Membrane localization of Arabidopsis acyl-CoA binding protein ACBP2

Cytosolic acyl-CoA binding proteins bind long-chain acyl-CoAs and act as intracellular acyl-CoA transporters and pool formers. Recently, we have characterized Arabidopsis thaliana cDNAs encoding novel forms of ACBP, designated ACBP1 and ACBP2, that contain a hydrophobic domain at the N-terminus and show conservation at the acyl-CoA binding domain to cytosolic ACBPs. We have previously demonstrated that ACBP1 is membrane-associated in Arabidopsis. Here, western blot analysis of anti-ACBP2 antibodies on A. thaliana protein showed that ACBP2 is located in the microsome-containing membrane fraction and in the subcellular fraction containing large particles (mitochondria, chloroplasts and peroxisomes), resembling the subcellular localization of ACBP1. To further investigate the subcellular localization of ACBP2, we fused ACBP2 translationally in-frame to GFP. By means of particle gene bombardment, ACBP2-GFP and ACBP1-GFP fusion proteins were observed transiently expressed at the plasma membrane and at the endoplasmic reticulum in onion epidermal cells. GFP fusions with deletion derivatives of ACBP1 or ACBP2 lacking the transmembrane domain were impaired in membrane targeting. Our investigations also showed that when the transmembrane domain of ACBP1 or that of ACBP2 was fused with GFP, the fusion protein was targeted to the plasma membrane, thereby establishing their role in membrane targeting. The localization of ACBP1-GFP is consistent with our previous observations using immunoelectron microscopy whereby ACBP1 was localized to the plasma membrane and vesicles. We conclude that ACBP2, like ACBP1, is a membrane protein that likely functions in membrane-associated acyl-CoA transfer/metabolism.     

The Golgi-localized Arabidopsis endomembrane protein12 contains both endoplasmic reticulum export and Golgi retention signals at its C terminus

Endomembrane proteins (EMPs), belonging to the evolutionarily conserved transmembrane nine superfamily in yeast and mammalian cells, are characterized by the presence of a large lumenal N terminus, nine transmembrane domains, and a short cytoplasmic tail. The Arabidopsis thaliana genome contains 12 EMP members (EMP1 to EMP12), but little is known about their protein subcellular localization and function. Here, we studied the subcellular localization and targeting mechanism of EMP12 in Arabidopsis and demonstrated that (1) both endogenous EMP12 (detected by EMP12 antibodies) and green fluorescent protein (GFP)-EMP12 fusion localized to the Golgi apparatus in transgenic Arabidopsis plants; (2) GFP fusion at the C terminus of EMP12 caused mislocalization of EMP12-GFP to reach post-Golgi compartments and vacuoles for degradation in Arabidopsis cells; (3) the EMP12 cytoplasmic tail contained dual sorting signals (i.e., an endoplasmic reticulum export motif and a Golgi retention signal that interacted with COPII and COPI subunits, respectively); and (4) the Golgi retention motif of EMP12 retained several post-Golgi membrane proteins within the Golgi apparatus in gain-of-function analysis. These sorting signals are highly conserved in all plant EMP isoforms and, thus, likely represent a general mechanism for EMP targeting in plant cells.     

Identification of a vacuolar sucrose transporter in barley and Arabidopsis mesophyll cells by a tonoplast proteomic approach

The vacuole is the main cellular storage pool, where sucrose (Suc) accumulates to high concentrations. While a limited number of vacuolar membrane proteins, such as V-type H(+)-ATPases and H(+)-pyrophosphatases, are well characterized, the majority of vacuolar transporters are still unidentified, among them the transporter(s) responsible for vacuolar Suc uptake and release. In search of novel tonoplast transporters, we used a proteomic approach, analyzing the tonoplast fraction of highly purified mesophyll vacuoles of the crop plant barley (Hordeum vulgare). We identified 101 proteins, including 88 vacuolar and putative vacuolar proteins. The Suc transporter (SUT) HvSUT2 was discovered among the 40 vacuolar proteins, which were previously not reported in Arabidopsis (Arabidopsis thaliana) vacuolar proteomic studies. To confirm the tonoplast localization of this Suc transporter, we constructed and expressed green fluorescent protein (GFP) fusion proteins with HvSUT2 and its closest Arabidopsis homolog, AtSUT4. Transient expression of HvSUT2-GFP and AtSUT4-GFP in Arabidopsis leaves and onion (Allium cepa) epidermal cells resulted in green fluorescence at the tonoplast, indicating that these Suc transporters are indeed located at the vacuolar membrane. Using a microcapillary, we selected mesophyll protoplasts from a leaf protoplast preparation and demonstrated unequivocally that, in contrast to the companion cell-specific AtSUC2, HvSUT2 and AtSUT4 are expressed in mesophyll protoplasts, suggesting that HvSUT2 and AtSUT4 are involved in transport and vacuolar storage of photosynthetically derived Suc.     

Trafficking pathways of Cx49-GFP in living mammalian cells

In the present study we examined the trafficking pathways of connexin49 (Cx49) fused to green fluorescent protein (GFP) in polar and non-polar cell lines. The Cx49 gene was isolated from ovine lens by RT-PCR. Cx49 cDNA was fused to GFP and the hybrid cDNA was transfected into several cell lines. After transfection of Cx49-GFP cDNA into HeLa cells, it was shown using the double whole-cell patch-clamp technique that the expressed fusion protein was still able to form conducting gap junction channels. Synthesis, assembly, and turnover of the Cx49-GFP hybrid protein were investigated using a pulse-chase protocol. A major 78-kDa protein band corresponding to Cx49-GFP could be detected with a turnover of 16-20 h and a half-life time of 10 h. The trafficking pathways of Cx49-GFP were monitored by confocal laser microscopy. Fusion proteins were localized in subcellular compartments, including the endoplasmic reticulum (ER), the ER-Golgi intermediate compartment, the Golgi apparatus, and the trans-Golgi network, as well as vesicles traveling towards the plasma membrane. Time-dependent sequential localization of Cx49-GFP in the ER and then the Golgi apparatus supports the notion of a slow turnover of Cx49-GFP compared to other connexins analyzed so far. Gap junction plaques resembling the usual punctuate distribution pattern could be demonstrated for COS-1 and MDCK cells. Basolateral distribution of Cx49-GFP was observed in polar MDCK cells, indicating specific sorting behavior of Cx49 in polarized cells. Together, this report describes the first characterization of biosynthesis and trafficking of lens Cx49.     

A plastid envelope location of Arabidopsis ent-kaurene oxidase links the plastid and endoplasmic reticulum steps of the gibberellin biosynthesis pathway

We have used fusions of gibberellin biosynthesis enzymes to green fluorescent protein (GFP) to determine the subcellular localization of the early steps of the pathway. Gibberellin biosynthesis from geranylgeranyl diphosphate is catalysed by enzymes of the terpene cyclase, cytochrome P450 mono-oxygenase and 2-oxoglutarate-dependent dioxygenase classes. We show that the N-terminal pre-sequences of the Arabidopsis thaliana terpene cyclases copalyl diphosphate synthase (AtCPS1) and ent-kaurene synthase (AtKS1) direct GFP to chloroplasts in transient assays following microprojectile bombardment of tobacco leaves. The AtKS1-GFP fusion is also imported by isolated pea chloroplasts. The N-terminal portion of the cytochrome P450 protein ent-kaurene oxidase (AtKO1) directs GFP to chloroplasts in tobacco leaf transient assays. Chloroplast import assays with 35S-labelled AtKO1 protein show that it is targeted to the outer face of the chloroplast envelope. The leader sequences of the two ent-kaurenoic acid oxidases (AtKAO1 and AtKAO2) from Arabidopsis direct GFP to the endoplasmic reticulum. These data suggest that the AtKO1 protein links the plastid- and endoplasmic reticulum-located steps of the gibberellin biosynthesis pathway by association with the outer envelope of the plastid.     

Oleosin expression and trafficking during oil body biogenesis in tobacco leaf cells

We have established a versatile method for studying the interaction of the oleosin gene product with oil bodies during oil body biogenesis in plants. Our approach has been to transiently express a green fluorescent protein (GFP)-tagged Arabidopsis oleosin gene fusion in tobacco leaf cells containing bona fide oil bodies and then to monitor oleosin-GFP expression using real-time confocal laser scanning microscopy. We show that normally non-oil-storing tobacco leaf cells are able to synthesize and then transport oleosin-GFP fusion protein to leaf oil bodies. Synthesis and transport of oleosin-GFP fusion protein to oil bodies occurred within the first 6 h posttransformation. Oleosin-GFP fusion protein exclusively associated with the endoplasmic reticulum and was trafficked in a Golgi-independent manner at speeds approaching 0.5 microm sec(-1) along highly dynamic endoplasmic reticulum positioned over essentially static polygonal cortical endoplasmic reticulum. Our data indicate that oil body biogenesis can occur outside of the embryo and that oleosin-GFP can be used to monitor early events in oil body biogenesis in real-time.     

Reassembly of peroxisomes in Hansenula polymorpha pex3 cells on reintroduction of Pex3p involves the nuclear envelope

The reassembly of peroxisomes in Hansenula polymorpha pex3 cells on reintroduction of Pex3p was examined. Using a Pex3-green fluorescent protein (Pex3-GFP) fusion protein, expressed under the control of an inducible promoter, it was observed that, initially on induction of Pex3-GFP synthesis, GFP fluorescence was localized to the endoplasmic reticulum and the nuclear envelope. Subsequently, a single organelle developed per cell that increased in size and multiplied by division. At these stages, GFP fluorescence was confined to peroxisomes. Fractionation experiments on homogenates of pex3 cells, in which the endoplasmic reticulum and nuclear envelope were marked with GFP, identified a small amount of GFP in peroxisomes present in the initial stage of peroxisome reassembly. Our data suggest a crucial role for the endoplasmic reticulum/nuclear envelope in peroxisome reintroduction on complementation of pex3 cells by the PEX3 gene.     

Tracking Glideosome-associated protein 50 reveals the development and organization of the inner membrane complex of Plasmodium falciparum

The most deadly of the human malaria parasites, Plasmodium falciparum, has different stages specialized for invasion of hepatocytes, erythrocytes, and the mosquito gut wall. In each case, host cell invasion is powered by an actin-myosin motor complex that is linked to an inner membrane complex (IMC) via a membrane anchor called the glideosome-associated protein 50 (PfGAP50). We generated P. falciparum transfectants expressing green fluorescent protein (GFP) chimeras of PfGAP50 (PfGAP50-GFP). Using immunoprecipitation and fluorescence photobleaching, we show that C-terminally tagged PfGAP50-GFP can form a complex with endogenous copies of the linker protein PfGAP45 and the myosin A tail domain-interacting protein (MTIP). Full-length PfGAP50-GFP is located in the endoplasmic reticulum in early-stage parasites and then redistributes to apical caps during the formation of daughter merozoites. In the final stage of schizogony, the PfGAP50-GFP profile extends further around the merozoite surface. Three-dimensional (3D) structured illumination microscopy reveals the early-stage IMC as a doubly punctured flat ellipsoid that separates to form claw-shaped apposed structures. A GFP fusion of PfGAP50 lacking the C-terminal membrane anchor is misdirected to the parasitophorous vacuole. Replacement of the acid phosphatase homology domain of PfGAP50 with GFP appears to allow correct trafficking of the chimera but confers a growth disadvantage.     

Mechanisms and effects of retention of over-expressed aquaporin AtPIP2;1 in the endoplasmic reticulum

Plasma membrane intrinsic proteins (PIPs) are aquaporins that mediate water transport across the plant plasma membrane (PM). The present work addresses, using Arabidopsis AtPIP2;1 as a model, the mechanisms and significance of trafficking of newly synthesized PIPs from the endoplasmic reticulum (ER) to the Golgi apparatus. A functional diacidic export motif (Asp4-Val5-Glu6) was identified in the N-terminal tail of AtPIP2;1, using expression in transgenic Arabidopsis of site-directed mutants tagged with the green fluorescent protein (GFP). Confocal fluorescence imaging and a novel fluorescence recovery after photobleaching application based on the distinct diffusion of PM and intracellular AtPIP2;1-GFP forms revealed a retention in the ER of diacidic mutated forms, but with quantitative differences. Thus, the individual role of the two acidic Asp4 and Glu6 residues was established. In addition, expression in transgenic Arabidopsis of ER-retained AtPIP2;1-GFP constructs reduced the root hydraulic conductivity. Co-expression of AtPIP2;1-GFP and AtPIP1;4-mCherry constructs suggested that ER-retained AtPIP2;1-GFP may interact with other PIPs to hamper their trafficking to the PM, thereby contributing to inhibition of root cell hydraulic conductivity.     

Life with and without AtTIP1;1, an Arabidopsis aquaporin preferentially localized in the apposing tonoplasts of adjacent vacuoles

The Arabidopsis thaliana Tonoplast Intrinsic Protein 1;1 (AtTIP1;1) is a member of the tonoplast aquaporin family. The tissue-specific expression pattern and intracellular localization of AtTIP1;1 were characterized using GUS and GFP fusion genes. Results indicate that AtTIP1;1 is expressed in almost all cell types with the notable exception of meristematic cells. The highest level of AtTIP1;1 expression was detected in vessel-flanking cells in vascular bundles. AtTIP1;1-GFP fusion protein labelled the tonoplast of the central vacuole and other smaller peripheral vacuoles. The fusion protein was not found evenly distributed along the tonoplast continuum but concentrated in contact zones of tonoplasts from adjacent vacuoles and in invaginations of the central vacuole. Such invaginations may result from partially engulfed small vacuoles. A knockout mutant was isolated and characterized to gain insight into AtTIP1;1 function. No phenotypic alteration was found under optimal growth conditions indicating that AtTIP1;1 function is not essential to the plant and that some members of the TIP family may act redundantly to facilitate water flow across the tonoplast. However, a conditional root phenotype was observed when mutant plants were grown on a glycerol-containing medium.     

Routes to the tonoplast: the sorting of tonoplast transporters in Arabidopsis mesophyll protoplasts

Vacuoles perform a multitude of functions in plant cells, including the storage of amino acids and sugars. Tonoplast-localized transporters catalyze the import and release of these molecules. The mechanisms determining the targeting of these transporters to the tonoplast are largely unknown. Using the paralogous Arabidopsis thaliana inositol transporters INT1 (tonoplast) and INT4 (plasma membrane), we performed domain swapping and mutational analyses and identified a C-terminal di-leucine motif responsible for the sorting of higher plant INT1-type transporters to the tonoplast in Arabidopsis mesophyll protoplasts. We demonstrate that this motif can reroute other proteins, such as INT4, SUCROSE TRANSPORTER2 (SUC2), or SWEET1, to the tonoplast and that the position of the motif relative to the transmembrane helix is critical. Rerouted INT4 is functionally active in the tonoplast and complements the growth phenotype of an int1 mutant. In Arabidopsis plants defective in the β-subunit of the AP-3 adaptor complex, INT1 is correctly localized to the tonoplast, while sorting of the vacuolar sucrose transporter SUC4 is blocked in cis-Golgi stacks. Moreover, we demonstrate that both INT1 and SUC4 trafficking to the tonoplast is sensitive to brefeldin A. Our data show that plants possess at least two different Golgi-dependent targeting mechanisms for newly synthesized transporters to the tonoplast.     

Redistribution of membrane proteins between the Golgi apparatus and endoplasmic reticulum in plants is reversible and not dependent on cytoskeletal networks

We have fused the signal anchor sequences of a rat sialyl transferase and a human galactosyl transferase along with the Arabidopsis homologue of the yeast HDEL receptor (AtERD2) to the jellyfish green fluorescent protein (GFP) and transiently expressed the chimeric genes in tobacco leaves. All constructs targeted the Golgi apparatus and co-expression with DsRed fusions along with immunolabelling of stably transformed BY2 cells indicated that the fusion proteins located all Golgi stacks. Exposure of tissue to brefeldin A (BFA) resulted in the reversible redistribution of ST-GFP into the endoplasmic reticulum. This effect occurred in the presence of a protein synthesis inhibitor and also in the absence of microtubules or actin filaments. Likewise, reformation of Golgi stacks on removal of BFA was not dependent on either protein synthesis or the cytoskeleton. These data suggest that ER to Golgi transport in the cell types observed does not require cytoskeletal-based mechanochemical motor systems. However, expression of an inhibitory mutant of Arabidopsis Rab 1b (AtRab1b(N121I) significantly slowed down the recovery of Golgi fluorescence in BFA treated cells indicating a role for Rab1 in regulating ER to Golgi anterograde transport.     

Loss of XRN4 function can trigger cosuppression in a sequence-dependent manner

OLE1 encodes an oleosin isoprotein, a major membrane protein of the lipid-reserve organelle in seeds known as the oil body. Transgenic Arabidopsis were generated to contain an artificial chimeric transgene composed of OLE1 and green fluorescent protein (GFP). Overexpression of the fusion protein allowed visualization of the oil body size and structure in living cells using fluorescence microscopy. Two mutants, xrn4-8(OleG) and xrn4-9(OleG), accumulating enlarged oil bodies with reduced GFP fluorescence were isolated from the mutagenized progeny of a transgenic plant. Both mutants contained a defect in EXORIBONUCLEASE4 (XRN4), a gene known to encode a ribonuclease that specifically degrades uncapped mRNAs. Transgene expression was silenced in these mutants, as demonstrated by the reduced levels of the transgene mRNA and its product, OLE1-GFP. XRN4 loss of function also triggered cosuppression, i.e. simultaneous reduction in expression of the transgene and an endogenous OLE1 gene that shared a region of identical sequence. The enlarged oil bodies exhibiting reduced GFP fluorescence were formed in the xrn4-8(OleG) and xrn4-9(OleG) mutants due to the reduction of the endogenous OLE1 and the transgene product, OLE1-GFP, respectively. Cosuppression triggered by the xrn4 mutation also occurs for other genes such as PYK10, which encodes an endoplasmic reticulum (ER) body-resident β-glucosidase. The overall results indicate that a loss of XRN4 function can potentially trigger the cosuppression in a sequence-dependent manner.     

Vacuolar sulfate transporters are essential determinants controlling internal distribution of sulfate in Arabidopsis

Uptake of external sulfate from the environment and use of internal vacuolar sulfate pools are two important aspects of the acquisition of sulfur for metabolism. In this study, we demonstrated that the vacuolar SULTR4-type sulfate transporter facilitates the efflux of sulfate from the vacuoles and plays critical roles in optimizing the internal distribution of sulfate in Arabidopsis thaliana. SULTR4;1-green fluorescent protein (GFP) and SULTR4;2-GFP fusion proteins were expressed under the control of their own promoters in transgenic Arabidopsis. The fusion proteins were accumulated specifically in the tonoplast membranes and were localized predominantly in the pericycle and xylem parenchyma cells of roots and hypocotyls. In roots, SULTR4;1 was constantly accumulated regardless of the changes of sulfur conditions, whereas SULTR4;2 became abundant by sulfur limitation. In shoots, both transporters were accumulated by sulfur limitation. Vacuoles isolated from callus of the sultr4;1 sultr4;2 double knockout showed excess accumulation of sulfate, which was substantially decreased by overexpression of SULTR4;1-GFP. In seedlings, the supplied [(35)S]sulfate was retained in the root tissue of the sultr4;1 sultr4;2 double knockout mutant. Comparison of the double and single knockouts suggested that SULTR4;1 plays a major role and SULTR4;2 has a supplementary function. Overexpression of SULTR4;1-GFP significantly decreased accumulation of [(35)S]sulfate in the root tissue, complementing the phenotype of the double mutant. These results suggested that SULTR4-type transporters, particularly SULTR4;1, actively mediate the efflux of sulfate from the vacuole lumen into the cytoplasm and influence the capacity for vacuolar storage of sulfate in the root tissue. The efflux function will promote rapid turnover of sulfate from the vacuoles particularly in the vasculature under conditions of low-sulfur supply, which will optimize the symplastic (cytoplasmic) flux of sulfate channeled toward the xylem vessels.     

Plant-specific insertions in the soybean aspartic proteinases, soyAP1 and soyAP2, perform different functions of vacuolar targeting

Most aspartic proteinases (APs) of plant origin are characterized by the presence of plant-specific insertion (PSI) in their primary structure. PSI has been reported to function as signals for both transport of AP molecules from the endoplasmic reticulum (ER) and for their targeting to the vacuole. To determine the functions of the PSIs in soyAP1 and soyAP2 identified in our previous study, we examined their subcellular localization by transient expression of a green fluorescent protein (GFP) fusion protein in the protoplasts of Arabidopsis suspension-cultured cells. Both soyAP1-GFP and soyAP2-GFP were targeted to the vacuole. To confirm the role of the PSI, we prepared PSI-deleted soyAP1 and soyAP2, and investigated their vacuolar targeting by the same method. While the former deletion mutant was always transported to the vacuole, the latter sometimes remained in the ER and was only sometimes transported to the vacuole. These observations indicated that, in the case of soyAP1, the PSI is not involved in vacuolar targeting, also suggesting that the function of the PSI differs depending on its origin.     

How are tonoplast proteins degraded?

Protein turnover is fundamental both for development and cellular homeostasis. The mechanisms responsible for the turnover of integral membrane proteins in plant cells are however still largely unknown. Recently, considerable attention has been devoted to the degradation of plasma membrane proteins. We have now studied the turnover of a tonoplast protein, the potassium channel TPK1, in fully differentiated Arabidopsis leaf cells and showed that its degradation occurs upon internalization into the vacuole. Here, we discuss the possible mechanisms and triggering events involved.     

FMDV-2A sequence and protein arrangement contribute to functionality of CYP2B1-reporter fusion protein

Two optimized forms of green fluorescence proteins (GFP), enhanced GFP (EGFP) and humanized Renilla GFP (hrGFP), were used to track expression of cytochrome P450 2B1 (CYP2B1), an endoplasmic reticulum membrane-bound protein. In transiently expressing HEK293 cells we show that CYP2B1-GFP fusion proteins are stable and functional, whereas the vice-versa-arranged GFP-CYP2B1 fusions are not. The CYP2B1-hrGFP fusion protein is characterized by reduction in mean fluorescence intensity (MFI) to less than 20% of that of the hrGFP protein alone, accompanied by a 50% loss of CYP2B1 activity. Exchanging the linker for an alpha-helical peptide structure between CYP2B1 and hrGFP does not improve fusion protein activity. Insertion of a short linker (five amino acids) increases reporter protein fluorescence intensity twofold without improving CYP2B1 activity. Introduction of the foot and mouth disease virus 2A sequence providing cotranslational cleavage led to an unstable hrGFP-2A protein, whereas the corresponding EGFP-2A protein was stable and yielded an MFI superior to those of all other fusion constructs tested. CYP2B1 activity of the EGFP-2A-CYP2B1 protein was in the range of that of the unmodified CYP2B1. These data indicate that the protein arrangement EGFP-2A-CYP2B1 is superior to others, since it is most active and visible, which is essential for an effective tracking of the CYP2B1 enzyme.     

Phosphate availability affects the tonoplast localization of PLDzeta2, an Arabidopsis thaliana phospholipase D

Under phosphate deprivation, higher plants change their lipid composition and recycle phosphate from phospholipids. A phospholipase D, PLDzeta2, is involved in this recycling and in other cellular functions related to plant development. We investigated the localization of Arabidopsis PLDzeta2 by cell fractionation and in vivo GFP confocal imaging. AtPLDzeta2 localizes to the tonoplast and the Nter regulatory domain is sufficient for its sorting. Under phosphate deprivation, AtPLDzeta2 remains located in the tonoplast but its distribution is uneven. We observed PLDzeta2-enriched tonoplast domains preferentially positioned close to mitochondria and beside chloroplasts. In absence of PLDzeta2, membrane developments were visualized inside vacuoles.     

The destination for single-pass membrane proteins is influenced markedly by the length of the hydrophobic domain

The tonoplast was proposed as a default destination of membrane-bound proteins without specific targeting signals. To investigate the nature of this targeting, we created type I fusion proteins with green fluorescent protein followed by the transmembrane domain of the human lysosomal protein LAMP1. We varied the length of the transmembrane domain from 23 to either 20 or 17 amino acids by deletion within the hydrophobic domain. The resulting chimeras, called TM23, TM20, and TM17, were expressed either transiently or stably in tobacco. TM23 clearly accumulated in the plasmalemma, as confirmed by immunoelectron microscopy. In contrast, TM17 clearly was retained in the endoplasmic reticulum, and TM20 accumulated in small mobile structures. The nature of the TM20-labeled compartments was investigated by coexpression with a marker localized mainly in the Golgi apparatus, AtERD2, fused to a yellow fluorescent protein. The strict colocalization of both fluorescent proteins indicated that TM20 accumulated in the Golgi apparatus. To further test the default destination of type I membrane proteins, green fluorescent protein was fused to the 19-amino acid transmembrane domain of the plant vacuolar sorting receptor BP-80. The resulting chimera also accumulated in the Golgi instead of in post-Golgi compartments, where native BP-80 localized. Additionally, when the transmembrane domain of BP-80 was lengthened to 22 amino acids, the reporter escaped the Golgi and accumulated in the plasma membrane. Thus, the tonoplast apparently is not a favored default destination for type I membrane proteins in plants. Moreover, the target membrane where the chimera concentrates is not unique and depends at least in part on the length of the membrane-spanning domain.     

A vacuolar carboxypeptidase mutant of Arabidopsis thaliana is degraded by the ERAD pathway independently of its N-glycan

Misfolded proteins produced in the endoplasmic reticulum (ER) are degraded by a mechanism, the ER-associated degradation (ERAD). Here we report establishment of the experimental system to analyze the ERAD in plant cells. Carboxypeptidase Y (CPY) is a vacuolar enzyme and its mutant CPY∗ is degraded by the ERAD in yeast. Since Arabidopsis thaliana has AtCPY, an ortholog of yeast CPY, we constructed and expressed fusion proteins consisting of AtCPY and GFP and of AtCPY∗, which carries a mutation homologous to yeast CPY∗, and GFP in A. thaliana cells. While AtCPY-GFP was efficiently transported to the vacuole, AtCPY∗-GFP was retained in the ER to be degraded in proteasome- and Cdc48-dependent manners. We also found that AtCPY∗-GFP was degraded by the ERAD in yeast cells, but that its single N-glycan did not function as a degradation signal in yeast or plant cells. Therefore, AtCPY∗-GFP can be used as a marker protein to analyze the ERAD pathway, likely for nonglycosylated substrates, in plant cells.