Interaction of a Golgi-associated kinesin-like protein with Rab6

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Dynamic association of cytoplasmic dynein heavy chain 1a with the Golgi apparatus and intermediate compartment

Microtubule motors, such as the minus end-directed motor, cytoplasmic dynein, play an important role in maintaining the integrity, intracellular location, and function of the Golgi apparatus, as well as in the translocation of membrane between the endoplasmic reticulum and Golgi apparatus. We have immunolocalised conventional cytoplasmic dynein heavy chain to the Golgi apparatus in cultured vertebrate cells. In addition, we present evidence that cytoplasmic dynein heavy chain cycles constitutively between the endoplasmic reticulum and Golgi apparatus: it colocalises partially with the intermediate compartment, it is found on nocodazole-induced peripheral Golgi elements and, most strikingly, on Brefeldin A-induced tubules that are moving towards microtubule plus ends. The direction of movement of membrane between the endoplasmic reticulum and Golgi apparatus is therefore unlikely to be regulated by controlling motor-membrane interactions: rather, the motors probably remain bound throughout the whole cycle, with their activity being modulated instead. We also report that the overexpression of p50/dynamitin results in the loss of cytoplasmic dynein heavy chain from the membrane of peripheral Golgi elements. These results explain how dynamitin overexpression causes the inhibition of endoplasmic reticulum-to-Golgi transport complex movement towards the centrosomal region, and support the general model that an intact dynactin complex is required for cytoplasmic dynein binding to all cargoes.

ER to Golgi transport: Requirement for p115 at a pre-Golgi VTC stage

The membrane transport factor p115 functions in the secretory pathway of mammalian cells. Using biochemical and morphological approaches, we show that p115 participates in the assembly and maintenance of normal Golgi structure and is required for ER to Golgi traffic at a pre-Golgi stage. Injection of antibodies against p115 into intact WIF-B cells caused Golgi disruption and inhibited Golgi complex reassembly after BFA treatment and wash-out. Addition of anti-p115 antibodies or depletion of p115 from a VSVtsO45 based semi-intact cell transport assay inhibited transport. The inhibition occurred after VSV glycoprotein (VSV-G) exit from the ER but before its delivery to the Golgi complex, and resulted in VSV-G protein accumulating in peripheral vesicular tubular clusters (VTCs). The p115-requiring step of transport followed the rab1-requiring step and preceded the Ca(2+)-requiring step. Unexpectedly, mannosidase I redistributed from the Golgi complex to colocalize with VSV-G protein arrested in pre-Golgi VTCs by p115 depletion. Redistribution of mannosidase I was also observed in cells incubated at 15 degrees C. Our data show that p115 is essential for the translocation of pre-Golgi VTCs from peripheral sites to the Golgi stack. This defines a previously uncharacterized function for p115 at the VTC stage of ER to Golgi traffic.

Promiscuity in Rab-SNARE interactions

Fusion of post-Golgi secretory vesicles with the plasma membrane in yeast requires the function of a Rab protein, Sec4p, and a set of v- and t-SNAREs, the Snc, Sso, and Sec9 proteins. We have tested the hypothesis that a selective interaction between Sec4p and the exocytic SNAREs is responsible for ensuring that secretory vesicles fuse with the plasma membrane but not with intracellular organelles. Assembly of Sncp and Ssop into a SNARE complex is defective in a sec4-8 mutant strain. However, Snc2p binds in vivo to many other syntaxin-like t-SNAREs, and binding of Sncp to the endosomal/Golgi t-SNARE Tlg2p is also reduced in sec4-8 cells. In addition, binding of Sncp to Ssop is reduced by mutations in two other Rab genes and four non-Rab genes that block the secretory pathway before the formation of secretory vesicles. In an alternate approach to look for selective Rab-SNARE interactions, we report that the nucleotide-free form of Sec4p coimmunoprecipitates with Ssop. However, Rab-SNARE binding is nonselective, because the nucleotide-free forms of six Rab proteins bind with similar low efficiency to three SNARE proteins, Ssop, Pep12p, and Sncp. We conclude that Rabs and SNAREs do not cooperate to specify the target membrane.

The kinesin-like motor protein KIF1C occurs in intact cells as a dimer and associates with proteins of the 14-3-3 family

Proteins of the kinesin superfamily are regulated in their motor activity as well as in their ability to bind to their cargo by carboxyl-terminal associating proteins and phosphorylation. KIF1C, a recently identified member of the KIF1/Unc104 family, was shown to be involved in the retrograde vesicle transport from the Golgi-apparatus to the endoplasmic reticulum. In a yeast two-hybrid screen using the carboxyl-terminal 350 amino acids of KIF1C as a bait, we identified as binding proteins 14-3-3 beta, gamma, epsilon, and zeta. In addition, a clone encoding the carboxyl-terminal 290 amino acids of KIF1C was found, indicating a potential for KIF1C to dimerize. Subsequent transient overexpression experiments showed that KIF1C can dimerize efficiently. However, in untransfected cells, only a small portion of KIF1C was detected as a dimer. The association of 14-3-3 proteins with KIF1C could be confirmed in transient expression systems and in untransfected cells and was dependent on the phosphorylation of serine 1092 located in a consensus binding sequence for 14-3-3 ligands. Serine 1092 was a substrate for the protein kinase casein kinase II in vitro, and inhibition of casein kinase II in cells diminished the association of KIF1C with 14-3-3gamma. Our data thus suggest that KIF1C can form dimers and is associated with proteins of the 14-3-3 family.

Rab6 coordinates a novel Golgi to ER retrograde transport pathway in live cells

We visualized a fluorescent-protein (FP) fusion to Rab6, a Golgi-associated GTPase, in conjunction with fluorescent secretory pathway markers. FP-Rab6 defined highly dynamic transport carriers (TCs) translocating from the Golgi to the cell periphery. FP-Rab6 TCs specifically accumulated a retrograde cargo, the wild-type Shiga toxin B-fragment (STB), during STB transport from the Golgi to the endoplasmic reticulum (ER). FP-Rab6 TCs associated intimately with the ER, and STB entered the ER via specialized peripheral regions that accumulated FP-Rab6. Microinjection of antibodies that block coatomer protein I (COPI) function inhibited trafficking of a KDEL-receptor FP-fusion, but not FP-Rab6. Additionally, markers of COPI-dependent recycling were excluded from FP-Rab6/STB TCs. Overexpression of Rab6:GDP (T27N mutant) using T7 vaccinia inhibited toxicity of Shiga holotoxin, but did not alter STB transport to the Golgi or Golgi morphology. Taken together, our results indicate Rab6 regulates a novel Golgi to ER transport pathway.

Reconstitution of membrane transport powered by a novel dimeric kinesin motor of the Unc104/KIF1A family purified from Dictyostelium

Motor-powered movement along microtubule tracks is important for membrane organization and trafficking. However, the molecular basis for membrane transport is poorly understood, in part because of the difficulty in reconstituting this process from purified components. Using video microscopic observation of organelle transport in vitro as an assay, we have purified two polypeptides (245 and 170 kD) from Dictyostelium extracts that independently reconstitute plus-end-directed membrane movement at in vivo velocities. Both polypeptides were found to be kinesin motors, and the 245-kD protein (DdUnc104) is a close relative of Caenorhabditis elegans Unc104 and mouse KIF1A, neuron-specific motors that deliver synaptic vesicle precursors to nerve terminals. A knockout of the DdUnc104 gene produces a pronounced defect in organelle transport in vivo and in the reconstituted assay. Interestingly, DdUnc104 functions as a dimeric motor, in contrast to other members of this kinesin subfamily, which are monomeric.

Clinical, molecular, and cell biological aspects of Chediak-Higashi syndrome

Chediak-Higashi syndrome (CHS) is a rare autosomal recessive disorder characterized by variable degrees of oculocutaneous albinism, easy bruisability, and bleeding as a result of deficient platelet dense bodies, and recurrent infections, with neutropenia, impaired chemotaxis and bactericidal activity, and abnormal NK cell function. Neurologic involvement is variable, but often includes peripheral neuropathy. Most patients also undergo an "accelerated phase," which is a nonmalignant lymphohistiocytic infiltration of multiple organs resembling lymphoma. Death often occurs in the first decade from infection, bleeding, or development of the accelerated phase. The hallmark of CHS is the presence of huge cytoplasmic granules in circulating granulocytes and many other cell types. These granules are peroxidase-positive and contain lysosomal enzymes, suggesting that they are giant lysosomes or, in the case of melanocytes, giant melanosomes. The underlying defect in CHS remains elusive, but the disorder can be considered a model for defects in vesicle formation, fusion, or trafficking. Because the beige mouse demonstrates many characteristics similar to those of human CHS patients, including dilution of coat color, recurrent infections, and the presence of giant granules, it is considered the animal homologue of CHS. The beige gene, Lyst, was mapped and sequenced in 1996, prompting identification of the human LYST gene on chromosome 1q42. Lyst and LYST show 86.5% sequence homology. LYST encodes a 429 kDa protein with a function that remains unknown, but the source of extensive speculation among students of cell biology.

Rab5 regulates motility of early endosomes on microtubules

The small GTPase Rab5 regulates membrane docking and fusion in the early endocytic pathway. Here we reveal a new role for Rab5 in the regulation of endosome interactions with the microtubule network. Using Rab5 fused to green fluorescent protein we show that Rab5-positive endosomes move on microtubules in vivo. In vitro, Rab5 stimulates both association of early endosomes with microtubules and early-endosome motility towards the minus ends of microtubules. Moreover, similarly to endosome membrane docking and fusion, Rab5-dependent endosome movement depends on the phosphatidylinositol-3-OH kinase hVPS34. Thus, Rab5 functionally links regulation of membrane transport, motility and intracellular distribution of early endosomes.

Arf proteins bind to mitotic kinesin-like protein 1 (MKLP1) in a GTP-dependent fashion

Arf proteins comprise a family of 21-kDa GTP-binding proteins with many proposed functions in mammalian cells, including the regulation of several steps of membrane transport, maintenance of organelle integrity, and activation of phospholipase D. We performed a yeast two-hybrid screen of human cDNA libraries using a dominant activating allele, [Q71L], of human Arf3 as bait. Eleven independent isolates contained plasmids encoding the C-terminal tail of mitotic kinesin-like protein-1 (MKLP1). Further deletion mapping allowed the identification of an 88 amino acid Arf3 binding domain in the C-terminus of MKLP1. This domain has no clear homology to other Arf binding proteins or to other proteins in the protein databases. The C-terminal domain of MKLP1 was expressed and purified from bacteria as a GST fusion protein and shown to bind Arf3 in a GTP-dependent fashion. A screen for mutations in Arf3 that specifically lost the ability to bind MKLP1 identified 10 of 14 point mutations in the GTP-sensitive switch I or switch II regions of Arf3. Two-hybrid assays of the C-terminal domain of MKLP1 with each of the human Arf isoforms revealed strong interaction with each. Taken together, these data are all supportive of the conclusion that activated Arf proteins bind to the C-terminal "tail" domain of MKLP1.

Rab11BP/Rabphilin-11, a downstream target of rab11 small G protein implicated in vesicle recycling

Rab11 small G protein has been implicated in vesicle recycling, but its upstream regulators or downstream targets have not yet been identified. We isolated here a downstream target of Rab11, named rabphilin-11, from bovine brain. Moreover, we isolated from a rat brain cDNA library its cDNA, which encoded a protein with a M(r) of 100,946 and 908 amino acids (aa). Rabphilin-11 bound GTP-Rab11 more preferentially than GDP-Rab11 at the N-terminal region and was specific for Rab11 and inactive for other Rab and Rho small G proteins. Both GTP-Rab11 and rabphilin-11 were colocalized at perinuclear regions, presumably the Golgi complex and recycling endosomes, in Madin-Darby canine kidney cells. In HeLa cells cultured on fibronectin, both the proteins were localized not only at perinuclear regions but also along microtubules, which were oriented toward membrane lamellipodia. Treatment of HeLa cells with nocodazole caused disruption of microtubules and dispersion of GTP-Rab11 and rabphilin-11. Overexpression of the C-terminal fragment of rabphilin-11 (aa 607-730), lacking the GTP-Rab11 binding domain, in HeLa cells reduced accumulation of transferrin at perinuclear regions and cell migration. Rabphilin-11 turned out to be a rat counterpart of recently reported bovine Rab11BP. These results indicate that rabphilin-11 is a downstream target of Rab11 which is involved in vesicle recycling.

Interactions of (-)-ilimaquinone with methylation enzymes: implications for vesicular-mediated secretion

BACKGROUND

The marine sponge metabolite (-)-ilimaquinone has antimicrobial, anti-HIV, anti-inflammatory and antimitotic activities, inhibits the cytotoxicity of ricin and diptheria toxin, and selectively fragments the Golgi apparatus. The range of activities demonstrated by this natural product provides a unique opportunity for studying these cellular processes.

RESULTS

Affinity chromatography experiments show that (-)-ilimaquinone interacts with enzymes of the activated methyl cycle: S-adenosylmethionine synthetase, S-adenosylhomocysteinase and methyl transferases. Known inhibitors of these enzymes were found to block vesicle-mediated secretion in a manner similar to (-)-ilimaquinone. Moreover, the antisecretory effects of (-)-ilimaquinone and inhibitors of methylation chemistry, but not brefeldin A, could be reversed in the presence of the cellular methylating agent S-adenosylmethionine. Of the enzymes examined in the activated methyl cycle, S-adenosylhomocysteinase was specifically inhibited by (-)-ilimaquinone. Consistent with these observations, (-)-ilimaquinone was shown to obstruct new methylation events in adrenocorticotrophic hormone (ACTH)-secreting pituitary cells.

CONCLUSIONS

(-)-ilimaquinone inhibits cellular methylations through its interactions with S-adenosylhomocysteinase. Furthermore, these studies indicate that the inhibition of secretion by ilimaquinone is the result of the natural product's antimethylation activity. It is likely that the ability to fragment the Golgi apparatus, as well as other activities, are also related to ilimaquinone's influence on methylation chemistry.

Expression of rab11a N124I in gastric parietal cells inhibits stimulatory recruitment of the H+-K+-ATPase

Stimulation of the gastric parietal cell results in a massive redistribution of H+-K+-ATPase from cytoplasmic tubulovesicles to the apical plasma membrane. Previous studies have implicated the small GTPase rab11 in this process. Using matrix-assisted laser desorption mass spectrometry, we confirmed that rab11 is associated with H+-K+-ATPase-enriched gastric microsomes. A stoichiometry of one rab11 per six copies of H+-K+-ATPase was estimated. Furthermore, rab11 exists in at least three forms on rabbit gastric microsomes: the two most prominent resemble rab11a, whereas the third resembles rab11b. Using an adenoviral expression system, we expressed the dominant negative mutant rab11a N124I in primary cultures of rabbit parietal cells under the control of the tetracycline transactivator protein (tTA). The mutant was well expressed with a distribution similar to that of the H+-K+-ATPase. Stimulation of these cultures with histamine and IBMX was assessed by measuring the aminopyrine (AP) uptake relative to resting cells (AP index). In experiments on six culture preparations, stimulated uninfected cells gave an AP index of 10.0 +/- 2.9, whereas parallel cultures expressing rab11a N124I were poorly responsive to stimulation, with a mean AP index of 3.2 +/- 0. 9. Control cultures expressing tTA alone or tTA plus actin responded equally well to stimulation, giving AP index values of 9.0 +/- 3.1 and 9.6 +/- 0.9, respectively. Thus inhibition by rab11a N124I is not simply due to adenoviral infection. The AP uptake data were confirmed by immunocytochemistry. In uninfected cells, H+-K+-ATPase demonstrated a broad cytoplasmic distribution, but it was cleared from the cytoplasm and associated with apically derived membranes on stimulation. In cells expressing rab11a N124I, H+-K+-ATPase maintained its resting localization on stimulation. Furthermore, this effect could be alleviated by culturing infected cells in the presence of tetracycline, which prevents expression of the mutant rab11. We therefore conclude that rab11a is the prominent GTPase associated with gastric microsomes and that it plays a role in parietal cell activation.

The developmental role of warthog, the notch modifier encoding Drab6

The warthog (wrt) gene, recovered as a modifier for Notch signaling, was found to encode the Drosophila homologue of rab6, Drab6. Vertebrate and yeast homologues of this protein have been shown to regulate Golgi network to TGN trafficking. To study the function of this protein in the development of a multicellular organism, we analyzed three different warthog mutants. The first was an R62C point mutation, the second a genomic null, and the third was an engineered GTP-bound form. Our studies show, contrary to yeast, that the Drosophila homologue of rab6 is an essential gene. However, it has limited effects on development beyond the larval stage. Only the mechanosensory bristles on the head, notum, and scutellum are affected by warthog mutations. We present models for the modifying effect of Drab6 on Notch signaling.

The role of ARF and Rab GTPases in membrane transport

Two key events of intracellular transport and membrane trafficking in eukaryotic cells, the formation of transport vesicles and their specific delivery to target membranes, are controlled by small GTPases of the ADP-ribosylation factor (ARF) and Rab families, respectively. The past 18 months have seen the identification of proteins that regulate ARF and Rab GDP/GTP cycle, as well as the characterization of their effectors, shedding light on the molecular mechanisms of ARF and Rab function.

Membrane motors

Research over the past 18 months has revealed that many membranous organelles move along both actin filaments and microtubules. It is highly likely that the activity of the microtubule motors, myosins and static linker proteins present on any organelle are co-ordinately regulated and that this control is linked to the processes of membrane traffic itself.

The Golgi-targeting sequence of the peripheral membrane protein p230

Vesicle transport requires the recruitment of cytosolic proteins to specific membrane compartments. We have previously characterised a brefeldin A-sensitive trans-Golgi network-localised protein (p230) that is associated with a population of non-clathrin-coated vesicles. p230 recycles between the cytosol and the cytoplasmic face of buds/vesicles of trans-Golgi network membranes in a G protein-regulated manner. Identifying the mechanism responsible for Golgi targeting of p230 is important for the elucidation of its function. By transfection of COS cells with deletion mutants of p230 we here demonstrate that the C-terminal domain is necessary for targeting to the Golgi. Furthermore, the C-terminal 98 amino acid domain of p230 attached to the green fluorescent protein (GFP-p230-C98aa) was efficiently Golgi-localised in transfected COS cells. Deletion mutants of GFP-p230-C98aa together with alanine scanning mutagenesis identified a minimum stretch of 42 amino acids that is essential for Golgi targeting, suggesting that the conformation of the domain is critical for efficient targeting. In COS cells expressing high levels of GFP-p230-C98aa fusion protein, endogenous p230 was no longer associated with Golgi membranes, suggesting that the GFP fusion protein and endogenous p230 may compete for the same membrane target structures. The Golgi binding of GFP-p230-C98aa is brefeldin A-sensitive and is regulated by G proteins. These studies have identified a minimal sequence responsible for specific targeting of p230 to the Golgi apparatus, which displays similar membrane binding characteristics to wild-type p230.

Synaptic vesicle docking and fusion

Neurotransmitter secretion shares many features with constitutive membrane trafficking. In both cases, vesicles are targeted to a specific acceptor membrane and fuse via a series of protein-protein interactions. Recent work has added to the list of protein complexes involved and is beginning to define the order in which they act. The rapid fusion, precise regulation and plasticity characteristic of synaptic exocytosis probably results from the addition of specialized regulators.

Interaction cloning and characterization of the cDNA encoding the human prenylated rab acceptor (PRA1)

Rab proteins are small GTPases involved in the regulation of intracellular membrane traffic in mammalian cells. In order to find Rab-interacting proteins we performed a two-hybrid screening using a human brain cDNA library. Here we report the isolation of a full-length human cDNA clone coding for a protein of 185 amino acids. This protein interacts strongly with the Rab4b, Rab5a, and Rab5c proteins and weakly with Rab4a, Rab6, Rab7, Rab17, and Rab22 in the two-hybrid assay. Comparison with the Data Bank revealed that this clone represents the human homolog of the previously isolated rat Prenylated Rab Acceptor (rPRA1). Analysis of mRNA expression shows a single abundant mRNA of about 0.8 kb ubiquitously expressed. Western blot analysis of the overexpressed protein shows a band of the expected size equally distributed between cytosol and membranes.

Vascular Endothelial Genes That Are Responsive to Tumor Necrosis Factor- In Vitro Are Expressed in Atherosclerotic Lesions, Including Inhibitor of Apoptosis Protein-1, Stannin, and Two Novel Genes

Activation and dysfunction of endothelial cells play a prominent role in patho-physiological processes such as atherosclerosis. We describe the identification by differential display of 106 cytokine-responsive gene fragments from endothelial cells, activated by monocyte conditioned medium or tumor necrosis factor-. A minority of the fragments (22/106) represent known genes involved in various processes, including leukocyte trafficking, vesicular transport, cell cycle control, apoptosis, and cellular protection against oxidative stress. Full-length cDNA clones were obtained for five novel transcripts that were induced or repressed more than 10-fold in vitro. These novel human cDNAs CA2_1, CG12_1, GG10_2, AG8_1, and GG2_1 encode inhibitor of apoptosis protein-1 (hIAP-1), homologues of apolipoprotein-L, mouse rabkinesin-6, rat stannin, and a novel 188 amino acid protein, respectively. Expression of 4 novel transcripts is shown by in situ hybridization on healthy and atherosclerotic vascular tissue, using monocyte chemotactic protein-1 as a marker for inflammation. CA2_1 (hIAP-1) and AG8_1 are expressed by endothelial cells and macrophage foam cells of the inflamed vascular wall. CG12_1 (apolipoprotein-L like) was specifically expressed in endothelial cells lining the normal and atherosclerotic iliac artery and aorta. These results substantiate the complex change in the gene expression pattern of vascular endothelial cells, which accompanies the inflammatory reaction of atherosclerotic lesions.

Transport-vesicle targeting: tethers before SNAREs

Protein secretion and the transport of proteins between membrane-bound compartments are mediated by small, membrane-bound vesicles. Here I review what is known about the process by which vesicles are targeted to the correct destination. A growing family of proteins, whose precise modes of action are far from established, is involved in targeting. Despite the wide diversity in the identities of the players, some common themes are emerging that may explain how vesicles can identify their targets and release their cargo at the correct time and place in eukaryotic cells.

Specific isoforms of actin-binding proteins on distinct populations of Golgi-derived vesicles

Golgi membranes and Golgi-derived vesicles are associated with multiple cytoskeletal proteins and motors, the diversity and distribution of which have not yet been defined. Carrier vesicles were separated from Golgi membranes, using an in vitro budding assay, and different populations of vesicles were separated using sucrose density gradients. Three main populations of vesicles labeled with beta-COP, gamma-adaptin, or p200/myosin II were separated and analyzed for the presence of actin/actin-binding proteins. beta-Actin was bound to Golgi cisternae and to all populations of newly budded vesicles. Centractin was selectively associated with vesicles co-distributing with beta-COP-vesicles, while p200/myosin II (non-muscle myosin IIA) and non-muscle myosin IIB were found on different vesicle populations. Isoforms of the Tm5 tropomyosins were found on selected Golgi-derived vesicles, while other Tm isoforms did not colocalize with Tm5 indicating the association of specialized actin filaments with Golgi-derived vesicles. Golgi-derived vesicles were shown to bind to F-actin polymerized from cytosol with Jasplakinolide. Thus, newly budded, coated vesicles derived from Golgi membranes can bind to actin and are customized for differential interactions with microfilaments by the presence of selective arrays of actin-binding proteins.

The GRIP domain - a novel Golgi-targeting domain found in several coiled-coil proteins

Many large coiled-coil proteins are being found associated peripherally with the cytoplasmic face of the organelles of the secretory pathway. Various roles have been proposed for these proteins, including the docking of donor vesicles or organelles to an acceptor organelle prior to fusion, and, in the case of the Golgi apparatus, the stacking of the cisternae [1] [2] [3] [4] [5]. Such critical roles require accurate recruitment to the correct organelle. For the endosomal coiled-coil protein EEA1, targeting requires a carboxy-terminal FYVE domain, which interacts with Rab5 and phosphatidylinositol 3-phosphate (PI(3)P), whereas the Golgi protein GM130 interacts with Golgi membranes via the protein GRASP65 [3] [6] [7]. In this paper, we show that two other mammalian Golgi coiled-coil proteins, golgin-245/p230 and golgin-97, have a conserved domain of about 50 amino acids at their carboxyl termini. This 'GRIP' domain is also found at the carboxyl terminus of several other large coiled-coiled proteins of unknown function, including two human proteins and proteins in the genomes of Caenorhabditis elegans and yeasts. The GRIP domains from several of these proteins, including that from the yeast protein Imh1p, were sufficient to specify Golgi targeting in mammalian cells when fused to green fluorescent protein (GFP). This result suggests that this small domain functions to recruit specific coiled-coil proteins to the Golgi by recognising a determinant that has been well conserved in eukaryotic evolution.

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.

Characterization of GAPCenA, a GTPase activating protein for Rab6, part of which associates with the centrosome

The Rab6 GTPase regulates intracellular transport at the level of the Golgi apparatus, probably in a retrograde direction. Here, we report the identification and characterization of a novel human Rab6-interacting protein named human GAPCenA (for 'GAP and centrosome-associated'). Primary sequence analysis indicates that GAPCenA displays similarities, within a central 200 amino acids domain, to both the yeast Rab GTPase activating proteins (GAPs) and to the spindle checkpoint proteins Saccharomyces cerevisiae Bub2p and Schizosaccharomyces pombe Cdc16p. We demonstrate that GAPCenA is indeed a GAP, specifically active in vitro on Rab6 and, to a lesser extent, on Rab4 and Rab2 proteins. Immunofluorescence and cell fractionation experiments showed that GAPCenA is mainly cytosolic but that a minor pool is associated with the centrosome. Moreover, GAPCenA was found to form complexes with cytosolic gamma-tubulin and to play a role in microtubule nucleation. Therefore, GAPCenA may be involved in the coordination of microtubule and Golgi dynamics during the cell cycle.

Cytoplasmic dynein and dynactin as likely candidates for microtubule-dependent apical targeting of pancreatic zymogen granules

The critical role of microtubules in vectorial delivery of post-Golgi carrier vesicles to the apical cell surface has been established for various polarized epithelial cell types. In the present study we used secretory granules of the rat and chicken pancreas, termed zymogen granules, as model system for apically bound post-Golgi carrier vesicles that underlie the regulated exocytotic pathway. We found that targeting of zymogen granules to the apical cell surface requires an intact microtubule system which contains its colchicine-resistant organizing center and, thus, the microtubular minus ends close to the apical membrane domain. Purified zymogen granules and their membranes were found to be associated with cytoplasmic dynein intermediate and heavy chain and to contain the major components of the dynein activator complex, dynactin, i.e. p150Glued, p62, p50, Arp1, and beta-actin. Kinesin heavy chain and the kinesin receptor, 160 kD kinectin, were not detected as components of zymogen granules. Immunofluorescence staining showed a zymogen granule-like distribution for dynein and dynactin (p150Glued, p62, p50, Arpl) in the apical cytoplasm, whereas kinesin and kinectin were largely concentrated in the basal half of the cells in a pattern similar to the distribution of calreticulin, a component of the endoplasmic reticulum. Secretory granules of non-polarized chromaffin cells of the bovine adrenal medulla, that are assumed to underlie microtubular plus end targeting from the Golgi apparatus to the cell periphery, were not found to be associated with dynein or dynactin. To our knowledge, this is the first demonstration of major components of the dynein-dynactin complex associated with the membrane of a biochemically and functionally well-defined organelle which is considered to underlie a vectorial minus end-driven microtubular transport critically involved in precise delivery of digestive enzymes to the apically located acinar lumen.

Identification of a putative effector protein for rab11 that participates in transferrin recycling

We have identified and cloned the cDNA for a 912-aa protein, rab11BP, that interacts with the GTP-containing active form of rab11, a GTP-binding protein that plays a critical role in receptor recycling. Although rab11BP is primarily cytosolic, a significant fraction colocalizes with rab11 in endosomal membranes of both the sorting and recycling subcompartments. In vitro binding of rab11 to native rab11BP requires partial denaturation of the latter to expose an internal binding site located between residues 334 and 504 that is apparently masked by the C-terminal portion of the protein, which includes six repeats known as WD40 domains. Within the cell, rab11BP must undergo a conformational change in which the rab11-binding site becomes exposed, because when coexpressed with rab11 in transfected cells the two proteins formed abundant complexes in association with membranes. Furthermore, although overexpression of rab11BP did not affect transferrin recycling, overexpression of a truncated form of the protein, rab11BP(1-504), that includes the rab11-binding site but lacks the WD40 domains inhibited recycling as strongly as does a dominant negative rab11 mutant protein that does not bind GTP. Strikingly, the inhibition caused by the truncated rab11BP was prevented completely when the cells also expressed a C-terminally deleted, nonprenylatable form of rab11 that, by itself, has no effect on recycling. We propose that rab11BP is an effector for rab11, whose association with this GTP-binding protein is dependent on the action of another membrane-associated factor that promotes the unmasking of the rab11-binding site in rab11BP.

The exocyst is an effector for Sec4p, targeting secretory vesicles to sites of exocytosis

Polarized secretion requires proper targeting of secretory vesicles to specific sites on the plasma membrane. Here we report that the exocyst complex plays a key role in vesicle targeting. Sec15p, an exocyst component, can associate with secretory vesicles and interact specifically with the rab GTPase, Sec4p, in its GTP-bound form. A chain of protein-protein interactions leads from Sec4p and Sec15p on the vesicle, through various subunits of the exocyst, to Sec3p, which marks the sites of exocytosis on the plasma membrane. Sec4p may control the assembly of the exocyst. The exocyst may therefore function as a rab effector system for targeted secretion.

Association of Rab25 and Rab11a with the apical recycling system of polarized Madin-Darby canine kidney cells

Recent evidence suggests that apical and basolateral endocytic pathways in epithelia converge in an apically located, pericentriolar endosomal compartment termed the apical recycling endosome. In this compartment, apically and basolaterally internalized membrane constituents are thought to be sorted for recycling back to their site of origin or for transcytosis to the opposite plasma membrane domain. We report here that in the epithelial cell line Madin-Darby Canine Kidney (MDCK), antibodies to Rab11a label an apical pericentriolar endosomal compartment that is dependent on intact microtubules for its integrity. Furthermore, this compartment is accessible to a membrane-bound marker (dimeric immunoglobulin A [IgA]) internalized from either the apical or basolateral pole, functionally defining it as the apical recycling endosome. We have also examined the role of a closely related epithelial-specific Rab, Rab25, in the regulation of membrane recycling and transcytosis in MDCK cells. When cDNA encoding Rab25 was transfected into MDCK cells, the protein colocalized with Rab11a in subapical vesicles. Rab25 transfection also altered the distribution of Rab11a, causing the coalescence of immunoreactivity into multiple denser vesicular structures not associated with the centrosome. Nevertheless, nocodazole still dispersed these vesicles, and dimeric IgA internalized from either the apical or basolateral membrane was detected in endosomes labeled with antibodies to both Rab11a and Rab25. Overexpression of Rab25 decreased the rate of IgA transcytosis and of apical, but not basolateral, recycling of internalized ligand. Conversely, expression of the dominant-negative Rab25T26N did not alter either apical recycling or transcytosis. These results indicate that both Rab11a and Rab25 associate with the apical recycling system of epithelial cells and suggest that Rab25 may selectively regulate the apical recycling and/or transcytotic pathways.

How to get to the right place at the right time: Rab/Ypt small GTPases and vesicle transport

Vesicles often must be transported over long distances in a very crowded cytoplasmic environment encumbered by the cytoskeleton and membranes of different origin that provide an important barrier to their free diffusion. In animal cells with specialised tasks, such as neurons or endothelial cells, vesicles that are directed to the cell periphery are linked to the microtubular cytoskeleton tracks via association with motor proteins that allow their vectorial movement. In lower eukaryotes the actin cytoskeleton plays a prominent role in organising vesicle movement during polarised growth and mating. The Ras-like small GTPases of the Rab/Ypt family play an essential role in vesicle trafficking and due to their diversity and specific localisation have long been implicated in the selective delivery of vesicles. Recent evidence has cast doubt on the classical point of view of how this class of proteins acts in vesicle transport and suggests their involvement also in the events that permit vesicle anchoring to the cytoskeleton. Therefore, after a brief review of what is known about how vesicle movement is achieved in mammalian and yeast systems, and how Rab/Ypt proteins regulate the vesicle predocking events, it is discussed how these proteins might participate in the events that lead to vesicle movement through association with the cytoskeleton machinery.

Nonuniform microtubular polarity established by CHO1/MKLP1 motor protein is necessary for process formation of podocytes

Podocytes are unique cells that are decisively involved in glomerular filtration. They are equipped with a complex process system consisting of major processes and foot processes whose function is insufficiently understood (Mundel, P., and W. Kriz. 1995. Anat. Embryol. 192:385-397). The major processes of podocytes contain a microtubular cytoskeleton. Taking advantage of a recently established cell culture system for podocytes with preserved ability to form processes (Mundel, P., J. Reiser, A. Zúñiga Mejía Borja, H. Pavenstädt, G.R. Davidson, W. Kriz, and R. Zeller. 1997b. Exp. Cell Res. 36:248-258), we studied the functional significance of the microtubular system in major processes. The following data were obtained: (a) Microtubules (MTs) in podocytes show a nonuniform polarity as revealed by hook-decoration. (b) CHO1/ MKLP1, a kinesin-like motor protein, is associated with MTs in podocytes. (c) Treatment of differentiating podocytes with CHO1/MKLP1 antisense oligonucleotides abolished the formation of processes and the nonuniform polarity of MTs. (d) During the recovery from taxol treatment, taxol-stabilized (nocodazole- resistant) MT fragments were distributed in the cell periphery along newly assembled nocodazole-sensitive MTs. A similar distribution pattern of CHO1/MKLP1 was found under these circumstances, indicating its association with MTs. (e) In the recovery phase after complete depolymerization, MTs reassembled exclusively at centrosomes. Taken together, these findings lead to the conclusion that the nonuniform MT polarity in podocytes established by CHO1/MKLP1 is necessary for process formation.

Role of xklp3, a subunit of the Xenopus kinesin II heterotrimeric complex, in membrane transport between the endoplasmic reticulum and the Golgi apparatus

The function of the Golgi apparatus is to modify proteins and lipids synthesized in the ER and sort them to their final destination. The steady-state size and function of the Golgi apparatus is maintained through the recycling of some components back to the ER. Several lines of evidence indicate that the spatial segregation between the ER and the Golgi apparatus as well as trafficking between these two compartments require both microtubules and motors. We have cloned and characterized a new Xenopus kinesin like protein, Xklp3, a subunit of the heterotrimeric Kinesin II. By immunofluorescence it is found in the Golgi region. A more detailed analysis by EM shows that it is associated with a subset of membranes that contain the KDEL receptor and are localized between the ER and Golgi apparatus. An association of Xklp3 with the recycling compartment is further supported by a biochemical analysis and the behavior of Xklp3 in BFA-treated cells. The function of Xklp3 was analyzed by transfecting cells with a dominant-negative form lacking the motor domain. In these cells, the normal delivery of newly synthesized proteins to the Golgi apparatus is blocked. Taken together, these results indicate that Xklp3 is involved in the transport of tubular-vesicular elements between the ER and the Golgi apparatus.

PAT1, a microtubule-interacting protein, recognizes the basolateral sorting signal of amyloid precursor protein

In epithelial cells, sorting of membrane proteins to the basolateral surface depends on the presence of a basolateral sorting signal (BaSS) in their cytoplasmic domain. Amyloid precursor protein (APP), a basolateral protein implicated in the pathogenesis of Alzheimer's disease, contains a tyrosine-based BaSS, and mutation of the tyrosine residue results in nonpolarized transport of APP. Here we report identification of a protein, termed PAT1 (protein interacting with APP tail 1), that interacts with the APP-BaSS but binds poorly when the critical tyrosine is mutated and does not bind the tyrosine-based endocytic signal of APP. PAT1 shows homology to kinesin light chain, which is a component of the plus-end directed microtubule-based motor involved in transporting membrane proteins to the basolateral surface. PAT1, a cytoplasmic protein, associates with membranes, cofractionates with APP-containing vesicles, and binds microtubules in a nucleotide-sensitive manner. Cotransfection of PAT1 with a reporter protein shows that PAT1 is functionally linked with intracellular transport of APP. We propose that PAT1 is involved in the translocation of APP along microtubules toward the cell surface.

Retrograde transport from the pre-Golgi intermediate compartment and the Golgi complex is affected by the vacuolar H+-ATPase inhibitor bafilomycin A1

The effect of the vacuolar H+-ATPase inhibitor bafilomycin A1 (Baf A1) on the localization of pre-Golgi intermediate compartment (IC) and Golgi marker proteins was used to study the role of acidification in the function of early secretory compartments. Baf A1 inhibited both brefeldin A- and nocodazole-induced retrograde transport of Golgi proteins to the endoplasmic reticulum (ER), whereas anterograde ER-to-Golgi transport remained largely unaffected. Furthermore, p58/ERGIC-53, which normally cycles between the ER, IC, and cis-Golgi, was arrested in pre-Golgi tubules and vacuoles, and the number of p58-positive approximately 80-nm Golgi (coatomer protein I) vesicles was reduced, suggesting that the drug inhibits the retrieval of the protein from post-ER compartments. In parallel, redistribution of beta-coatomer protein from the Golgi to peripheral pre-Golgi structures took place. The small GTPase rab1p was detected in short pre-Golgi tubules in control cells and was efficiently recruited to the tubules accumulating in the presence of Baf A1. In contrast, these tubules showed no enrichment of newly synthesized, anterogradely transported proteins, indicating that they participate in retrograde transport. These results suggest that the pre-Golgi structures contain an active H+-ATPase that regulates retrograde transport at the ER-Golgi boundary. Interestingly, although Baf A1 had distinct effects on peripheral pre-Golgi structures, only more central, p58-containing elements accumulated detectable amounts of 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP), a marker for acidic compartments, raising the possibility that the lumenal pH of the pre-Golgi structures gradually changes in parallel with their translocation to the Golgi region.

In vitro reconstitution of microtubule plus end-directed, GTPgammaS-sensitive motility of Golgi membranes

Purified Golgi membranes were mixed with cytosol and microtubules (MTs) and observed by video enhanced light microscopy. Initially, the membranes appeared as vesicles that moved along MTs. As time progressed, vesicles formed aggregates from which membrane tubules emerged, traveled along MTs, and eventually generated extensive reticular networks. Membrane motility required ATP, occurred mainly toward MT plus ends, and was inhibited almost completely by the H1 monoclonal antibody to kinesin heavy chain, 5'-adenylylimidodiphosphate, and 100 microM but not 20 microM vanadate. Motility was also blocked by GTPgammaS or A1F4- but was insensitive to A1C13, NaF, staurosporin, or okadaic acid. The targets for GTPgammaS and A1F4- were evidently of cytosolic origin, did not include kinesin or MTs, and were insensitive to several probes for trimeric G proteins. Transport of Golgi membranes along MTs mediated by a kinesin has thus been reconstituted in vitro. The motility is regulated by one or more cytosolic GTPases but not by protein kinases or phosphatases that are inhibited by staurosporin or okadaic acid, respectively. The pertinent GTPases are likely to be small G proteins or possibly dynamin. The in vitro motility may correspond to Golgi-to-ER or Golgi-to-cell surface transport in vivo.

Specific binding to a novel and essential Golgi membrane protein (Yip1p) functionally links the transport GTPases Ypt1p and Ypt31p

The regulation of vesicular transport in eukaryotic cells involves Ras-like GTPases of the Ypt/Rab family. Studies in yeast and mammalian cells indicate that individual family members act in vesicle docking/fusion to specific target membranes. Using the two-hybrid system, we have now identified a 248 amino acid, integral membrane protein, termed Yip1, that specifically binds to the transport GTPases Ypt1p and Ypt31p. Evidence for physical interaction of these GTPases with Yip1p was also demonstrated by affinity chromatography and/or co-immunoprecipitation. Like the two GTPases, Yip1p is essential for yeast cell viability and, according to subcellular fractionation and indirect immunofluorescence, is located to Golgi membranes at steady state. Mutant cells depleted of Yip1p and conditionally lethal yip1 mutants at the non-permissive temperature massively accumulate endoplasmic reticulum membranes and display aberrations in protein secretion and glycosylation of secreted invertase. The results suggests for a role for Yip1p in recruiting the two GTPases to Golgi target membranes in preparation for fusion.

The rod cGMP phosphodiesterase delta subunit dissociates the small GTPase Rab13 from membranes

Small Rab GTPases are involved in the regulation of membrane trafficking. They cycle between cytosolic and membrane-bound forms. These membrane association/dissociation are tightly controlled by regulatory proteins. To search for proteins interacting with Rab13, a small GTPase associated with vesicles in fibroblasts and predominantly with tight junctions in epithelial cells, we screened a HeLa two-hybrid cDNA library and isolated a clone encoding a protein of 17.4 kDa. This protein, almost identical to the bovine rod cGMP phosphodiesterase delta subunit, was named human delta-PDE. The delta-PDE binds specifically to Rab13. It exhibits two putative C-terminal sequences necessary for the interaction with PDZ (PSD95, Dlg, ZO-1) domains contained in many proteins localized to specific plasma membrane microdomains. Immunofluorescence microscopic studies revealed that the vesicular stomatitis virus (VSV)-tagged delta-PDE is localized in vesicular structures accumulated near the plasma membrane in epithelial cells. Deletion of the PDZ binding motifs impair VSV-delta-PDE subcellular distribution. Purified recombinant delta-PDE had the capacity to dissociate Rab13 from cellular membranes. Our data support the proposal that delta-PDE, but not GDP dissociation inhibitor, may serve to control the dynamic of the association of Rab13 with cellular membranes.

Vesicle budding on Golgi membranes: regulation by G proteins and myosin motors

One of the main functions of the Golgi complex is to generate transport vesicles for the post-Golgi trafficking of proteins in secretory pathways. Many different populations of vesicles are distinguished by unique sets of structural and regulatory proteins which participate in vesicle budding and fusion. Monomeric and heterotrimeric G proteins regulate vesicle budding and secretory traffic into and out of the Golgi complex. An inventory of G protein alpha subunits associated with Golgi membranes highlights their diverse involvement and potential for coupling Golgi trafficking, through various signal transduction pathways, to cell growth or other more specialized cell functions. Cytoskeletal proteins are now also known to associate specifically with the Golgi complex and Golgi-derived vesicles. Amongst these, conventional and unconventional myosins are recruited to vesicle membranes. Several roles in vesicle budding and vesicle trafficking can be proposed for these actin-based motors.

Rab proteins

Rab proteins form the largest branch of the Ras superfamily of GTPases. They are localized to the cytoplasmic face of organelles and vesicles involved in the biosynthetic/secretory and endocytic pathways in eukaryotic cells. It is now well established that Rab proteins play an essential role in the processes that underlie the targeting and fusion of transport vesicles with their appropriate acceptor membranes. However, the recent discovery of several putative Rab effectors, which are not related to each other and which fulfil diverse functions, suggests a more complex role for Rab proteins. At least two Rab proteins act at the level of the Golgi apparatus. Rab1 and its yeast counterpart Ypt1 control transport events through early Golgi compartments. Work from our laboratory points out a role for Rab6 in intra-Golgi transport, likely in a retrograde direction.

The organisation of the Golgi apparatus

The past year has seen considerable progress in understanding the mechanism of COPI (coatomer protein I) vesicle docking and SNARE (soluble NSF attachment protein receptor) mediated fusion, the mechanism of cisternal growth and stacking and the regulation of Golgi architecture. The route taken by cargo proteins through the Golgi apparatus is still a matter of some dispute.

Hydrolysis of GTP on rab11 is required for the direct delivery of transferrin from the pericentriolar recycling compartment to the cell surface but not from sorting endosomes

Rab11 is a small GTP-binding protein that in cultured mammalian cells has been shown to be concentrated in the pericentriolar endosomal recycling compartment and to play a key role in passage of the recycling transferrin receptor through that compartment [Ullrich, O., Reinsch, S., Urbé, S., Zerial, M. & Parton, R. G. (1996) J. Cell Biol. 135, 913-924]. To obtain insights into the site(s) of action of rab11 within the recycling pathway, we have now compared the effects on recycling at 37 degreesC of overexpression of wild-type rab11 and various mutant forms of this protein in cells that had been loaded with transferrin at either 37 degreesC or 16 degreesC. We show that incubation at 16 degreesC blocks passage of endocytosed transferrin into the recycling compartment and that, whereas the rab11 dominant negative mutant form (S25N) inhibits transferrin recycling after interiorization at either temperature, the wild-type rab11 and constitutively active mutant (Q70L) have no inhibitory effect on the recycling of molecules that were interiorized at 16 degreesC. This differential inhibitory effect shows that two distinct pathways for recycling are followed by the bulk of the transferrin molecules interiorized at the two different temperatures. The incapacity of the constitutively active form of rab11 (Q70L) to inhibit recycling of molecules interiorized at 16 degreesC is consistent with their recycling taking place directly from sorting endosomes, in a process that does not require hydrolysis of GTP on rab11. The fact that the dominant negative (S25N) form of rab11 inhibits recycling of molecules interiorized at both temperatures indicates that activation of rab11 by GTP is required for exit of transferrin from sorting endosomes, regardless of whether this exit is toward the recycling compartment or directly to the plasma membrane.