant, and comprehensively characterized its structure by combining X-ray crystallography, NMR spectroscopy and molecular dynamics simulations. We found that the catalytically-inactive SpyCEP antigen conferred protection similar to wild-type SpyCEP in a mouse infection model. Further, a new higher-resolution crystal structure of the inactive SpyCEP mutant provided new insights into this large chemokine protease comprising nine domains derived from two non-covalently linked fragments. NMR spectroscopy and molecular simulation analyses revealed conformational flexibility that is likely important for optimal substrate recognition and overall function. These combined immunogenicity and structural data demonstrate that the full-length SpyCEP inactive mutant is a strong candidate human vaccine antigen. These findings show how a multi-disciplinary study was used to overcome obstacles in the development of a GAS vaccine, an approach applicable to other future vaccine programs. Moreover, the information provided may also facilitate the structure-based discovery of small-molecule therapeutics targeting SpyCEP protease inhibition.

Trafficking to the seed protein storage vacuole

The processing and subcellular trafficking of seed storage proteins is a critical area of physiological, agricultural and biotechnological research. Trafficking to the lytic vacuole has been extensively discussed in recent years, without substantial distinction from trafficking to the protein storage vacuole (PSV). However, despite some overlap between these pathways, there are several examples of unique processing and machinery in the PSV pathway. Moreover, substantial new data has recently come to light regarding the important players in this pathway, in particular, the intracellular NHX proteins and their role in regulating lumenal pH. In some cases, these new data are limited to genetic evidence, with little mechanistic understanding. As such, the implications of these data in the current paradigm of PSV trafficking is perhaps yet unclear. Although it has generally been assumed that the major classes of storage proteins are trafficked via the same pathway, there is mounting evidence that the 12S globulins and 2S albumins may be trafficked independently. Advances in identification of vacuolar targeting signals, as well as an improved mechanistic understanding of various vacuolar sorting receptors, may reveal the differences in these trafficking pathways.

From structure to function - a family portrait of plant subtilases

Contents Summary 901 I. Introduction 901 II. Biochemistry and structure of plant SBTs 902 III. Phylogeny of plant SBTs and family organization 903 IV. Physiological roles of plant SBTs 905 V. Conclusions and outlook 911 Acknowledgements 912 References 912 SUMMARY: Subtilases (SBTs) are serine peptidases that are found in all three domains of life. As compared with homologs in other Eucarya, plant SBTs are more closely related to archaeal and bacterial SBTs, with which they share many biochemical and structural features. However, in the course of evolution, functional diversification led to the acquisition of novel, plant-specific functions, resulting in the present-day complexity of the plant SBT family. SBTs are much more numerous in plants than in any other organism, and include enzymes involved in general proteolysis as well as highly specific processing proteases. Most SBTs are targeted to the cell wall, where they contribute to the control of growth and development by regulating the properties of the cell wall and the activity of extracellular signaling molecules. Plant SBTs affect all stages of the life cycle as they contribute to embryogenesis, seed development and germination, cuticle formation and epidermal patterning, vascular development, programmed cell death, organ abscission, senescence, and plant responses to their biotic and abiotic environments. In this article we provide a comprehensive picture of SBT structure and function in plants.

Plant Vacuoles

Plant vacuoles are multifunctional organelles. On the one hand, most vegetative tissues develop lytic vacuoles that have a role in degradation. On the other hand, seed cells have two types of storage vacuoles: protein storage vacuoles (PSVs) in endosperm and embryonic cells and metabolite storage vacuoles in seed coats. Vacuolar proteins and metabolites are synthesized on the endoplasmic reticulum and then transported to the vacuoles via Golgi-dependent and Golgi-independent pathways. Proprotein precursors delivered to the vacuoles are converted into their respective mature forms by vacuolar processing enzyme, which also regulates various kinds of programmed cell death in plants. We summarize two types of vacuolar membrane dynamics that occur during defense responses: vacuolar membrane collapse to attack viral pathogens and fusion of vacuolar and plasma membranes to attack bacterial pathogens. We also describe the chemical defense against herbivores brought about by the presence of PSVs in the idioblast myrosin cell.

Nanobody-triggered lockdown of VSRs reveals ligand reloading in the Golgi

Protein degradation in lytic compartments is crucial for eukaryotic cells. At the heart of this process, vacuolar sorting receptors (VSRs) bind soluble hydrolases in the secretory pathway and release them into the vacuolar route. Sorting efficiency is suggested to result from receptor recycling. However, how and to where plant VSRs recycle remains controversial. Here we present a nanobody-epitope interaction-based protein labeling and tracking approach to dissect their anterograde and retrograde transport routes in vivo. We simultaneously employ two different nanobody-epitope pairs: one for the location-specific post-translational fluorescence labeling of receptors and the other pair to trigger their compartment-specific lockdown via an endocytosed dual-epitope linker protein. We demonstrate VSR recycling from the TGN/EE, thereby identifying the cis-Golgi as the recycling target and show that recycled VSRs reload ligands. This is evidence that bidirectional VSR-mediated sorting of vacuolar proteins exists and occurs between the Golgi and the TGN/EE.

Nanobody-triggered lockdown of VSRs reveals ligand reloading in the Golgi

Protein degradation in lytic compartments is crucial for eukaryotic cells. At the heart of this process, vacuolar sorting receptors (VSRs) bind soluble hydrolases in the secretory pathway and release them into the vacuolar route. Sorting efficiency is suggested to result from receptor recycling. However, how and to where plant VSRs recycle remains controversial. Here we present a nanobody-epitope interaction-based protein labeling and tracking approach to dissect their anterograde and retrograde transport routes in vivo. We simultaneously employ two different nanobody-epitope pairs: one for the location-specific post-translational fluorescence labeling of receptors and the other pair to trigger their compartment-specific lockdown via an endocytosed dual-epitope linker protein. We demonstrate VSR recycling from the TGN/EE, thereby identifying the cis-Golgi as the recycling target and show that recycled VSRs reload ligands. This is evidence that bidirectional VSR-mediated sorting of vacuolar proteins exists and occurs between the Golgi and the TGN/EE.

The Multivesicular Body and Autophagosome Pathways in Plants

In eukaryotic cells, the endomembrane system consists of multiple membrane-bound organelles, which play essential roles in the precise transportation of various cargo proteins. In plant cells, vacuoles are regarded as the terminus of catabolic pathways whereas the selection and transport of vacuolar cargoes are mainly mediated by two types of organelles, multivesicular bodies (MVBs) also termed prevacuolar compartments (PVCs) and autophagosomes. MVBs are single-membrane bound organelles with intraluminal vesicles and mediate the transport between the trans-Golgi network (TGN) and vacuoles, while autophagosomes are double-membrane bound organelles, which mediate cargo delivery to the vacuole for degradation and recycling during autophagy. Great progress has been achieved recently in identification and characterization of the conserved and plant-unique regulators involved in the MVB and autophagosome pathways. In this review, we present an update on the current knowledge of these key regulators and pay special attention to their conserved protein domains. In addition, we discuss the possible interplay between the MVB and autophagosome pathways in regulating vacuolar degradation in plants.

RMR (Receptor Membrane RING-H2) type 1 and 2 show different promoter activities and subcellular localizations in Arabidopsis thaliana

Soluble vacuolar proteins reach their compartments of final accumulation through the binding with specific transmembrane cargo receptors. In Arabidopsis thaliana two different families of receptors have been characterized. The AtVSRs (Vacuolar Sorting Receptor), which are known to be involved in the protein sorting to lytic vacuoles (LV), and the AtRMRs (Receptor Membrane RING-H2), for which there is less evidence for a role in the traffic to the protein storage vacuole (PSV). In this study we investigated the localization and tissue expression of two RMRs (AtRMR1 and 2) in their species of origin, A. thaliana. Our experiments using leaf protoplasts and transgenic plants supported previous results of subcellular localization in Nicotiana benthamiana that visualized AtRMR1 and 2 in the cisternae of endoplasmic reticulum (ER) and in the trans-Golgi network (TGN), respectively. The promoter activities of AtRMR1 and AtRMR2 detected in transgenic A. thaliana lines suggest that the expression of these two receptors only partially overlap in some organs and tissues. These results suggest that AtRMR1 and 2 are not functionally redundant, but could also interact and participate in the same cellular process in tissues with an overlapping expression.

Trafficking routes to the plant vacuole: connecting alternative and classical pathways

Due to the numerous roles plant vacuoles play in cell homeostasis, detoxification, and protein storage, the trafficking pathways to this organelle have been extensively studied. Recent evidence, however, suggests that our vision of transport to the vacuole is not as simple as previously imagined. Alternative routes have been identified and are being characterized. Intricate interconnections between routes seem to occur in various cases, complicating the interpretation of data. In this review, we aim to summarize the published evidence and link the emerging data with previous findings. We discuss the current state of information on alternative and classical trafficking routes to the plant vacuole.

Subcellular Trafficking of Mammalian Lysosomal Proteins: An Extended View

Lysosomes clear macromolecules, maintain nutrient and cholesterol homeostasis, participate in tissue repair, and in many other cellular functions. To assume these tasks, lysosomes rely on their large arsenal of acid hydrolases, transmembrane proteins and membrane-associated proteins. It is therefore imperative that, post-synthesis, these proteins are specifically recognized as lysosomal components and are correctly sorted to this organelle through the endosomes. Lysosomal transmembrane proteins contain consensus motifs in their cytosolic regions (tyrosine- or dileucine-based) that serve as sorting signals to the endosomes, whereas most lysosomal acid hydrolases acquire mannose 6-phosphate (Man-6-P) moieties that mediate binding to two membrane receptors with endosomal sorting motifs in their cytosolic tails. These tyrosine- and dileucine-based motifs are tickets for boarding in clathrin-coated carriers that transport their cargo from the trans-Golgi network and plasma membrane to the endosomes. However, increasing evidence points to additional mechanisms participating in the biogenesis of lysosomes. In some cell types, for example, there are alternatives to the Man-6-P receptors for the transport of some acid hydrolases. In addition, several “non-consensus” sorting motifs have been identified, and atypical transport routes to endolysosomes have been brought to light. These “unconventional” or “less known” transport mechanisms are the focus of this review.

Dimerization of the Vacuolar Receptors AtRMR1 and -2 from Arabidopsis thaliana Contributes to Their Localization in the trans-Golgi Network

In Arabidopsis thaliana, different types of vacuolar receptors were discovered. The AtVSR (Vacuolar Sorting Receptor) receptors are well known to be involved in the traffic to lytic vacuole (LV), while few evidences demonstrate the involvement of the receptors from AtRMR family (Receptor Membrane RING-H2) in the traffic to the protein storage vacuole (PSV). In this study we focused on the localization of two members of AtRMR family, AtRMR1 and -2, and on the possible interaction between these two receptors in the plant secretory pathway. Our experiments with agroinfiltrated Nicotiana benthamiana leaves demonstrated that AtRMR1 was localized in the endoplasmic reticulum (ER), while AtRMR2 was targeted to the trans-Golgi network (TGN) due to the presence of a cytosolic 23-amino acid sequence linker. The fusion of this linker to an equivalent position in AtRMR1 targeted this receptor to the TGN, instead of the ER. By using a Bimolecular Fluorescent Complementation (BiFC) technique and experiments of co-localization, we demonstrated that AtRMR2 can make homodimers, and can also interact with AtRMR1 forming heterodimers that locate to the TGN. Such interaction studies strongly suggest that the transmembrane domain and the few amino acids surrounding it, including the sequence linker, are essential for dimerization. These results suggest a new model of AtRMR trafficking and dimerization in the plant secretory pathway.

Receptor-mediated sorting of soluble vacuolar proteins: myths, facts, and a new model

To prevent their being released to the cell exterior, acid hydrolases are recognized by receptors at some point in the secretory pathway and diverted towards the lytic compartment of the cell (lysosome or vacuole). In animal cells, the receptor is called the mannosyl 6-phosphate receptor (MPR) and it binds hydrolase ligands in the trans-Golgi network (TGN). These ligands are then sequestered into clathrin-coated vesicles (CCVs) because of motifs in the cytosolic tail of the MPR which interact first with monomeric adaptors (Golgi-localized, Gamma-ear-containing, ARF-binding proteins, GGAs) and then with tetrameric (adaptin) adaptor complexes. The CCVs then fuse with an early endosome, whose more acidic lumen causes the ligands to dissociate. The MPRs are then recycled back to the TGN via retromer-coated carriers. Plants have vacuolar sorting receptors (VSRs) which were originally identified in CCVs isolated from pea (Pisum sativum L.) cotyledons. It was therefore assumed that VSRs would have an analogous function in plants to MPRs in animals. Although this dogma has enjoyed wide support over the last 20 years there are many inconsistencies. Recently, results have been published which are quite contrary to it. It now emerges that VSRs and their ligands can interact very early in the secretory pathway, and dissociate in the TGN, which, in contrast to its mammalian counterpart, has a pH of 5.5. Multivesicular endosomes in plants lack proton pump complexes and consequently have an almost neutral internal pH, which discounts them as organelles of pH-dependent receptor-ligand dissociation. These data force a critical re-evaluation of the role of CCVs at the TGN, especially considering that vacuolar cargo ligands have never been identified in them. We propose that one population of TGN-derived CCVs participate in retrograde transport of VSRs from the TGN. We also present a new model to explain how secretory and vacuolar cargo proteins are effectively separated after entering the late Golgi/TGN compartments.

Lysosomal and vacuolar sorting: not so different after all!

Soluble hydrolases represent the main proteins of lysosomes and vacuoles and are essential to sustain the lytic properties of these organelles typical for the eukaryotic organisms. The sorting of these proteins from ER residents and secreted proteins is controlled by highly specific receptors to avoid mislocalization and subsequent cellular damage. After binding their soluble cargo in the early stage of the secretory pathway, receptors rely on their own sorting signals to reach their target organelles for ligand delivery, and to recycle back for a new round of cargo recognition. Although signals in cargo and receptor molecules have been studied in human, yeast and plant model systems, common denominators and specific examples of diversification have not been systematically explored. This review aims to fill this niche by comparing the structure and the function of lysosomal/vacuolar sorting receptors (VSRs) from these three organisms.

Receptor-mediated sorting of soluble vacuolar proteins ends at the trans-Golgi network/early endosome

The sorting of soluble proteins for degradation in the vacuole is of vital importance in plant cells, and relies on the activity of vacuolar sorting receptors (VSRs). In the plant endomembrane system, VSRs bind vacuole-targeted proteins and facilitate their transport to the vacuole. Where exactly these interactions take place has remained controversial, however. Here, we examine the potential for VSR-ligand interactions in all compartments of the vacuolar transport system in tobacco mesophyll protoplasts. To do this, we developed compartment-specific VSR sensors that assemble as a result of a nanobody-epitope interaction, and monitored the degree of ligand binding by analysing Förster resonance energy transfer using fluorescence lifetime imaging microscopy (FRET-FLIM). We show that VSRs bind ligands in the endoplasmic reticulum (ER) and in the Golgi, but not in the trans-Golgi network/early endosome (TGN/EE) or multivesicular late endosomes, suggesting that the post-TGN/EE trafficking of ligands towards the vacuole is VSR independent. We verify this by showing that non-VSR-ligands are also delivered to the vacuole from the TGN/EE after endocytic uptake. We conclude that VSRs are required for the transport of ligands from the ER and the Golgi to the TGN/EE, and suggest that the onward transport to the vacuole occurs by default.