The International Barley Sequencing Consortium—At the Threshold of Efficient Access to the Barley Genome

Arabidopsis (Arabidopsis thaliana) tryptophan-proline-proline (WPP)-domain proteins, WPP1 and WPP2, are plant-unique, nuclear envelope-associated proteins of unknown function. They have sequence similarity to the nuclear envelope-targeting domain of plant RanGAP1, the GTPase activating protein of the small GTPase Ran. WPP domain-interacting tail-anchored protein 1 (WIT1) and WIT2 are two Arabidopsis proteins containing a coiled-coil domain and a C-terminal predicted transmembrane domain. They are required for RanGAP1 association with the nuclear envelope in root tips. Here, we show that WIT1 also binds WPP1 and WPP2 in planta, we identify the chaperone heat shock cognate protein 70-1 (HSC70-1) as in vivo interaction partner of WPP1 and WPP2, and we show that HSC70-1 interacts in planta with WIT1. WIT1 and green fluorescent protein (GFP)-WIT1 are targeted to the nuclear envelope in Arabidopsis. In contrast, GFP-WIT1 forms large cytoplasmic aggregates when overexpressed transiently in Nicotiana benthamiana leaf epidermis cells. Coexpression of HSC70-1 significantly reduces GFP-WIT1 aggregation and permits association of most GFP-WIT1 with the nuclear envelope. Significantly, WPP1 and WPP2 show the same activity. A WPP1 mutant with reduced affinity for GFP-WIT1 fails to decrease its aggregation. While the WPP-domain proteins act on a region of WIT1 containing the coiled-coil domain, HSC70-1 additionally acts on the C-terminal transmembrane domain. Taken together, our data suggest that both HSC70-1 and the WPP-domain proteins play a role in facilitating WIT1 nuclear envelope targeting, which is, to our knowledge, the first described in planta activity for the WPP-domain proteins.The cytoplasmic Ran GTPase activating protein RanGAP is critical to establishing a functional RanGTP/RanGDP gradient across the nuclear envelope (NE) and is associated with the outer surface of the NE in metazoan and higher plant cells (Matunis et al., 1996; Rose and Meier, 2001). Plant RanGAP1 association with the NE requires a plant-specific targeting domain, named the Trp-Pro-Pro (WPP) domain (Rose and Meier, 2001). Arabidopsis (Arabidopsis thaliana) WPP1 and WPP2 are small (155- and 180-amino-acid residues, respectively) plant-unique proteins of unknown function, which are similar to the WPP domain of RanGAP proteins. WPP1 and WPP2 are located in the cytoplasm, with a concentration at the NE (Patel et al., 2004). They are characterized by a 104-amino-acid-long WPP domain, predicted to consist of a β-strand and three α-helices and shown to be sufficient for NE targeting (Patel et al., 2004). They are also associated with cytoplasmic speckles most likely representing Golgi (Patel et al., 2005). Reduced expression of the WPP protein family causes decreased mitotic activity in roots of Arabidopsis, resulting in shortening of primary roots and decreased number of lateral roots (Patel et al., 2004). RanGAP1 association with the NE in the Arabidopsis root tip requires two families of NE-localized, plant-specific, WPP domain-interacting proteins (WPP domain-interacting protein [WIP] and WPP domain-interacting tail-anchored protein [WIT] families) that are characterized by the presence of a coiled-coil domain and a C-terminal predicted transmembrane domain (TMD; Xu et al., 2007; Zhao et al., 2008). Based on sequence analysis, both the WIP and WIT protein family were classified as putative tail-anchored (TA) proteins, proteins that associate with membranes posttranslationally (Borgese et al., 2003).The heat shock protein 70 family (HSP70) contains both heat-inducible and constitutively expressed members, called heat shock cognate proteins (HSC70). HSC70 chaperones assist in folding newly synthesized proteins (Bukau and Horwich, 1998), are involved in posttranslational translocation of secretory proteins across endoplasmic reticulum (ER) and mitochondrial membranes (Chirico et al., 1988; Deshaies et al., 1988), prevent irreversible aggregation of their substrates (Ngosuwan et al., 2003), and facilitate degradation of misfolded proteins (Meacham et al., 2001). Recently, mammalian HSC70 has also been implied in assisting the membrane insertion of a subset of TA proteins (Abell et al., 2007).The Arabidopsis genome encodes five different cytosolic HSP70s, three of which are expressed constitutively (HSC70-1, HSC70-2, and HSC70-3). While expressed in all organs, Hsc70-1 and Hsc70-2 expression levels are highest in leaves and Hsc70-3 in leaves and roots. All three genes can be further induced by heat shock and cold stress (Sung et al., 2001). Constitutive overexpression of Arabidopsis Hsc70-1 in transgenic plants leads to changes in growth and development, increases thermotolerance (Sung and Guy, 2003), and decreases the plant''s ability to respond to pathogen attack (Noel et al., 2007). Recently, specific interactions of HSC70-1 with SGT1 (for Suppressor of G2 allele of skp1; Noel et al., 2007) and HSC70-3 with turnip mosaic virus RNA-dependent RNA polymerase (Dufresne et al., 2008) were identified, suggesting a role of HSC70 in viral replication and pathogenesis. Both HSC70-1 and HSC70-3 can be detected in the nuclei and the cytoplasm of Nicotiana benthamiana epidermal cells (Noel et al., 2007; Dufresne et al., 2008).Here, we identified Arabidopsis HSC70-1 as an in vivo interaction partner of WPP1 and WPP2 and demonstrated that HSC70-1 associates with WIT1. Using transient expression in N. benthamiana, we show that when expressed at a high level, WIT1 accumulates in large fluorescent bodies in the cytoplasm that may represent aggregates. Upon coexpression in the same system, WPP1, WPP2, and HSC70-1 are all able to prevent the aggregation of overexpressed WIT1 and enable WIT1 association with the NE. While WPP-domain proteins act on a region of WIT1 containing the coiled-coil domain, HSC70-1 additionally acts on the C-terminal TMD. We propose that WPP1 and WPP2 play a chaperone-like role reflected in preventing the aggregation of the coiled-coil region of WIT1 and possibly other coiled-coil TA-type proteins, either in conjunction or independently of HSC70-type chaperones.     

Arabidopsis WPP-domain proteins are developmentally associated with the nuclear envelope and promote cell division

The nuclear envelope (NE) acts as a selective barrier to macromolecule trafficking between the nucleus and the cytoplasm and undergoes a complex reorganization during mitosis. Different eukaryotic kingdoms show specializations in NE function and composition. In contrast with vertebrates, the protein composition of the NE and the function of NE proteins are barely understood in plants. MFP1 attachment factor 1 (MAF1) is a plant-specific NE-associated protein first identified in tomato (Lycopersicon esculentum). Here, we demonstrate that two Arabidopsis thaliana MAF1 homologs, WPP1 and WPP2, are associated with the NE specifically in undifferentiated cells of the root tip. Reentry into cell cycle after callus induction from differentiated root segments reprograms their NE association. Based on green fluorescent protein fusions and immunogold labeling data, the proteins are associated with the outer NE and the nuclear pores in interphase cells and with the immature cell plate during cytokinesis. RNA interference-based suppression of the Arabidopsis WPP family causes shorter primary roots, a reduced number of lateral roots, and reduced mitotic activity of the root meristem. Together, these data demonstrate the existence of regulated NE targeting in plants and identify a class of plant-specific NE proteins involved in mitotic activity.     

Two distinct interacting classes of nuclear envelope-associated coiled-coil proteins are required for the tissue-specific nuclear envelope targeting of Arabidopsis RanGAP

Ran GTPase plays essential roles in multiple cellular processes, including nucleocytoplasmic transport, spindle formation, and postmitotic nuclear envelope (NE) reassembly. The cytoplasmic Ran GTPase activating protein RanGAP is critical to establish a functional RanGTP/RanGDP gradient across the NE and is associated with the outer surface of the NE in metazoan and higher plant cells. Arabidopsis thaliana RanGAP association with the root tip NE requires a family of likely plant-specific nucleoporins combining coiled-coil and transmembrane domains (CC-TMD) and WPP domain-interacting proteins (WIPs). We have now identified, by tandem affinity purification coupled with mass spectrometry, a second family of CC-TMD proteins, structurally similar, yet clearly distinct from the WIP family, that is required for RanGAP NE association in root tip cells. A combination of loss-of-function mutant analysis and protein interaction data indicates that at least one member of each NE-associated CC-TMD protein family is required for RanGAP targeting in root tip cells, while both families are dispensable in other plant tissues. This suggests an unanticipated complexity of RanGAP NE targeting in higher plant cells, contrasting both the single nucleoporin anchor in metazoans and the lack of targeting in fungi and proposes an early evolutionary divergence of the underlying plant and animal mechanisms.     

Isolation and characterization of a novel v-SNARE family protein that interacts with a calcium-dependent protein kinase from the common ice plant, <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis>

McCPK1 (Mesembryanthemum crystallinum calcium-dependent protein kinase 1) mRNA expression is transiently salinity- and dehydration-stress responsive. The enzyme also undergoes dynamic subcellular localization changes in response to these same stresses. Using the yeast-two hybrid system, we have isolated and characterized a M. crystallinum CPK1 Adaptor Protein 2 (McCAP2). We show that McCPK1 interacts with the C-terminal, coiled-coil containing region of McCAP2 in the yeast two-hybrid system. This interaction was confirmed in vitro between the purified recombinant forms of each of the proteins and in vivo by coimmunoprecipitation experiments from plant extracts. McCAP2, however, was not a substrate for McCPK1. Computational threading analysis suggested that McCAP2 is a member of a novel family of proteins with unknown function also found in rice and Arabidopsis. These proteins contain coiled-coil spectrin repeat domains present in the syntaxin superfamily that participate in vesicular and protein trafficking. Consistent with the interaction data, subcellular localization and fractionation studies showed that McCAP2 colocalizes with McCPK1 to vesicular structures located on the actin cytoskeleton and within the endoplasmic reticulum in cells subjected to low humidity stress. McCAP2 also colocalizes with AtVTI1a, an Arabidopsis v-SNARE [vesicle-soluble N-ethyl maleimide-sensitive factor (NSF) attachment protein (SNAP) receptor] present in the trans-Golgi network (TGN) and prevacuolar compartments (PVCs). Both interaction and subcellular localization studies suggest that McCAP2 may possibly serve as an adaptor protein responsible for vesicle-mediated trafficking of McCPK1 to or from the plasma membrane along actin microfilaments of the cytoskeleton. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

CASP,the alternatively spliced product of the gene encoding the CCAAT-displacement protein transcription factor,is a Golgi membrane protein related to giantin

Large coiled-coil proteins are being found in increasing numbers on the membranes of the Golgi apparatus and have been proposed to function in tethering of transport vesicles and in the organization of the Golgi stack. Members of one class of Golgi coiled-coil protein, comprising giantin and golgin-84, are anchored to the bilayer by a single C-terminal transmembrane domain (TMD). In this article, we report the characterization of another mammalian coiled-coil protein, CASP, that was originally identified as an alternatively spliced product of the CUTL1 gene that encodes CCAAT-displacement protein (CDP), the human homologue of the Drosophila homeodomain protein Cut. We find that the Caenorhabditis elegans homologues of CDP and CASP are also generated from a single gene. CASP lacks the DNA binding motifs of CDP and was previously reported to be a nuclear protein. Herein, we show that it is in fact a Golgi protein with a C-terminal TMD and shares with giantin and golgin-84 a conserved histidine in its TMD. However, unlike these proteins, CASP has a homologue in Saccharomyces cerevisiae, which we call COY1. Deletion of COY1 does not affect viability, but strikingly restores normal growth to cells lacking the Golgi soluble N-ethylmaleimide-sensitive factor attachment protein receptor Gos1p. The conserved histidine is necessary for Coy1p's activity in cells lacking Gos1p, suggesting that the TMD of these transmembrane Golgi coiled-coil proteins is directly involved in their function.     

Novel plant SUN-KASH bridges are involved in RanGAP anchoring and nuclear shape determination

Inner nuclear membrane Sad1/UNC-84 (SUN) proteins interact with outer nuclear membrane (ONM) Klarsicht/ANC-1/Syne homology (KASH) proteins, forming linkers of nucleoskeleton to cytoskeleton conserved from yeast to human and involved in positioning of nuclei and chromosomes. Defects in SUN-KASH bridges are linked to muscular dystrophy, progeria, and cancer. SUN proteins were recently identified in plants, but their ONM KASH partners are unknown. Arabidopsis WPP domain-interacting proteins (AtWIPs) are plant-specific ONM proteins that redundantly anchor Arabidopsis RanGTPase-activating protein 1 (AtRanGAP1) to the nuclear envelope (NE). In this paper, we report that AtWIPs are plant-specific KASH proteins interacting with Arabidopsis SUN proteins (AtSUNs). The interaction is required for both AtWIP1 and AtRanGAP1 NE localization. AtWIPs and AtSUNs are necessary for maintaining the elongated nuclear shape of Arabidopsis epidermal cells. Together, our data identify the first KASH members in the plant kingdom and provide a novel function of SUN-KASH complexes, suggesting that a functionally diverged SUN-KASH bridge is conserved beyond the opisthokonts.     

Characterization of SUN-domain proteins at the higher plant nuclear envelope

Sad1/UNC-84 (SUN)-domain proteins are inner nuclear membrane (INM) proteins that are part of bridging complexes linking cytoskeletal elements with the nucleoskeleton, and have been shown to be conserved in non-plant systems. In this paper, we report the presence of members of this family in the plant kingdom, and investigate the two Arabidopsis SUN-domain proteins, AtSUN1 and AtSUN2. Our results indicate they contain the highly conserved C-terminal SUN domain, and share similar structural features with animal and fungal SUN-domain proteins including a functional coiled-coil domain and nuclear localization signal. Both are expressed in various tissues with AtSUN2 expression levels relatively low but upregulated in proliferating tissues. Further, we found AtSUN1 and AtSUN2 expressed as fluorescent protein fusions, to localize to and show low mobility in the nuclear envelope (NE), particularly in the INM. Deletion of various functional domains including the N terminus and coiled-coil domain affect the localization and increase the mobility of AtSUN1 and AtSUN2. Finally, we present evidence that AtSUN1 and AtSUN2 are present as homomers and heteromers in vivo , and that the coiled-coil domains are required for this. The study provides evidence suggesting the existence of cytoskeletal–nucleoskeletal bridging complexes at the plant NE.     

Identification of unique SUN-interacting nuclear envelope proteins with diverse functions in plants

Although a plethora of nuclear envelope (NE) transmembrane proteins (NETs) have been identified in opisthokonts, plant NETs are largely unknown. The only known NET homologues in plants are Sad1/UNC-84 (SUN) proteins, which bind Klarsicht/ANC-1/Syne-1 homology (KASH) proteins. Therefore, de novo identification of plant NETs is necessary. Based on similarities between opisthokont KASH proteins and the only known plant KASH proteins, WPP domain–interacting proteins, we used a computational method to identify the KASH subset of plant NETs. Ten potential plant KASH protein families were identified, and five candidates from four of these families were verified for their NE localization, depending on SUN domain interaction. Of those, Arabidopsis thaliana SINE1 is involved in actin-dependent nuclear positioning in guard cells, whereas its paralogue SINE2 contributes to innate immunity against an oomycete pathogen. This study dramatically expands our knowledge of plant KASH proteins and suggests that plants and opisthokonts have recruited different KASH proteins to perform NE regulatory functions.     

AtCSLD2 is an integral Golgi membrane protein with its N-terminus facing the cytosol

Cellulose synthase-like proteins in the D family share high levels of sequence identity with the cellulose synthase proteins and also contain the processive beta-glycosyltransferase motifs conserved among all members of the cellulose synthase superfamily. Consequently, it has been hypothesized that members of the D family function as either cellulose synthases or glycan synthases involved in the formation of matrix polysaccharides. As a prelude to understanding the function of proteins in the D family, we sought to determine where they are located in the cell. A polyclonal antibody against a peptide located at the N-terminus of the Arabidopsis D2 cellulose synthase-like protein was generated and purified. After resolving Golgi vesicles from plasma membranes using endomembrane purification techniques including two-phase partitioning and sucrose density gradient centrifugation, we used antibodies against known proteins and marker enzyme assays to characterize the various membrane preparations. The Arabidopsis cellulose synthase-like D2 protein was found mostly in a fraction that was enriched with Golgi membranes. In addition, versions of the Arabidopsis cellulose synthase-like D2 proteins tagged with a green fluorescent protein was observed to co-localize with a DsRed-tagged Golgi marker protein, the rat alpha-2,6-sialyltransferase. Therefore, we postulate that the majority of Arabidopsis cellulose synthase-like D proteins, under our experimental conditions, are likely located at the Golgi membranes. Furthermore, protease digestion of Golgi-rich vesicles revealed almost complete loss of reaction with the antibodies, even without detergent treatment of the Golgi vesicles. Therefore, the N-terminus of the Arabidopsis cellulose synthase-like D2 protein likely faces the cytosol. Combining this observation with the transmembrane domain predictions, we postulate that the large hydrophilic domain of this protein also faces the cytosol.     

Plant-specific mitotic targeting of RanGAP requires a functional WPP domain

The small GTPase Ran is involved in nucleocytoplasmic transport, spindle formation, nuclear envelope (NE) formation, and cell-cycle control. In vertebrates, these functions are controlled by a three-dimensional gradient of Ran-GTP to Ran-GDP, established by the spatial separation of Ran GTPase-activating protein (RanGAP) and the Ran guanine nucleotide exchange factor RCC1. While this spatial separation is established by the NE during interphase, it is orchestrated during mitosis by association of RCC1 with the chromosomes and RanGAP with the spindle and kinetochores. SUMOylation of vertebrate RanGAP1 is required for NE, spindle, and centromere association. Arabidopsis RanGAP1 (AtRanGAP1) lacks the SUMOylated C-terminal domain of vertebrate RanGAP, but contains a plant-specific N-terminal domain (WPP domain), which is necessary and sufficient for its targeting to the NE in interphase. Here we show that the human and plant RanGAP-targeting domains are kingdom specific. AtRanGAP1 has a mitotic trafficking pattern uniquely different from that of vertebrate RanGAP, which includes targeting to the outward-growing rim of the cell plate. The WPP domain is necessary and sufficient for this targeting. Point mutations in conserved residues of the WPP domain also abolish targeting to the nuclear rim and the cell plate, suggesting that the same mechanism is involved in both targeting events. These results indicate that plant and animal RanGAPs undergo different migration patterns during cell division, which require their kingdom-specific targeting domains.     

Identification and characterization of AtCASP,a plant transmembrane Golgi matrix protein

Golgins are a family of coiled-coil proteins that are associated with the Golgi apparatus. They are necessary for tethering events in membrane fusion and may act as structural support for Golgi cisternae. Here we report on the identification of an Arabidopsis golgin which is a homologue of CASP, a known transmembrane mammalian and yeast golgin. Similar to its homologues, the plant CASP contains a long N-terminal coiled-coil region protruding into the cytosol and a C-terminal transmembrane domain with amino acid residues which are highly conserved across species. Through fluorescent protein tagging experiments, we show that plant CASP localizes at the plant Golgi apparatus and that the C-terminus of this protein is sufficient for its localization, as has been shown for its mammalian counterpart. In addition, we demonstrate that the plant CASP is able to localize at the mammalian Golgi apparatus. However, mutagenesis of a conserved tyrosine in the transmembrane domain revealed that it is necessary for ER export and Golgi localization of the Arabidopsis CASP in mammalian cells, but is not required for its correct localization in plant cells. These data suggest that mammalian and plant cells have different mechanisms for concentrating CASP in the Golgi apparatus.†These authors have contributed equally to the work     

A novel Rab6-interacting domain defines a family of Golgi-targeted coiled-coil proteins

In recent years, a large number of coiled-coil proteins localised to the Golgi apparatus have been identified using antisera from human patients with a variety of autoimmune conditions [1]. Because of their common method of discovery and extensive regions of coiled-coil, they have been classified as a family of proteins, the golgins [1]. This family includes golgin-230/245/256, golgin-97, GM130/golgin-95, golgin-160/MEA-2/GCP170, giantin/macrogolgin and a related group of proteins - possibly splice variants - GCP372 and GCP364[2][3][4][5][6][7][8][9][10][11]. GM130 and giantin have been shown to function in the p115-mediated docking of vesicles with Golgi cisternae [12]. In this process, p115, another coiled-coil protein, is though to bind to giantin on vesicles and to GM130 on cisternae, thus acting as a tether holding the two together [12] [13]. Apart from giantin and GM130, none of the golgins has yet been assigned a function in the Golgi apparatus. In order to obtain clues as to the functions of the golgins, the targeting to the Golgi apparatus of two members of this family, golgin-230/245/256 and golgin-97, was investigated. Each of these proteins was shown to target to the Golgi apparatus through a carboxy-terminal domain containing a conserved tyrosine residue, which was critical for targeting. The domain preferentially bound to Rab6 on protein blots, and mutations that abolished Golgi targeting resulted in a loss of this interaction. Sequence analysis revealed that a family of coiled-coil proteins from mammals, worms and yeast contain this domain at their carboxyl termini. One of these proteins, yeast Imh1p, has previously been shown to have a tight genetic interaction with Rab6 [14]. On the basis of these data, it is proposed that this family of coiled-coil proteins functions in Rab6-regulated membrane-tethering events.     

Subclass-specific localization and trafficking of Arabidopsis p24 proteins in the ER-Golgi interface

We describe a comprehensive analysis of the subcellular localization and in vivo trafficking of Arabidopsis p24 proteins. In Arabidopsis, there are 11 p24 proteins, which fall into only δ and β subfamilies. Interestingly, the δ subfamily of p24 proteins in Arabidopsis is elaborated spectacularly in evolution, which can be grouped into two subclasses: p24δ1 and p24δ2. We found that, although all p24δ proteins possess classic COPII/COPI binding motifs in their cytosolic C-termini, p24δ1 proteins are localized to the endoplasmic reticulum (ER), p24δ2 proteins are localized to both ER and Golgi. Two p24β proteins reside largely in Golgi. Similar to Atp24 (termed p24δ1c in this study), p24δ2d also cycles between the ER and Golgi. Interestingly, coexpression with p24β1 could retain p24δ2d, but not p24δ1d in Golgi. We revealed that the lumenal coiled-coil domain of p24δ2d is required for its steady-state localization in Golgi, probably through its interaction with p24β1. In p24β1, there is no classic COPII or COPI binding motif in its C-terminus. However, the protein also cycles between the ER and Golgi. We found that a conserved RV motif located at the extreme end of the C-terminus of p24β1 plays an important role in its Golgi target.     

Holding it all together? Candidate proteins for the plant Golgi matrix

A combination of electron microscopy and fluorescence microscopy has provided us with a global picture of the structure of the plant Golgi apparatus. However, the components that shape this structure remain elusive. In other organisms, members of the golgin family of coiled-coil proteins are essential for Golgi structure and organisation. Putative Arabidopsis and rice homologues of some golgin family members can be identified using database searches. Likewise, the heterogeneous group of multi-subunit-tethering complexes is responsible for crucial transport steps that affect Golgi structure and cisternal organisation in animals and yeasts. The Arabidopsis genome harbours possible homologues for the majority of the subunits of these complexes, suggesting that they also operate in the plant kingdom.     

The Golgi-associated long coiled-coil protein NECC1 participates in the control of the regulated secretory pathway in PC12 cells

Golgi-associated long coiled-coil proteins, often referred to as golgins, are involved in the maintenance of the structural organization of the Golgi apparatus and the regulation of membrane traffic events occurring in this organelle. Little information is available on the contribution of golgins to Golgi function in cells specialized in secretion such as endocrine cells or neurons. In the present study, we characterize the intracellular distribution as well as the biochemical and functional properties of a novel long coiled-coil protein present in neuroendocrine tissues, NECC1 (neuroendocrine long coiled-coil protein 1). The present study shows that NECC1 is a peripheral membrane protein displaying high stability to detergent extraction, which distributes across the Golgi apparatus in neuroendocrine cells. In addition, NECC1 partially localizes to post-Golgi carriers containing secretory cargo in PC12 cells. Overexpression of NECC1 resulted in the formation of juxtanuclear aggregates together with a slight fragmentation of the Golgi and a decrease in K+-stimulated hormone release. In contrast, NECC1 silencing did not alter Golgi architecture, but enhanced K+-stimulated hormone secretion in PC12 cells. In all, the results of the present study identify NECC1 as a novel component of the Golgi matrix and support a role for this protein as a negative modulator of the regulated trafficking of secretory cargo in neuroendocrine cells.     

Involvement of a Golgi-resident GPI-anchored protein in maintenance of the Golgi structure

The Golgi apparatus consists of a series of flattened cisternal membranes that are aligned in parallel to form stacks. Cytosolic-oriented Golgi-associated proteins have been identified that may coordinate or maintain the Golgi architecture. Here, we describe a novel GPI-anchored protein, Golgi-resident GPI-anchored protein (GREG) that has a brefeldin A-sensitive Golgi localization. GREG resides in the Golgi lumen as a cis-oriented homodimer, due to strong interactions between coiled-coil regions in the C termini. Dimerization of GREG as well as its Golgi localization depends on a unique tandem repeat sequence within the coiled-coil region. RNA-mediated interference of GREG expression or expression of GREG mutants reveals an essential role for GREG in maintenance of the Golgi integrity. Under these conditions, secretion of the vesicular stomatitis virus glycoprotein protein as a marker for protein transport along the secretory pathway is inhibited, suggesting a loss of Golgi function as well. These results imply the involvement of a luminal protein in Golgi structure and function.     

Identification and characterization of a novel Golgi protein, golgin-67

In the course of screening a lambdagt11 human leukemic T-cell cDNA expression library with an antibody specific to the mitotic target of Src, Sam68, we identified and cloned a cDNA encoding a novel protein with a predicted molecular mass of 51.4 kDa. Polyclonal antibodies raised to a His(6)-tagged construct of this protein, detected a approximately 67-kDa protein in immunoprecipitation experiments, and cytological studies showed that this protein localized to the Golgi complex, through colocalization experiments with specific Golgi markers. Therefore, we designated this protein golgin-67. Sequence analysis revealed that golgin-67 is a highly coiled-coil protein, with potential Cdc2 and Src kinase phosphorylation motifs. It has sequence homologies to other Golgi proteins, including the coatamer complex I vesicle docking protein, GM130. Structurally, golgin-67 resembles, golgin-84, an integral membrane Golgi protein with an N-terminal coiled-coil domain and a single C-terminal transmembrane domain. The C-terminal region of golgin-67, which contains a predicted transmembrane domain, was demonstrated to be essential for its Golgi localization.     

Anchorage of plant RanGAP to the nuclear envelope involves novel nuclear-pore-associated proteins

The Ran GTPase controls multiple cellular processes including nucleocytoplasmic transport, spindle assembly, and nuclear envelope (NE) formation [1-4]. Its roles are accomplished by the asymmetric distribution of RanGTP and RanGDP enabled by the specific locations of the Ran GTPase-activating protein RanGAP and the nucleotide exchange factor RCC1 [5-8]. Mammalian RanGAP1 targeting to the NE and kinetochores requires interaction of its sumoylated C-terminal domain with the nucleoporin Nup358/RanBP2 [9-14]. In contrast, Arabidopsis RanGAP1 is associated with the NE and cell plate, mediated by an N-terminal, plant-specific WPP domain [15-18]. In the absence of RanBP2 in plants, the mechanism for spatially sequestering plant RanGAP is unknown. Here, Arabidopsis WPP-domain interacting proteins (WIPs) that interact with RanGAP1 in vivo and colocalize with RanGAP1 at the NE and cell plate were identified. Immunogold labeling indicates that WIP1 is associated with the outer NE. In a wip1-1/wip2-1/wip3-1 triple mutant, RanGAP1 is dislocated from the NE in undifferentiated root-tip cells, whereas NE targeting in differentiated root cells and targeting to the cell plate remain intact. We propose that WIPs are novel plant nucleoporins involved in RanGAP1 NE anchoring in specific cell types. Our data support a separate evolution of RanGAP targeting mechanisms in different kingdoms.     

ERG30, a VAP-33-related protein, functions in protein transport mediated by COPI vesicles.

Intracellular transport of newly synthesized and mature proteins via vesicles is controlled by a large group of proteins. Here we describe a ubiquitous rat protein-endoplasmic reticulum (ER) and Golgi 30-kD protein (ERG30)-which shares structural characteristics with VAP-33, a 33-kD protein from Aplysia californica which was shown to interact with the synaptic protein VAMP. The transmembrane topology of the 30-kD ERG30 corresponds to a type II integral membrane protein, whose cytoplasmic NH(2) terminus contains a predicted coiled-coil motif. We localized ERG30 to the ER and to pre-Golgi intermediates by biochemical and immunocytochemical methods. Consistent with a role in vesicular transport, anti-ERG30 antibodies specifically inhibit intra-Golgi transport in vitro, leading to significant accumulation of COPI-coated vesicles. It appears that ERG30 functions early in the secretory pathway, probably within the Golgi and between the Golgi and the ER.     

Targeting of the Arf-like GTPase Arl3p to the Golgi requires N-terminal acetylation and the membrane protein Sys1p

The GTPase Arl3p is required to recruit a second GTPase, Arl1p, to the Golgi in Saccharomyces cerevisiae. Arl1p binds to the GRIP domain, which is present in a number of long coiled-coil proteins or 'golgins'. Here we show that Arl3p is not myristoylated like most members of the Arf family, but is instead amino-terminally acetylated by the NatC complex. Targeting of Arl3p also requires a Golgi membrane protein Sys1p. The human homologues of Arl3p (Arf-related protein 1 (ARFRP1)) and Sys1p (hSys1) can be isolated in a complex after chemical cross-linking. This suggests that the targeting of ARFRP1/Arl3p to the Golgi is mediated by a direct interaction between its acetylated N terminus and Sys1p/hSys1.