Identification of sorting motifs of AtβFruct4 for trafficking from the ER to the vacuole through the Golgi and PVC

Tonoplast and Peroxisome Targeting of γ-tocopherol N-methyltransferase Homologs Involved in the Synthesis of Monoterpene Indole Alkaloids

Many plant species from the Apocynaceae, Loganiaceae and Rubiaceae families evolved a specialized metabolism leading to the synthesis of a broad palette of monoterpene indole alkaloids (MIAs). These compounds are believed to constitute a cornerstone of the plant chemical arsenal but above all several MIAs display pharmacological properties that have been exploited for decades by humans to treat various diseases. It is established that MIAs are produced in planta due to complex biosynthetic pathways engaging a multitude of specialized enzymes but also a complex tissue and subcellular organization. In this context, N-methyltransferases (NMTs) represent an important family of enzymes indispensable for MIA biosynthesis but their characterization has always remained challenging. In particular, little is known about the subcellular localization of NMTs in MIA-producing plants. Here, we performed an extensive analysis on the subcellular localization of NMTs from four distinct medicinal plants but also experimentally validated that two putative NMTs from Catharanthus roseus exhibit NMT activity. Apart from providing unprecedented data regarding the targeting of these enzymes in planta, our results point out an additional layer of complexity to the subcellular organization of the MIA biosynthetic pathway by introducing tonoplast and peroxisome as new actors of the final steps of MIA biosynthesis.

Go your own way: membrane-targeting sequences

Membrane-targeting sequences, connected targeting mechanisms, and co-factors orchestrate primary targeting of proteins to membranes.

OsINV3 and Its Homolog, OsINV2, Control Grain Size in Rice

Vacuolar invertase is involved in sugar metabolism and plays a crucial role in plant growth and development, thus regulating seed size. However, information linking vacuolar invertase and seed size in rice is limited. Here we characterized a small grain mutant sg2 (grain size on chromosome 2) that showed a reduced in grain size and 1000-grain weight compared to the wild type. Map-based cloning and genetic complementation showed that OsINV3 is responsible for the observed phenotype. Loss-of-function of OsINV3 resulted in grains of smaller size when compared to the wild type, while overexpression showed increased grain size. We also obtained a T-DNA insertion mutant of OsINV2, which is a homolog of OsINV3 and generated double knockout (KO) mutants of OsINV2 and OsINV3 using CRISPR/Cas9. Genetic data showed that OsINV2, that has no effect on grain size by itself, reduces grain length and width in the absence of OsINV3. Altered sugar content with increased sucrose and decreased hexose levels, as well as changes vacuolar invertase activities and starch constitution in INV3^KO, INV2^KO, INV3^KOINV2^KO mutants indicate that OsINV2 and OsINV3 affect sucrose metabolism in sink organs. In summary, we identified OsINV3 as a positive regulator of grain size in rice, and while OsINV2 has no function on grain size by itself. In the absence of OsINV3, it is possible to detect a role of OsINV2 in the regulation of grain size. Both OsINV3 and OsINV2 are involved in sucrose metabolism, and thus regulate grain size. Our findings increase our understanding of the role of OsINV3 and its homolog, OsINV2, in grain size development and also suggest a potential strategy to improve grain yield in rice.

OsVIN2 encodes a vacuolar acid invertase that affects grain size by altering sugar metabolism in rice

OsVIN2, a vacuolar invertase, affects grain size and yield by altering sugar composition, transport, and starch accumulation in rice. Grain size, a major determinant of rice yield, is influenced by many developmental and environmental factors. Sugar metabolism plays vital roles in plant development. However, the way in which sugar metabolism affects rice grain size remains largely elusive. In this study, we characterized the small grain-size rice mutant sgs1. Histological analyses showed that reduced spikelet hull and endosperm size results from decreased cell size rather than cell number. Map-based cloning and complementation tests revealed that a DaiZ7 transposon insertion in a vacuolar invertase gene OsVIN2 is responsible for the mutant phenotype. Subcellular distribution and biochemical analysis indicated that OsVIN2 is located in the vacuolar lumen, and that its sucrose hydrolysis activity is maintained under acidic conditions. Furthermore, an altered sugar content with increased sucrose and decreased hexose levels, as well as changes in invertase and sucrose synthase activities, sugar transport gene expression, and starch constitution in sgs1 implies that OsVIN2 affects sucrose metabolism, including sugar composition, transport, and conversion from the source to the sink organs. Collectively, OsVIN2 is involved in sugar metabolism, and thus regulates grain size; our findings provide insights into grain development and also suggest a potential strategy to improve grain quality and yield in rice.

AtCAP2 is crucial for lytic vacuole biogenesis during germination by positively regulating vacuolar protein trafficking

Protein trafficking is a fundamental mechanism of subcellular organization and contributes to organellar biogenesis. AtCAP2 is an Arabidopsis homolog of the Mesembryanthemum crystallinum calcium-dependent protein kinase 1 adaptor protein 2 (McCAP2), a member of the syntaxin superfamily. Here, we show that AtCAP2 plays an important role in the conversion to the lytic vacuole (LV) during early plant development. The AtCAP2 loss-of-function mutant atcap2-1 displayed delays in protein storage vacuole (PSV) protein degradation, PSV fusion, LV acidification, and biosynthesis of several vacuolar proteins during germination. At the mature stage, atcap2-1 plants accumulated vacuolar proteins in the prevacuolar compartment (PVC) instead of the LV. In wild-type plants, AtCAP2 localizes to the PVC as a peripheral membrane protein and in the PVC compartment recruits glyceraldehyde-3-phosphate dehydrogenase C2 (GAPC2) to the PVC. We propose that AtCAP2 contributes to LV biogenesis during early plant development by supporting the trafficking of specific proteins involved in the PSV-to-LV transition and LV acidification during early stages of plant development.

Dual-targeting of Arabidopsis DMP1 isoforms to the tonoplast and the plasma membrane

The reports of dual-targeted proteins in plants have steadily increased over the past years. The vast majority of these proteins are soluble proteins distributed between compartments of the non-secretory pathway, predominantly chloroplasts and mitochondria. In contrast, dual-targeted transmembrane proteins, especially of the secretory pathway, are rare and the mechanisms leading to their differential targeting remain largely unknown. Here, we report dual-targeting of the Arabidopsis DUF679 Membrane Protein 1 (DMP1) to the tonoplast (TP) and the plasma membrane (PM). In Arabidopsis and tobacco two equally abundant DMP1 isoforms are synthesized by alternative translation initiation: a full length protein, DMP1.1, and a truncated one, DMP1.2, which lacks the N-terminal 19 amino acids including a TP-targeting dileucine motif. Accumulation of DMP1.1 and DMP1.2 in the TP and the PM, respectively, is Brefeldin A-sensitive, indicating transit via the Golgi. However, DMP1.2 interacts with DMP1.1, leading to extensive rerouting of DMP1.2 to the TP and “eclipsed” localization of DMP1.2 in the PM where it is barely visible by confocal laser scanning microscopy but clearly detectable by membrane fractionation. It is demonstrated that eGFP fusion to either DMP1 terminus can cause mistargeting artifacts: C-terminal fusion to DMP1.1 or DMP1.2 results in altered ER export and N-terminal fusion to DMP1.1 causes mistargeting to the PM, presumably by masking of the TP targeting signal. These results illustrate how the interplay of alternative translation initiation, presence or absence of targeting information and rerouting due to protein-protein interaction determines the ultimate distribution of a transmembrane protein between two membranes.

Allelic differences in a vacuolar invertase affect Arabidopsis growth at early plant development

Improving carbon fixation in order to enhance crop yield is a major goal in plant sciences. By quantitative trait locus (QTL) mapping, it has been demonstrated that a vacuolar invertase (vac-Inv) plays a key role in determining the radical length in Arabidopsis. In this model, variation in vac-Inv activity was detected in a near isogenic line (NIL) population derived from a cross between two divergent accessions: Landsberg erecta (Ler) and Cape Verde Island (CVI), with the CVI allele conferring both higher Inv activity and longer radicles. The aim of the current work is to understand the mechanism(s) underlying this QTL by analyzing structural and functional differences of vac-Inv from both accessions. Relative transcript abundance analyzed by quantitative real-time PCR (qRT-PCR) showed similar expression patterns in both accessions; however, DNA sequence analyses revealed several polymorphisms that lead to changes in the corresponding protein sequence. Moreover, activity assays revealed higher vac-Inv activity in genotypes carrying the CVI allele than in those carrying the Ler allele. Analyses of purified recombinant proteins showed a similar K m for both alleles and a slightly higher V max for that of Ler. Treatment of plant extracts with foaming to release possible interacting Inv inhibitory protein(s) led to a large increase in activity for the Ler allele, but no changes for genotypes carrying the CVI allele. qRT-PCR analyses of two vac-Inv inhibitors in seedlings from parental and NIL genotypes revealed different expression patterns. Taken together, these results demonstrate that the vac-Inv QTL affects root biomass accumulation and also carbon partitioning through a differential regulation of vac-Inv inhibitors at the mRNA level.

Vacuolar Sorting Receptor-Mediated Trafficking of Soluble Vacuolar Proteins in Plant Cells

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.

Targeting of tonoplast proteins to the vacuole

Vacuoles are essential for plant growth and development, and are dynamic compartments that require constant deposition of integral membrane proteins. These membrane proteins carry out many critical functions of the vacuole such as transporting ions and metabolites for vacuolar storage. Understanding the mechanisms for targeting proteins to the vacuolar membrane, or tonoplast, is important for developing novel applications for biotechnology. The mechanisms to target tonoplast proteins to the vacuole are quite complex. Multiple routes, including both Golgi-dependent and Golgi-independent mechanisms, have been implicated in tonoplast protein trafficking. A few endomembrane proteins that regulate this traffic at the level of the endoplasmic reticulum, the pre-vacuolar compartment and the tonoplast are now known. Recent reports indicate that the Golgi-dependent and independent pathways may merge at the level of the pre-vacuolar compartment. Finally, the small GTP-binding protein Rab7 and the SNARE protein SYP21 have been implicated in the traffic of tonoplast proteins from the pre-vacuolar compartment to the tonoplast. With multiple cargo proteins being analyzed under a variety of experimental systems, a clearer picture for targeting mechanisms for tonoplast proteins is starting to emerge.

Trafficking of plant vacuolar invertases: from a membrane-anchored to a soluble status. Understanding sorting information in their complex N-terminal motifs

Vacuolar invertases (VIs) are highly expressed in young tissues and organs. They may have a substantial regulatory influence on whole-plant metabolism as well as on photosynthetic efficiency. Therefore, they are emerging as potentially interesting biotechnological targets to increase plant biomass production, especially under stress. On the one hand, VIs are well known as soluble and extractable proteins. On the other hand, they contain complex N-terminal propeptide (NTPP) regions with a basic region (BR) and a transmembrane domain (TMD). Here we analyzed in depth the Arabidopsis thaliana VI2 (AtVI2) NTPP by mutagenesis. It was found that correct sorting to the lytic vacuole (LV) depends on the presence of intact dileucine (SSDALLPIS), BR (RRRR) and TMD motifs. AtVI2 remains inserted into membranes on its way to the LV, and the classical sorting pathway (endoplasmic reticulum→Golgi→LV) is followed. However, our data suggest that VIs might follow an alternative, adaptor protein 3 (AP3)-dependent route as well. Membrane-anchored transport and a direct recognition of the dileucine motif in the NTPP of VIs might have evolved as a simple and more efficient sorting mechanism as compared with the vacuolar sorting receptor 1/binding protein of 80 kDa (VSR1/BP80)-dependent sorting mechanism followed by those proteins that travel to the vacuole as soluble proteins.

Arabidopsis μ-adaptin subunit AP1M of adaptor protein complex 1 mediates late secretory and vacuolar traffic and is required for growth

Adaptor protein (AP) complexes are the predominant coat proteins of membrane vesicles in post-Golgi trafficking of mammalian cells. Each AP complex contains a specific medium subunit, μ-adaptin, that selects cargo proteins bearing sequence-specific sorting motifs. Much less is known about the AP complexes and their μ subunits in plants. Because of uncertain homology, the μ-adaptins of Arabidopsis have been designated muA through muD [Happel et al. (2004) Plant J 37(5):678-693]. Furthermore, only muD has been assigned to a specific AP complex, AP-3, involved in Golgi-vacuolar trafficking [Niihama et al. (2009) Plant Cell Physiol 50(12):2057-2068, Zwiewka et al. (2011) Cell Res 21(12):1711-1722, and Wolfenstetter et al. (2012) Plant Cell 24(1):215-232]. In contrast, the μ subunit of neither the post-Golgi trafficking AP-1 complex nor the endocytic AP-2 complex has been identified. Here, we report the functional analysis of redundant AP-1 μ-adaptins AP1M1 (also known as muB1) and AP1M2 (also known as muB2). Coimmunoprecipitation revealed that both AP1M2 and its less strongly expressed isoform AP1M1 are complexed with the large subunit γ-adaptin of AP-1. In addition, AP1M2 was localized at or near the trans-Golgi network. Knockout mutations of AP1M2 impaired pollen function and arrested plant growth whereas the ap1m1 ap1m2 double mutant was nearly pollen-lethal. At the cellular level, the absence of AP1M2 entailed inhibition of multiple trafficking pathways from the trans-Golgi network to the vacuole and to the plasma membrane in interphase and to the plane of cell division in cytokinesis. Thus, AP-1 is crucial in post-Golgi trafficking in plant cells and required for cell division and plant growth.

ScRALF3, a secreted RALF-like peptide involved in cell-cell communication between the sporophyte and the female gametophyte in a solanaceous species

Small peptides have been shown to regulate numerous aspects of plant development through cell-cell communication. These signaling events are particularly important during reproduction, regulating gamete development and embryogenesis. Rapid alkalinization factor (RALF)-like genes, a large gene family that encodes secreted peptides, have specific or ubiquitous expression patterns. Previously, five RALF-like genes with potential involvement during reproduction were isolated from Solanum chacoense. Here, we show that ScRALF3 is an important peptide regulator of female gametophyte development. Its expression, which is auxin-inducible, is strictly regulated before and after fertilization. Down-regulation of ScRALF3 expression by RNA interference leads to the production of smaller fruits that produce fewer seeds, due to improper development of the embryo sacs. Defects include loss of embryo sac nuclei polarization, as well as an increase in asynchronous division, accounting for cellular dysfunctions and premature embryo sac development arrest during megagametogenesis. ScRALF3 is expressed in the sporophytic tissue surrounding the embryo sac, the integument and the nucellus, as revealed by in situ hybridization and GUS staining. As expected for a secreted peptide, fluorescence from an ScRALF3-GFP fusion construct is detected throughout the secretory pathway. Therefore, the ScRALF3 secreted peptide may be directly involved in the regulation of multiple aspects of cell-cell communication between the female gametophyte and its surrounding sporophytic tissue during ovule development.

Functional identification of sorting receptors involved in trafficking of soluble lytic vacuolar proteins in vegetative cells of Arabidopsis

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 (Arabidopsis thaliana) 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, the identity of VSRs for soluble proteins of the lytic vacuole in vegetative cells remains controversial. 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, and rmr1rmr2 and rmr3rmr4 double mutants, soluble lytic vacuolar (Arabidopsis aleurain-like protein:green fluorescent protein [GFP] and carboxypeptidase Y:GFP and PSV (phaseolin) proteins, but not the vacuolar membrane protein Arabidopsis βFructosidase4:GFP, exhibited defects in their trafficking; they accumulated to the endoplasmic reticulum 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 Arabidopsis aleurain-like protein: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.

Trafficking of vacuolar proteins: the crucial role of Arabidopsis vacuolar protein sorting 29 in recycling vacuolar sorting receptor

The retromer is involved in recycling lysosomal sorting receptors in mammals. A component of the retromer complex in Arabidopsis thaliana, vacuolar protein sorting 29 (VPS29), plays a crucial role in trafficking storage proteins to protein storage vacuoles. However, it is not known whether or how vacuolar sorting receptors (VSRs) are recycled from the prevacuolar compartment (PVC) to the trans-Golgi network (TGN) during trafficking to the lytic vacuole (LV). Here, we report that VPS29 plays an essential role in the trafficking of soluble proteins to the LV from the TGN to the PVC. maigo1-1 (mag1-1) mutants, which harbor a knockdown mutation in VPS29, were defective in trafficking of two soluble proteins, Arabidopsis aleurain-like protein (AALP):green fluorescent protein (GFP) and sporamin:GFP, to the LV but not in trafficking membrane proteins to the LV or plasma membrane or via the secretory pathway. AALP:GFP and sporamin:GFP in mag1-1 protoplasts accumulated in the TGN but were also secreted into the medium. In mag1-1 mutants, VSR1 failed to recycle from the PVC to the TGN; rather, a significant proportion was transported to the LV; VSR1 overexpression rescued this defect. Moreover, endogenous VSRs were expressed at higher levels in mag1-1 plants. Based on these results, we propose that VPS29 plays a crucial role in recycling VSRs from the PVC to the TGN during the trafficking of soluble proteins to the LV.