Isolation of functional Golgi-derived vesicles with a possible role in retrograde transport

Syntaxin-5's flexibility in SNARE pairing supports Golgi functions

Deficiency in the conserved oligomeric Golgi (COG) complex that orchestrates SNARE-mediated tethering/fusion of vesicles that recycle the Golgi's glycosylation machinery results in severe glycosylation defects. Although two major Golgi v-SNAREs, GS28/GOSR1, and GS15/BET1L, are depleted in COG-deficient cells, the complete knockout of GS28 and GS15 only modestly affects Golgi glycosylation, indicating the existence of an adaptation mechanism in Golgi SNARE. Indeed, quantitative mass-spectrometry analysis of STX5-interacting proteins revealed two novel Golgi SNARE complexes-STX5/SNAP29/VAMP7 and STX5/VTI1B/STX8/YKT6. These complexes are present in wild-type cells, but their usage is significantly increased in both GS28- and COG-deficient cells. Upon GS28 deletion, SNAP29 increased its Golgi residency in a STX5-dependent manner. While STX5 depletion and Retro2-induced diversion from the Golgi severely affect protein glycosylation, GS28/SNAP29 and GS28/VTI1B double knockouts alter glycosylation similarly to GS28 KO, indicating that a single STX5-based SNARE complex is sufficient to support Golgi glycosylation. Importantly, co-depletion of three Golgi SNARE complexes in GS28/SNAP29/VTI1B TKO cells resulted in severe glycosylation defects and a reduced capacity for glycosylation enzyme retention at the Golgi. This study demonstrates the remarkable plasticity in SXT5-mediated membrane trafficking, uncovering a novel adaptive response to the failure of canonical intra-Golgi vesicle tethering/fusion machinery.

Acute COG complex inactivation unveiled its immediate impact on Golgi and illuminated the nature of intra-Golgi recycling vesicles

Conserved Oligomeric Golgi (COG) complex controls Golgi trafficking and glycosylation, but the precise COG mechanism is unknown. The auxin-inducible acute degradation system was employed to investigate initial defects resulting from COG dysfunction. We found that acute COG inactivation caused a massive accumulation of COG-dependent (CCD) vesicles that carry the bulk of Golgi enzymes and resident proteins. v-SNAREs (GS15, GS28) and v-tethers (giantin, golgin84, and TMF1) were relocalized into CCD vesicles, while t-SNAREs (STX5, YKT6), t-tethers (GM130, p115), and most of Rab proteins remained Golgi-associated. Airyscan microscopy and velocity gradient analysis revealed that different Golgi residents are segregated into different populations of CCD vesicles. Acute COG depletion significantly affected three Golgi-based vesicular coats-COPI, AP1, and GGA, suggesting that COG uniquely orchestrates tethering of multiple types of intra-Golgi CCD vesicles produced by different coat machineries. This study provided the first detailed view of primary cellular defects associated with COG dysfunction in human cells.

Mannose glycosylation is an integral step for NIS localization and function in human breast cancer cells

Chasing an intriguing biological question on the disparity of sodium iodide symporter (NIS, officially known as SLC5A5) expression and function in the clinical scenario of breast cancer, this study addresses key molecular defects involved. NIS in cancer patients has primarily been recorded to be a cytoplasmic protein, thus limiting the scope for targeted radio-iodine therapy. We developed NIS transgene-overexpressing MCF-7 breast cancer cells, and found a few clonal derivatives that show predominant expression of NIS in the plasma membrane. The majority of clones, however, showed cytosolic NIS expression over long passages. Cells expressing membranous NIS show unperturbed dynamic trafficking of NIS through secretory pathway organelles when compared to cells expressing cytoplasmic NIS or to parental cells. Further, treatment of cells expressing membranous NIS with specific glycosylation inhibitors highlighted the importance of inherent glycosylation processing and an 84 gene signature glycosylation RT-Profiler array revealed that clones expressing NIS in their membrane cluster separately compared to the other cells. We further confirm a role of three differentially expressed genes, i.e. MAN1B1, MAN1A1 and MAN2A1, in regulating NIS localization by RNA interference. Thus, this study shows the important role of mannosidase in N-glycosylation processing in order to correctly traffic NIS to the plasma membrane in breast cancer cells.This article has an associated First Person interview with the first author of the paper.

Overexpression of trans-Golgi network t-SNAREs rescues vacuolar trafficking and TGN morphology defects in a putative tethering factor mutant

The trans-Golgi network (TGN) is a major site for sorting of cargo to either the vacuole or apoplast. The TGN-localized coiled-coil protein TNO1 is a putative tethering factor that interacts with the TGN t-SNARE SYP41 and is required for correct localization of the SYP61 t-SNARE. An Arabidopsis thaliana tno1 mutant is hypersensitive to salt stress and partially mislocalizes vacuolar proteins to the apoplast, indicating a role in vacuolar trafficking. Here, we show that overexpression of SYP41 or SYP61 significantly increases SYP41-SYP61 complex formation in a tno1 mutant, and rescues the salt sensitivity and defective vacuolar trafficking of the tno1 mutant. The TGN is disrupted and vesicle budding from Golgi cisternae is reduced in the tno1 mutant, and these defects are also rescued by overexpression of SYP41 or SYP61. Our results suggest that the trafficking and Golgi morphology defects caused by loss of TNO1 can be rescued by increasing SYP41-SYP61 t-SNARE complex formation, implicating TNO1 as a tethering factor mediating efficient vesicle fusion at the TGN.

Cell-free Fluorescent Intra-Golgi Retrograde Vesicle Trafficking Assay

Intra-Golgi retrograde vesicle transport is used to traffic and sort resident Golgi enzymes to their appropriate cisternal locations. An assay was established to investigate the molecular details of vesicle targeting in a cell-free system. Stable cell lines were generated in which the trans-Golgi enzyme galactosyltransferase (GalT) was tagged with either CFP or YFP. Given that GalT is recycled to the cisterna where it is located at steady state, GalT-containing vesicles target GalT-containing cisternal membranes. Golgi membranes were therefore isolated from GalT-CFP expressing cells, while vesicles were prepared from GalT-YFP expressing ones. Incubating CFP-labelled Golgi with YFP-labelled vesicles in the presence of cytosol and an energy regeneration mixture at 37 °C produced a significant increase in CFP-YFP co-localization upon fluorescent imaging of the mixture compared to incubation on ice. The assay was validated to require energy, proteins and physiologically important trafficking components such as Rab GTPases and the conserved oligomeric Golgi tethering complex. This assay is useful for the investigation of both physiological and pathological changes that affect the Golgi trafficking machinery, in particular, vesicle tethering.

Dissecting functions of the conserved oligomeric Golgi tethering complex using a cell-free assay

Vesicle transport sorts proteins between compartments and is thereby responsible for generating the non-uniform protein distribution along the eukaryotic secretory and endocytic pathways. The mechanistic details of specific vesicle targeting are not yet well characterized at the molecular level. We have developed a cell-free assay that reconstitutes vesicle targeting utilizing the recycling of resident enzymes within the Golgi apparatus. The assay has physiological properties, and could be used to show that the two lobes of the conserved oligomeric Golgi tethering complex play antagonistic roles in trans-Golgi vesicle targeting. Moreover, we can show that the assay is sensitive to several different congenital defects that disrupt Golgi function and therefore cause glycosylation disorders. Consequently, this assay will allow mechanistic insight into the targeting step of vesicle transport at the Golgi, and could also be useful for characterizing some novel cases of congenital glycosylation disorders.

Inter-Golgi transport mediated by COPI-containing vesicles carrying small cargoes

A core prediction of the vesicular transport model is that COPI vesicles are responsible for trafficking anterograde cargoes forward. In this study, we test this prediction by examining the properties and requirements of inter-Golgi transport within fused cells, which requires mobile carriers in order for exchange of constituents to occur. We report that both small soluble and membrane-bound secretory cargo and exogenous Golgi resident glycosyl-transferases are exchanged between separated Golgi. Large soluble aggregates, which traverse individual stacks, do not transfer between Golgi, implying that small cargoes (which can fit in a typical transport vesicle) are transported by a different mechanism. Super-resolution microscopy reveals that the carriers of both anterograde and retrograde cargoes are the size of COPI vesicles, contain coatomer, and functionally require ARF1 and coatomer for transport. The data suggest that COPI vesicles traffic both small secretory cargo and steady-state Golgi resident enzymes among stacked cisternae that are stationary.

DOI:http://dx.doi.org/10.7554/eLife.01296.001

All eukaryotic cells contain an organelle called the Golgi apparatus, which consists of a series of four to six flattened structures called cisternae. Proteins that are intended for secretion from the cell, or proteins that go on to become part of the cell membrane, must pass through the Golgi, where they undergo modifications that ensure they are targeted to the correct place.

There are two main models for how proteins are transported from the entry side of the Golgi, known as the cis face, to the exit side (trans face), through a process known as anterograde transport. One possibility is that the cargo protein matures within a single cisterna, which gradually moves from the cis to the trans face without the protein ever leaving it. Alternatively, the cisternae may remain fixed in position, while individual proteins are carried between them by specialized transport vesicles called COPI vesicles.

Now, Pellett et al. have used modern molecular biology techniques to revisit this question, more than 25 years after members of the same group first obtained evidence suggesting the involvement of COPI vesicles. To do this, they labelled the proteins that reside within the Golgi of one cell green, and those within the Golgi of another cell, red. They then fused the two cells together, and traced the movement of labelled proteins between the two organelles.

Proteins that are known to undergo anterograde transport were also transported between the two Golgi, whereas large protein aggregates were not. Super-resolution microscopy revealed that the transported proteins were carried in vesicles the size of COPI vesicles and surrounded by a coat protein that resembles COPI. Moreover, transport involved the adaptor protein ARF, which helps to load cargo into COPI vesicles.

By providing evidence that Golgi resident proteins and proteins that normally undergo anterograde transport can be carried by COPI vesicles between two physically separate Golgi, Pellett et al. increase the weight of evidence that COPI vesicles may also be responsible for both retrograde and anterograde transport within the Golgi itself.

DOI:http://dx.doi.org/10.7554/eLife.01296.002

The dynamics of engineered resident proteins in the mammalian Golgi complex relies on cisternal maturation

After leaving the endoplasmic reticulum, secretory proteins traverse several membranous transport compartments before reaching their destinations. How they move through the Golgi complex, a major secretory station composed of stacks of membranous cisternae, is a central yet unsettled issue in membrane biology. Two classes of mechanisms have been proposed. One is based on cargo-laden carriers hopping across stable cisternae and the other on "maturing" cisternae that carry cargo forward while progressing through the stack. A key difference between the two concerns the behavior of Golgi-resident proteins. Under stable cisternae models, Golgi residents remain in the same cisterna, whereas, according to cisternal maturation, Golgi residents recycle from distal to proximal cisternae via retrograde carriers in synchrony with cisternal progression. Here, we have engineered Golgi-resident constructs that can be polymerized at will to prevent their recycling via Golgi carriers. Maturation models predict the progress of such polymerized residents through the stack along with cargo, but stable cisternae models do not. The results support the cisternal maturation mechanism.

Segregation of the Qb-SNAREs GS27 and GS28 into Golgi vesicles regulates intra-Golgi transport

The Golgi apparatus is the main glycosylation and sorting station along the secretory pathway. Its structure includes the Golgi vesicles, which are depleted of anterograde cargo, and also of at least some Golgi-resident proteins. The role of Golgi vesicles remains unclear. Here, we show that Golgi vesicles are enriched in the Qb-SNAREs GS27 (membrin) and GS28 (GOS-28), and depleted of nucleotide sugar transporters. A block of intra-Golgi transport leads to accumulation of Golgi vesicles and partitioning of GS27 and GS28 into these vesicles. Conversely, active intra-Golgi transport induces fusion of these vesicles with the Golgi cisternae, delivering GS27 and GS28 to these cisternae. In an in vitro assay based on a donor compartment that lacks UDP-galactose translocase (a sugar transporter), the segregation of Golgi vesicles from isolated Golgi membranes inhibits intra-Golgi transport; re-addition of isolated Golgi vesicles devoid of UDP-galactose translocase obtained from normal cells restores intra-Golgi transport. We conclude that this activity is due to the presence of GS27 and GS28 in the Golgi vesicles, rather than the sugar transporter. Furthermore, there is an inverse correlation between the number of Golgi vesicles and the number of inter-cisternal connections under different experimental conditions. Finally, a rapid block of the formation of vesicles via COPI through degradation of ϵCOP accelerates the cis-to-trans delivery of VSVG. These data suggest that Golgi vesicles, presumably with COPI, serve to inhibit intra-Golgi transport by the extraction of GS27 and GS28 from the Golgi cisternae, which blocks the formation of inter-cisternal connections.

Non-muscle myosin IIA transports a Golgi glycosyltransferase to the endoplasmic reticulum by binding to its cytoplasmic tail

The mechanism of the Golgi-to-ER transport of Golgi glycosyltransferases is not clear. We utilize a cell line expressing the core 2 N-acetylglucosaminyltransferase-M (C2GnT-M) tagged with c-Myc to explore this mechanism. By immunoprecipitation using anti-c-Myc antibodies coupled with proteomics analysis, we have identified several proteins including non-muscle myosin IIA (NMIIA), heat shock protein (HSP)-70 and ubiquitin activating enzyme E1 in the immunoprecipitate. Employing yeast-two-hybrid analysis and pulldown experiments, we show that the C-terminal region of the NMIIA heavy chain binds to the 1-6 amino acids in the cytoplasmic tail of C2GnT-M. We have found that NMIIA co-localizes with C2GnT-M at the periphery of the Golgi. In addition, inhibition or knockdown of NMIIA prevents the brefeldin A-induced collapse of the Golgi as shown by the inhibition of the migration of both Giantin, a Golgi matrix protein, and C2GnT-M, a Golgi non-matrix protein, to the ER. In contrast, knockdown of HSP70 retains Giantin in the Golgi but moves C2GnT-M to the ER, a process also blocked by inhibition or knockdown of NMIIA. Also, the intracellular distribution of C2GnT-M is not affected by knockdown of β-coatomer protein with or without inhibition of HSPs, suggesting that the Golgi-to-ER trafficking of C2GnT-M does not depend on coat protein complex-I. Further, inhibition of proteasome results in accumulation of ubiquitinated C2GnT-M, suggesting its degradation by proteasome. Therefore, NMIIA and not coat protein complex-I is responsible for transporting the Golgi glycosyltransferase to the ER for proteasomal degradation. The data suggest that NMIIA is involved in the Golgi remodeling.

Irradiation-induced protein inactivation reveals Golgi enzyme cycling to cell periphery

Acute inhibition is a powerful technique to test proteins for direct roles and order their activities in a pathway, but as a general gene-based strategy, it is mostly unavailable in mammalian systems. As a consequence, the precise roles of proteins in membrane trafficking have been difficult to assess in vivo. Here we used a strategy based on a genetically encoded fluorescent protein that generates highly localized and damaging reactive oxygen species to rapidly inactivate exit from the endoplasmic reticulum (ER) during live-cell imaging and address the long-standing question of whether the integrity of the Golgi complex depends on constant input from the ER. Light-induced blockade of ER exit immediately perturbed Golgi membranes, and surprisingly, revealed that cis-Golgi-resident proteins continuously cycle to peripheral ER-Golgi intermediate compartment (ERGIC) membranes and depend on ER exit for their return to the Golgi. These experiments demonstrate that ER exit and extensive cycling of cis-Golgi components to the cell periphery sustain the mammalian Golgi complex.

Golgi and related vesicle proteomics: simplify to identify

Despite more than six decades of successful Golgi research, the fundamental question as to how biosynthetic material is transported through the secretory pathway remains unanswered. New technologies such as live cell imaging and correlative microscopy have highlighted the plastic nature of the Golgi, one that is sensitive to perturbation yet highly efficient in regaining both structure and function. Single molecule-microscopy and super resolution-microscopy further adds to this picture. Various models for protein transport have been put forward, each with its own merits and pitfalls but we are far from resolving whether one is more correct than the other. As such, our laboratory considers multiple mechanisms of Golgi transport until proven otherwise. This includes the two classical modes of transport, vesicular transport and cisternal progression/maturation as well as more recent models such as tubular inter- and intra-cisternal connections (long lasting or transient) and inter-Golgi stack transport. In this article, we focus on an emerging inductive technology, mass spectrometry-based proteomics that has already enabled insight into the relative composition of compartments and subcompartments of the secretory pathway including mechanistic aspects of protein transport. We note that proteomics, as with any other technology, is not a stand-alone technology but one that works best alongside complementary approaches.

Interaction of Golgin-84 with the COG complex mediates the intra-Golgi retrograde transport

The coiled-coil Golgi membrane protein golgin-84 functions as a tethering factor for coat protein I (COPI) vesicles. Protein interaction analyses have revealed that golgin-84 interacts with another tether, the conserved oligomeric Golgi (COG) complex, through its subunit Cog7. Therefore, we explored the function of golgin-84 as the tether for COPI vesicles of intra-Golgi retrograde traffic. First, glycosylic maturation of both plasma membrane (CD44) and lysosomal (lamp1) glycoproteins was distorted in golgin-84 knockdown (KD) cells. The depletion of golgin-84 caused fragmentation of the Golgi with the mislocalization of Golgi resident proteins, resulting in the accumulation of vesicles carrying intra-Golgi soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and cis-Golgi membrane protein GPP130. Similar observations were obtained by diminution of the COG complex, suggesting a strong correlation between the two tethers. Indeed, COG complex-dependent (CCD) vesicles that accumulate in Cog3 or Cog7 KD cells carried golgin-84. Surprisingly, the interaction between golgin-84 and another candidate tethering partner CASP (CDP/cut alternatively spliced product) decreased in Cog3 KD cells. These results indicate that golgin-84 on COPI vesicles interact with the COG complex before SNARE assembly, suggesting that the interaction of golgin-84 with COG plays an important role in the tethering process of intra-Golgi retrograde vesicle traffic.

Membrane traffic within the Golgi apparatus

Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry secretory cargoes forward and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional endoplasmic reticulum sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell-type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key mechanistic questions remain about the formation and maturation of Golgi cisternae, the recycling of resident Golgi proteins, the origins of Golgi compartmental identity, the establishment of Golgi architecture, and the roles of Golgi structural elements in membrane traffic.

PtdIns4P recognition by Vps74/GOLPH3 links PtdIns 4-kinase signaling to retrograde Golgi trafficking

Targeting and retention of resident integral membrane proteins of the Golgi apparatus underly the function of the Golgi in glycoprotein and glycolipid processing and sorting. In yeast, steady-state Golgi localization of multiple mannosyltransferases requires recognition of their cytosolic domains by the peripheral Golgi membrane protein Vps74, an orthologue of human GOLPH3/GPP34/GMx33/MIDAS (mitochondrial DNA absence sensitive factor). We show that targeting of Vps74 and GOLPH3 to the Golgi apparatus requires ongoing synthesis of phosphatidylinositol (PtdIns) 4-phosphate (PtdIns4P) by the Pik1 PtdIns 4-kinase and that modulation of the levels and cellular location of PtdIns4P leads to mislocalization of these proteins. Vps74 and GOLPH3 bind specifically to PtdIns4P, and a sulfate ion in a crystal structure of GOLPH3 indicates a possible phosphoinositide-binding site that is conserved in Vps74. Alterations in this site abolish phosphoinositide binding in vitro and Vps74 function in vivo. These results implicate Pik1 signaling in retention of Golgi-resident proteins via Vps74 and show that GOLPH3 family proteins are effectors of Golgi PtdIns 4-kinases.

Functional analysis of GS28, an intra-Golgi SNARE, in Caenorhabditis elegans

Intra-Golgi retrograde transport is assumed to maintain Golgi function by recycling Golgi-resident proteins to younger cisternae in the progression of entire Golgi stack from cis to trans. GS28 (Golgi SNARE of 28 kDa, also known as GOS28) is a Golgi-localized SNARE protein and has been implicated in intra-Golgi retrograde transport. However, the in vivo functions of GS28, and consequently, the roles of the intra-Golgi retrograde transport in animal development are largely unknown. In this study, we generated deletion mutants of Caenorhabditis elegans GS28 and performed a synthetic lethal RNAi screen using GS28 mutants. We found that another Golgi-localized SNARE, Ykt6, functions cooperatively with GS28 in embryonic development. During post-embryonic development, GS28 mutants exhibited reduced seam cell numbers and a missing ray phenotype under Ykt6 knockdown conditions, suggesting that cell proliferation and/or differentiation of stem cell-like seam cells are impaired in GS28- and Ykt6-depleted worms. We also demonstrated that GS28 and Ykt6 act redundantly for the proper expression of Golgi-resident proteins in adult intestinal cells. This study reveals the in vivo importance of the Golgi-localized SNAREs GS28 and Ykt6.

Rab1b interacts with GBF1 and modulates both ARF1 dynamics and COPI association

Assembly of the cytosolic coat protein I (COPI) complex at the ER-Golgi interface is directed by the ADP ribosylation factor1 (Arf1) and its guanine nucleotide exchange factor (GBF1). Rab1b GTPase modulates COPI recruitment, but the molecular mechanism underlying this action remains unclear. Our data reveal that in vivo expression of the GTP-restricted Rab1b mutant (Rab1Q67L) increased the association of GBF1 and COPI to peripheral structures localized at the ER exit sites (ERES) interface. Active Rab1b also stabilized Arf1 on Golgi membranes. Furthermore, we characterized GBF1 as a new Rab1b effector, and showed that its N-terminal domain was involved in this interaction. Rab1b small interfering RNA oligonucleotide assays suggested that Rab1b was required for GBF1 membrane association. To further understand how Rab1b functions in ER-to-Golgi transport, we analyzed GFP-Rab1b dynamics in HeLa cells. Time-lapse microscopy indicated that the majority of the Rab1b-labeled punctuated structures are relatively short-lived with limited-range movements. FRAP of Golgi GFP-Rab1bwt showed rapid recovery (t(1/2) 120 s) with minimal dependence on microtubules. Our data support a model where Rab1b-GTP induces GBF1 recruitment at the ERES interface and at the Golgi complex where it is required for COPII/COPI exchange or COPI vesicle formation, respectively.

New Insights into Membrane Trafficking and Protein Sorting

Protein transport in the secretory and endocytic pathways is a multistep process involving the generation of transport carriers loaded with defined sets of cargo, the shipment of the cargo-loaded transport carriers between compartments, and the specific fusion of these transport carriers with a target membrane. The regulation of these membrane-mediated processes involves a complex array of protein and lipid interactions. As the machinery and regulatory processes of membrane trafficking have been defined, it is increasingly apparent that membrane transport is intimately connected with a number of other cellular processes, such as quality control in the endoplasmic reticulum (ER), cytoskeletal dynamics, receptor signaling, and mitosis. The fidelity of membrane trafficking relies on the correct assembly of components on organelles. Recruitment of peripheral proteins plays a critical role in defining organelle identity and the establishment of membrane subdomains, essential for the regulation of vesicle transport. The molecular mechanisms for the biogenesis of membrane subdomains are also central to understanding how cargo is sorted and segregated and how different populations of transport carriers are generated. In this review we will focus on the emerging themes of organelle identity, membrane subdomains, regulation of Golgi trafficking, and advances in dissecting pathways in physiological systems.

COG complex-mediated recycling of Golgi glycosyltransferases is essential for normal protein glycosylation

Defects in conserved oligomeric Golgi (COG) complex result in multiple deficiencies in protein glycosylation. On the other hand, acute knock-down (KD) of Cog3p (COG3 KD) causes accumulation of intra-Golgi COG complex-dependent (CCD) vesicles. Here, we analyzed cellular phenotypes at different stages of COG3 KD to uncover the molecular link between COG function and glycosylation disorders. For the first time, we demonstrated that medial-Golgi enzymes are transiently relocated into CCD vesicles in COG3 KD cells. As a result, Golgi modifications of both plasma membrane (CD44) and lysosomal (Lamp2) glycoproteins are distorted. Localization of these proteins is not altered, indicating that the COG complex is not required for anterograde trafficking and accurate sorting. COG7 KD and double COG3/COG7 KD caused similar defects with respect to both Golgi traffic and glycosylation, suggesting that the entire COG complex orchestrates recycling of medial-Golgi-resident proteins. COG complex-dependent docking of isolated CCD vesicles was reconstituted in vitro, supporting their role as functional trafficking intermediates. Altogether, the data suggest that constantly cycling medial-Golgi enzymes are transported from distal compartments in CCD vesicles. Dysfunction of COG complex leads to separation of glycosyltransferases from anterograde cargo molecules passing along secretory pathway, thus affecting normal protein glycosylation.

Rab1 defines a novel pathway connecting the pre-Golgi intermediate compartment with the cell periphery

The function of the pre-Golgi intermediate compartment (IC) and its relationship with the endoplasmic reticulum (ER) and Golgi remain only partially understood. Here, we report striking segregation of IC domains in polarized PC12 cells that develop neurite-like processes. Differentiation involves expansion of the IC and movement of Rab1-containing tubules to the growth cones of the neurites, whereas p58- and COPI-positive IC elements, like rough ER and Golgi, remain in the cell body. Exclusion of Rab1 effectors p115 and GM130 from the neurites further indicated that the centrifugal, Rab1-mediated pathway has functions that are not directly related to ER-to-Golgi trafficking. Disassembly of COPI coats did not affect this pathway but resulted in missorting of p58 to the neurites. Live cell imaging showed that green fluorescent protein (GFP)-Rab1A-containing IC elements move bidirectionally both within the neurites and cell bodies, interconnecting different ER exit sites and the cis-Golgi region. Moreover, in nonpolarized cells GFP-Rab1A-positive tubules moved centrifugally towards the cell cortex. Hydroxymethylglutaryl-CoA reductase, the key enzyme of cholesterol biosynthesis, colocalized with slowly sedimenting, Rab1-enriched membranes when the IC subdomains were separated by velocity sedimentation. These results reveal a novel pathway directly connecting the IC with the cell periphery and suggest that this Rab1-mediated pathway is linked to the dynamics of smooth ER.

Role of tethering factors in secretory membrane traffic

Coiled-coil and multisubunit tethers have emerged as key regulators of membrane traffic and organellar architecture. The restricted subcellular localization of tethers and their ability to interact with Rabs and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) suggests that tethers participate in determining the specificity of membrane fusion. An accepted model of tether function considers them molecular “bridges” that link opposing membranes before SNARE pairing. This model has been extended by findings in various experimental systems, suggesting that tethers may have other functions. Recent reports implicate tethers in the assembly of SNARE complexes, cargo selection and transit, cytoskeletal events, and localized attachment of regulatory proteins. A concept of tethers as scaffolding machines that recruit protein components involved in varied cellular responses is emerging. In this model, tethers function as integration switches that simultaneously transmit information to coordinate distinct processes required for membrane traffic.

The role of ARF1 and rab GTPases in polarization of the Golgi stack

The organization and sorting of proteins within the Golgi stack to establish and maintain its cis to trans polarization remains an enigma. The function of Golgi compartments involves coat assemblages that facilitate vesicle traffic, Rab-tether-SNAP receptor (SNARE) machineries that dictate membrane identity, as well as matrix components that maintain structure. We have investigated how the Golgi complex achieves compartmentalization in response to a key component of the coat complex I (COPI) coat assembly pathway, the ARF1 GTPase, in relationship to GTPases-regulating endoplasmic reticulum (ER) exit (Sar1) and targeting fusion (Rab1). Following collapse of the Golgi into the ER in response to inhibition of activation of ARF1 by Brefeldin A, we found that Sar1- and Rab1-dependent Golgi reformation took place at multiple peripheral and perinuclear ER exit sites. These rapidly converged into immature Golgi that appeared as onion-like structures composed of multiple concentrically arrayed cisternae of mixed enzyme composition. During clustering to the perinuclear region, Golgi enzymes were sorted to achieve the degree of polarization within the stack found in mature Golgi. Surprisingly, we found that sorting of Golgi enzymes into their subcompartments was insensitive to the dominant negative GTP-restricted ARF1 mutant, a potent inhibitor of COPI coat disassembly and vesicular traffic. We suggest that a COPI-independent, Rab-dependent mechanism is involved in the rapid reorganization of resident enzymes within the Golgi stack following synchronized release from the ER, suggesting an important role for Rab hubs in directing Golgi polarization.

Commuting between Golgi cisternae—Mind the GAP!

Intracellular transport has remained central to cell biology now for more than 40 years. Despite this, we still lack an overall mechanistic framework that describes transport in different parts of the cell. In the secretory pathway, basic questions, such as how biosynthetic cargo traverses the pathway, are still debated. Historically, emphasis was first put on interpreting function from morphology at the ultrastructural level revealing membrane structures such as the transitional ER, vesicular carriers, vesicular tubular clusters, Golgi cisternae, Golgi stacks and the Golgi ribbon. This emphasis on morphology later switched to biochemistry and yeast genetics yielding many of the key molecular players and their associated functions that we know today. More recently, microscopy studies of living cells incorporating biophysics and system analysis has proven useful and is often used to readdress earlier findings, sometimes with surprising outcomes.

Procollagen trafficking, processing and fibrillogenesis

Collagen fibrils in the extracellular matrix allow connective tissues such as tendon, skin and bone to withstand tensile forces. The fibrils are indeterminate in length, insoluble and form elaborate three-dimensional arrays that extend over numerous cell lengths. Studies of the molecular basis of collagen fibrillogenesis have provided insight into the trafficking of procollagen (the precursor of collagen) through the cellular secretory pathway, the conversion of procollagen to collagen by the procollagen metalloproteinases, and the directional deposition of fibrils involving the plasma membrane and late secretory pathway. Fibril-associated molecules are targeted to the surface of collagen fibrils, and these molecules play an important role in regulating the diameter and interactions between the fibrils.

Cog3p depletion blocks vesicle-mediated Golgi retrograde trafficking in HeLa cells

The conserved oligomeric Golgi (COG) complex is an evolutionarily conserved multi-subunit protein complex that regulates membrane trafficking in eukaryotic cells. In this work we used short interfering RNA strategy to achieve an efficient knockdown (KD) of Cog3p in HeLa cells. For the first time, we have demonstrated that Cog3p depletion is accompanied by reduction in Cog1, 2, and 4 protein levels and by accumulation of COG complex-dependent (CCD) vesicles carrying v-SNAREs GS15 and GS28 and cis-Golgi glycoprotein GPP130. Some of these CCD vesicles appeared to be vesicular coat complex I (COPI) coated. A prolonged block in CCD vesicles tethering is accompanied by extensive fragmentation of the Golgi ribbon. Fragmented Golgi membranes maintained their juxtanuclear localization, cisternal organization and are competent for the anterograde trafficking of vesicular stomatitis virus G protein to the plasma membrane. In a contrast, Cog3p KD resulted in inhibition of retrograde trafficking of the Shiga toxin. Furthermore, the mammalian COG complex physically interacts with GS28 and COPI and specifically binds to isolated CCD vesicles.

Regulation of microtubule-dependent recycling at the trans-Golgi network by Rab6A and Rab6A'

The small GTPase rab6A but not the isoform rab6A' has previously been identified as a regulator of the COPI-independent recycling route that carries Golgi-resident proteins and certain toxins from the Golgi to the endoplasmic reticulum (ER). The isoform rab6A' has been implicated in Golgi-to-endosomal recycling. Because rab6A but not A', binds rabkinesin6, this motor protein is proposed to mediate COPI-independent recycling. We show here that both rab6A and rab6A' GTP-restricted mutants promote, with similar efficiency, a microtubule-dependent recycling of Golgi resident glycosylation enzymes upon overexpression. Moreover, we used small interfering RNA mediated down-regulation of rab6A and A' expression and found that reduced levels of rab6 perturbs organization of the Golgi apparatus and delays Golgi-to-ER recycling. Rab6-directed Golgi-to-ER recycling seems to require functional dynactin, as overexpression of p50/dynamitin, or a C-terminal fragment of Bicaudal-D, both known to interact with dynactin inhibit recycling. We further present evidence that rab6-mediated recycling seems to be initiated from the trans-Golgi network. Together, this suggests that a recycling pathway operates at the level of the trans-Golgi linking directly to the ER. This pathway would be the preferred route for both toxins and resident Golgi proteins.

Golgi enzymes are enriched in perforated zones of golgi cisternae but are depleted in COPI vesicles

In the most widely accepted version of the cisternal maturation/progression model of intra-Golgi transport, the polarity of the Golgi complex is maintained by retrograde transport of Golgi enzymes in COPI-coated vesicles. By analyzing enzyme localization in relation to the three-dimensional ultrastructure of the Golgi complex, we now observe that Golgi enzymes are depleted in COPI-coated buds and 50- to 60-nm COPI-dependent vesicles in a variety of different cell types. Instead, we find that Golgi enzymes are concentrated in the perforated zones of cisternal rims both in vivo and in a cell-free system. This lateral segregation of Golgi enzymes is detectable in some stacks during steady-state transport, but it was significantly prominent after blocking endoplasmic reticulum-to-Golgi transport. Delivery of transport carriers to the Golgi after the release of a transport block leads to a diminution in Golgi enzyme concentrations in perforated zones of cisternae. The exclusion of Golgi enzymes from COPI vesicles and their transport-dependent accumulation in perforated zones argues against the current vesicle-mediated version of the cisternal maturation/progression model.

Intracellular sorting and transport of proteins

The secretory and endocytic pathways of eukaryotic organelles consist of multiple compartments, each with a unique set of proteins and lipids. Specific transport mechanisms are required to direct molecules to defined locations and to ensure that the identity, and hence function, of individual compartments are maintained. The localisation of proteins to specific membranes is complex and involves multiple interactions. The recent dramatic advances in understanding the molecular mechanisms of membrane transport has been due to the application of a multi-disciplinary approach, integrating membrane biology, genetics, imaging, protein and lipid biochemistry and structural biology. The aim of this review is to summarise the general principles of protein sorting in the secretory and endocytic pathways and to highlight the dynamic nature of these processes. The molecular mechanisms involved in this transport along the secretory and endocytic pathways are discussed along with the signals responsible for targeting proteins to different intracellular locations.

ER-to-Golgi transport: COP I and COP II function (Review)

COP I and COP II coat proteins direct protein and membrane trafficking in between early compartments of the secretory pathway in eukaryotic cells. These coat proteins perform the dual, essential tasks of selecting appropriate cargo proteins and deforming the lipid bilayer of appropriate donor membranes into buds and vesicles. COP II proteins are required for selective export of newly synthesized proteins from the endoplasmic reticulum (ER). COP I proteins mediate a retrograde transport pathway that selectively recycles proteins from the cis-Golgi complex to the ER. Additionally, COP I coat proteins have complex functions in intra-Golgi trafficking and in maintaining the normal structure of the mammalian interphase Golgi complex.

Cisternal maturation and vesicle transport: join the band wagon! (Review)

'No cellular organelle has been the subject of as many, as long-lasting or as diverse polemics as the Golgi apparatus'. This statement was made by Whaley almost 30 years ago in the book The Golgi Apparatus and still holds true today, perhaps more then ever. Why is this? How come something as mundane as a series of intracellular membrane bound structures continues to fascinate and captivate a large section of the cell biology community? One simple reason (putting polemics aside) is that the secretory pathway appears deceptively simple. Once probed, however, it has a persistent habit of developing into an enigma. Is one then not closer than 30 years ago? In a sense yes, in that one has more components and a better understanding of inherent membrane dynamics, but it is still not known how newly synthesized proteins and lipids make their way from the ER to the plasma membrane. Is it by vesicles, cisternal carriers or transient tubular connections? It has also been learned that newly synthesized proteins are segregated away from the resident components throughout the pathway, but not how. Do coat proteins hold the key? It is understood that the cytoskeleton is important, but not really why. It is known that each Golgi stack is a fully functional unit, but not why stacks are connected laterally into a large ribbon (the Golgi apparatus). This review focuses on how proteins make their way through the pathway, a basic question that remains to be answered.

In vitro transport on cis and trans sides of the Golgi involves two distinct types of coatomer and ADP-ribosylation factor-independent transport intermediates

The cisternal maturation model proposes that secretory proteins transit the Golgi in cisternae that mature by the continuous retrograde transport of Golgi enzymes in vesicles. We have tested the hypothesis that de novo generation of transport intermediates containing medial, trans, and trans Golgi network (TGN) enzymes is reconstituted in vitro. Our analysis shows that the majority of transport is mediated by a steady state of transport intermediate production and consumption by Golgi cisternae, with only a minor contribution of pre-existing transport intermediates. Transport in the medial and trans regions of the stack involved intermediates containing Golgi enzymes, apparently moving in a retrograde direction. In contrast, transport between the trans Golgi and TGN was exclusively mediated by intermediates containing secretory protein, as expected for anterograde transport. These intermediates may be physiologically relevant, because only these two specific types of intermediates can be detected in cell homogenates. By analogy to the coatomer (COPI)-independent transport of Golgi enzymes to the endoplasmic reticulum, the steady-state production of intra-Golgi transport intermediates was not impaired by inhibition of COPI vesicle formation. These data suggest a model for COPI-independent intra-Golgi transport by cisternal maturation with a shift in mechanism to anterograde transport at the trans Golgi and TGN boundary.

The Golgi apparatus: balancing new with old

Most models put forward to explain cellular processes do not stand the test of time. The 'lucky' few that are able to survive extensive experimental tests and peer critique may eventually become dogmas or paradigms. When this happens, the amount of experimental data required to overturn the paradigm is extensive. To some, such inertia may seem prohibitive to scientific progress but rather, in our opinion, this helps to maintain a degree of coherence. It is needed so that experiments and interpretations may be conducted within relatively safe boundaries. In the field of protein transport in the secretory pathway, we have enjoyed a relatively stable and productive period for quite some time (more than 30 years!). It is only very recently that the field has entered into a phase where all bets seem to be off. As in any paradigm shift, the accumulation of experimental observations inconsistent with the old dogma eventually reached a critical point. As we 'reluctantly' dispense with the long-standing paradigm of forward vesicular transport, we face a time that is bound to be trying as well as exciting.

Characterization of a mammalian Golgi-localized protein complex, COG, that is required for normal Golgi morphology and function

Multiprotein complexes are key determinants of Golgi apparatus structure and its capacity for intracellular transport and glycoprotein modification. Three complexes that have previously been partially characterized include (a) the Golgi transport complex (GTC), identified in an in vitro membrane transport assay, (b) the ldlCp complex, identified in analyses of CHO cell mutants with defects in Golgi-associated glycosylation reactions, and (c) the mammalian Sec34 complex, identified by homology to yeast Sec34p, implicated in vesicular transport. We show that these three complexes are identical and rename them the conserved oligomeric Golgi (COG) complex. The COG complex comprises four previously characterized proteins (Cog1/ldlBp, Cog2/ldlCp, Cog3/Sec34, and Cog5/GTC-90), three homologues of yeast Sec34/35 complex subunits (Cog4, -6, and -8), and a previously unidentified Golgi-associated protein (Cog7). EM of ldlB and ldlC mutants established that COG is required for normal Golgi morphology. "Deep etch" EM of purified COG revealed an approximately 37-nm-long structure comprised of two similarly sized globular domains connected by smaller extensions. Consideration of biochemical and genetic data for mammalian COG and its yeast homologue suggests a model for the subunit distribution within this complex, which plays critical roles in Golgi structure and function.

The Golgi apparatus: going round in circles?

Recent studies have questioned the idea that the Golgi complex is a stable organelle with a unique identity through which secretory cargo is transported by vesicles. Instead, it is proposed that Golgi apparatus proteins continuously recycle via the endoplasmic reticulum by vesicle transport, whereas cargo molecules remain in maturing cisternal structures. Rather than forming a rigid matrix, structural Golgi proteins might be highly dynamic and recycle via the cytoplasm. I will discuss the evidence for these claims and consider whether or not they really disprove older ideas on how the Golgi apparatus is structured and performs its function.

Peri-Golgi vesicles contain retrograde but not anterograde proteins consistent with the cisternal progression model of intra-Golgi transport

A cisternal progression mode of intra-Golgi transport requires that Golgi resident proteins recycle by peri-Golgi vesicles, whereas the alternative model of vesicular transport predicts anterograde cargo proteins to be present in such vesicles. We have used quantitative immuno-EM on NRK cells to distinguish peri-Golgi vesicles from other vesicles in the Golgi region. We found significant levels of the Golgi resident enzyme mannosidase II and the transport machinery proteins giantin, KDEL-receptor, and rBet1 in coatomer protein I-coated cisternal rims and peri-Golgi vesicles. By contrast, when cells expressed vesicular stomatitis virus protein G this anterograde marker was largely absent from the peri-Golgi vesicles. These data suggest a role of peri-Golgi vesicles in recycling of Golgi residents, rather than an important role in anterograde transport.

Sorting of Golgi resident proteins into different subpopulations of COPI vesicles: a role for ArfGAP1

We present evidence for two subpopulations of coatomer protein I vesicles, both containing high amounts of Golgi resident proteins but only minor amounts of anterograde cargo. Early Golgi proteins p24alpha2, beta1, delta1, and gamma3 are shown to be sorted together into vesicles that are distinct from those containing mannosidase II, a glycosidase of the medial Golgi stack, and GS28, a SNARE protein of the Golgi stack. Sorting into each vesicle population is Arf-1 and GTP hydrolysis dependent and is inhibited by aluminum and beryllium fluoride. Using synthetic peptides, we find that the cytoplasmic domain of p24beta1 can bind Arf GTPase-activating protein (GAP)1 and cause direct inhibition of ArfGAP1-mediated GTP hydrolysis on Arf-1 bound to liposomes and Golgi membranes. We propose a two-stage reaction to explain how GTP hydrolysis constitutes a prerequisite for sorting of resident proteins, yet becomes inhibited in their presence.

Small cargo proteins and large aggregates can traverse the Golgi by a common mechanism without leaving the lumen of cisternae

Procollagen (PC)-I aggregates transit through the Golgi complex without leaving the lumen of Golgi cisternae. Based on this evidence, we have proposed that PC-I is transported across the Golgi stacks by the cisternal maturation process. However, most secretory cargoes are small, freely diffusing proteins, thus raising the issue whether they move by a transport mechanism different than that used by PC-I. To address this question we have developed procedures to compare the transport of a small protein, the G protein of the vesicular stomatitis virus (VSVG), with that of the much larger PC-I aggregates in the same cell. Transport was followed using a combination of video and EM, providing high resolution in time and space. Our results reveal that PC-I aggregates and VSVG move synchronously through the Golgi at indistinguishable rapid rates. Additionally, not only PC-I aggregates (as confirmed by ultrarapid cryofixation), but also VSVG, can traverse the stack without leaving the cisternal lumen and without entering Golgi vesicles in functionally relevant amounts. Our findings indicate that a common mechanism independent of anterograde dissociative carriers is responsible for the traffic of small and large secretory cargo across the Golgi stack.

Mitotic Golgi is in a dynamic equilibrium between clustered and free vesicles independent of the ER

Golgi inheritance during cell division involves Golgi disassembly but it remains unclear whether the breakdown product is dispersed vesicles, clusters of vesicles or a fused ER/Golgi network. Evidence against the fused ER/Golgi hypothesis was previously obtained from subcellular fractionation studies, but left concerns about the means used to obtain and disrupt mitotic cells. Here, we performed velocity gradient analysis on otherwise untreated cells shaken from plates 9 h after release from an S-phase block. In addition, we used digitonin and freeze/thaw permeabilization as alternatives to mechanical homogenization. Under each of these conditions, approximately 75% of the Golgi was recovered in a population of small vesicles that lacked detectable ER. We also used multilabel fluorescent microscopy with optical sectioning by deconvolution to compare the 3D metaphase staining pattern of endogenous Golgi and ER markers. Although both ER and Golgi staining were primarily diffuse, only the ER was excluded from the mitotic spindle region. Surprisingly, only 2% of the Golgi fluorescence was present as resolvable structures previously characterized as vesicle clusters. These were not present in the ER pattern. Significantly, a portion of the diffuse Golgi fluorescence, presumably representing dispersed 60-nm vesicles, underwent an apparent rapid aggregation with the larger Golgi structures upon treatments that impaired microtubule integrity. Therefore, mitotic Golgi appears to be in a dynamic equilibrium between clustered and free vesicles, and accurate partitioning may be facilitated by microtubule-based motors acting on the clusters to insure random and uniform distribution of the vesicles.

Evidence that the entire Golgi apparatus cycles in interphase HeLa cells: sensitivity of Golgi matrix proteins to an ER exit block

We tested whether the entire Golgi apparatus is a dynamic structure in interphase mammalian cells by assessing the response of 12 different Golgi region proteins to an endoplasmic reticulum (ER) exit block. The proteins chosen spanned the Golgi apparatus and included both Golgi glycosyltransferases and putative matrix proteins. Protein exit from ER was blocked either by microinjection of a GTP-restricted Sar1p mutant protein in the presence of a protein synthesis inhibitor, or by plasmid-encoded expression of the same dominant negative Sar1p. All Golgi region proteins examined lost juxtanuclear Golgi apparatus-like distribution as scored by conventional and confocal fluorescence microscopy in response to an ER exit block, albeit with a differential dependence on Sar1p concentration. Redistribution of GalNAcT2 was more sensitive to low Sar1p(dn) concentrations than giantin or GM130. Redistribution was most rapid for p27, COPI, and p115. Giantin, GM130, and GalNAcT2 relocated with approximately equal kinetics. Distinct ER accumulation could be demonstrated for all integral membrane proteins. ER-accumulated Golgi region proteins were functional. Photobleaching experiments indicated that Golgi-to-ER protein cycling occurred in the absence of any ER exit block. We conclude that the entire Golgi apparatus is a dynamic structure and suggest that most, if not all, Golgi region-integral membrane proteins cycle through ER in interphase cells.

Evidence that Golgi structure depends on a p115 activity that is independent of the vesicle tether components giantin and GM130

Inhibition of the putative coatomer protein I (COPI) vesicle tethering complex, giantin-p115-GM130, may contribute to mitotic Golgi breakdown. However, neither this, nor the role of the giantin-p115-GM130 complex in the maintenance of Golgi structure has been demonstrated in vivo. Therefore, we generated antibodies directed against the mapped binding sites in each protein of the complex and injected these into mammalian tissue culture cells. Surprisingly, the injected anti-p115 and antigiantin antibodies caused proteasome-mediated degradation of the corresponding antigens. Reduction of p115 levels below detection led to COPI-dependent Golgi fragmentation and apparent accumulation of Golgi-derived vesicles. In contrast, neither reduction of giantin below detectable levels, nor inhibition of p115 binding to GM130, had any detectable effect on Golgi structure or Golgi reassembly after cell division or brefeldin A washout. These observations indicate that inhibition of p115 can induce a mitotic-like Golgi disassembly, but its essential role in Golgi structure is independent of its Golgi-localized binding partners giantin and GM130.

Lumenal endosomal and Golgi-retrieval determinants involved in pH-sensitive targeting of an early Golgi protein

Despite the potential importance of retrieval-based targeting, few Golgi cisternae-localized proteins have been demonstrated to be targeted by retrieval, and the putative retrieval signals remain unknown. Golgi phosphoprotein of 130 kDa (GPP130) is a cis-Golgi protein that allows assay of retrieval-based targeting because it redistributes to endosomes upon treatment with agents that disrupt lumenal pH, and it undergoes endosome-to-Golgi retrieval upon drug removal. Analysis of chimeric molecules containing domains from GPP130 and the plasma membrane protein dipeptidylpeptidase IV indicated that GPP130 targeting information is contained entirely within its lumenal domain. Dissection of the lumenal domain indicated that a predicted coiled-coil stem domain adjacent to the transmembrane domain was both required and sufficient for pH-sensitive Golgi localization and endosome-to-Golgi retrieval. Further dissection of this stem domain revealed two noncontiguous stretches that each conferred Golgi localization separated by a stretch that conferred endosomal targeting. Importantly, in the absence of the endosomal determinant the Golgi targeting of constructs containing either or both of the Golgi determinants became insensitive to pH disruption by monensin. Because monensin blocks endosome-to-Golgi transport, the finding that the endosomal determinant confers monensin sensitivity suggests that the endosomal determinant causes GPP130 to traffic to endosomes from which it is normally retrieved. Thus, our observations identify Golgi and endosomal targeting determinants within a lumenal predicted coiled-coil domain that appear to act coordinately to mediate retrieval-based targeting of GPP130.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Steady-state localization of a medial-Golgi glycosyltransferase involves transit through the trans-Golgi network

The steady-state localization of medial-Golgi enzymes is likely to involve retrograde transport pathways; however, the trafficking of these resident enzymes through the Golgi stack is unclear. To investigate if the medial-Golgi enzyme beta-1,2-N-acetylglucosaminyltransferase I (GlcNAc-TI) is transported to the late Golgi, a modified GlcNAc-TI bearing an N-glycan site on the C-terminus was constructed. The modified GlcNAc-TI was demonstrated to be functionally active in vivo, and was localized to the Golgi stack of transfected cells. In stable Chinese-hamster ovary (CHO) cell clones, the N-glycosylated GlcNAc-TI carried sialylated complex N-glycan chains. Pulse-chase studies showed that the majority of GlcNAc-TI was sialylated within 60 min of synthesis. Treatment of transfected CHO cells with Brefeldin A resulted in the glycosylated GlcNAc-TI bearing endo-beta-N-acetylglucosaminidase H resistant chains; however, the sialylation of glycosylated GlcNAc-TI was dramatically reduced. These data imply that, in CHO cells, newly synthesized GlcNAc-TI is transported rapidly through the Golgi stack to the trans-Golgi network, suggesting that GlcNAc-TI continuously recycles from the late Golgi. Furthermore, this data suggests that retrograde transport pathways play an important role in establishing the asymmetric distribution of GlcNAc-TI within the Golgi stack.

Trafficking and localisation of resident Golgi glycosylation enzymes

The localisation of glycosylation enzymes within the Golgi apparatus is fundamental to the regulation of glycoprotein and glycolipid biosynthesis. Regions responsible for specifying Golgi localisation have been identified in numerous Golgi resident enzymes. The transmembrane domain of Golgi glycosyltransferases provides a dominant localisation signal and in many cases there are also major contributions from the lumenal domain. The mechanism by which these targeting domains function in maintaining an asymmetric distribution of Golgi resident glycosylation enzymes has been intensely debated in recent years. It is now clear that the targeting of Golgi resident enzymes is intimately associated with the organisation of Golgi membranes and the control of protein and lipid traffic in both anterograde and retrograde directions. Here we discuss the recent advances into how Golgi targeting signals of glycosylation enzymes function, and propose a model for maintaining the steady-state localisation of Golgi glycosyltransferases.

Ykt6 forms a SNARE complex with syntaxin 5, GS28, and Bet1 and participates in a late stage in endoplasmic reticulum-Golgi transport

The yeast SNARE Ykt6p has been implicated in several trafficking steps, including vesicular transport from the endoplasmic reticulum (ER) to the Golgi, intra-Golgi transport, and homotypic vacuole fusion. The functional role of its mammalian homologue (Ykt6) has not been established. Using antibodies specific for mammalian Ykt6, it is revealed that it is found mainly in Golgi-enriched membranes. Three SNAREs, syntaxin 5, GS28, and Bet1, are specifically associated with Ykt6 as revealed by co-immunoprecipitation, suggesting that these four SNAREs form a SNARE complex. Double labeling of Ykt6 and the Golgi marker mannosidase II or the ER-Golgi recycling marker KDEL receptor suggests that Ykt6 is primarily associated with the Golgi apparatus. Unlike the KDEL receptor, Ykt6 does not cycle back to the peripheral ER exit sites. Antibodies against Ykt6 inhibit in vitro ER-Golgi transport of vesicular stomatitis virus envelope glycoprotein (VSVG) only when they are added before the EGTA-sensitive stage. ER-Golgi transport of VSVG in vitro is also inhibited by recombinant Ykt6. In the presence of antibodies against Ykt6, VSVG accumulates in peri-Golgi vesicular structures and is prevented from entering the mannosidase II compartment, suggesting that Ykt6 functions at a late stage in ER-Golgi transport. Golgi apparatus marked by mannosidase II is fragmented into vesicular structures in cells microinjected with Ykt6 antibodies. It is concluded that Ykt6 functions in a late step of ER-Golgi transport, and this role may be important for the integrity of the Golgi complex.

Stoichiometry and kinetics of transport vesicle fusion with Golgi membranes

The in vitro complementation assay established by Rothman and co-workers continues to be an important tool to study intra-Golgi transport. In this study, kinetic modeling is used to identify four main parameters that, together, explain the basic features of an assay that is a modification of the original assay. First, the assay signal depends on the ratio of Golgi membranes to transport intermediates in the assay. Secondly, an inactivation rate describes how the activity of transport intermediates decreases over time. Thirdly, the rate at which transport intermediates irreversibly bind to Golgi membranes is measured independently of membrane fusion, thus allowing a quantitative distinction between these two steps. Fourthly, a single rate constant describes the remaining reactions, which result in membrane fusion. This approach of kinetic modeling of experiments is generally applicable to other in vitro assays of cell biological phenomena, permitting quantitative interpretations and an increased resolution of the experiments.

In vitro assays of vesicular transport

Movement of proteins and lipids between the various compartments of eukaryotic cells is fundamental to the maintenance of cellular homeostasis, and an understanding of the molecular mechanisms that govern these processes remains a key goal of cell biological research. This aim has been greatly facilitated by the development of assays that recapitulate specific events in vitro. In the following article we provide an overview of some of the currently used assays that measure the movement of proteins within the exocytic and endocytic pathways, and provide a starting point for those wishing to establish their own systems to study other vesicular transport steps.

The transmembrane domain of murine alpha-mannosidase IB is a major determinant of Golgi localization

Murine alpha1,2-mannosidase IB is a type II transmembrane protein localized to the Golgi apparatus where it is involved in the biogenesis of complex and hybrid N-glycans. This enzyme consists of a cytoplasmic tail, a transmembrane domain followed by a "stem" region and a large C-terminal catalytic domain. To analyze the determinants of targeting, we constructed various deletion mutants of murine alpha1,2-mannosidase IB as well as alpha1,2-mannosidase IB/yeast alpha1,2-mannosidase and alpha1,2-mannosidase IB/GFP chimeras and localized these proteins by fluorescence microscopy, when expressed transiently in COS7 cells. Replacing the catalytic domain of alpha1,2-mannosidase IB with that of the homologous yeast alpha1,2-mannosidase and deleting the "stem" region in this chimera had no effect on Golgi targeting, but caused increased cell surface localization. The N-terminal tagged protein lacking a catalytic domain was also localized to the Golgi. In the latter case, when the stem region was partially or completely removed, the protein was found in both the ER and the Golgi. A chimera consisting of the alpha1,2-mannosidase IB N-terminal region (cytoplasmic and transmembrane domains plus 10 amino acids of the "stem" region) and GFP was localized mainly to the Golgi. Deletion of 30 out of 35 amino acids in the cytoplasmic tail had no effect on Golgi localization. A GFP chimera lacking the entire cytoplasmic tail was found in both the ER and the Golgi. These results indicate that the transmembrane domain of alpha1,2-mannosidase IB is a major determinant of Golgi localization.

Protein sorting in the Golgi apparatus: a consequence of maturation and triggered sorting

To explain how resident proteins distribute in peak-like patterns at various positions in the secretory pathway, Glick and co-workers postulated that resident proteins comprise different populations (termed kin populations) and that these compete with each other for entering retrograde transport carriers [Glick et al. (1997) FEBS Lett. 414, 177-181]. Using modelling and computer simulation, they could demonstrate that differences in competitiveness sufficed to generate overlapping but distinct peak-like steady state distributions of the different kin populations across the Golgi stack. In this study, we have tested the robustness of the competition model and find that over-expression or changes in the number of kin populations affect their overall steady state distributions. To increase the robustness of the system, we have introduced a milieu-induced trigger for recycling. This allows for a decrease in the coupling between kin populations permitting both over-expression as well as changes in the number of kin populations. We have also extended the model to include a Golgi to endoplasmic reticulum (ER) recycling pathway and find that only a small amount of resident proteins may recycle at any time without upsetting their observed distributions in the Golgi stack. The biological relevance of a trigger-induced sorting mechanism and ER recycling is discussed.