TRANSPORT THROUGH GOLGI APPARATUS
Golgi apparatus (aka Golgi body and Golgi complex) first discovered by Italian scientist Camillo Golgi is an important sub cellular organelle in both plants and animals. Present in cytoplasm, Golgi complex is made up of flattened, sac like membranous structure called cisternae (dicytosome in plants). The number of cisternae in a stack differs from species to species.
Structure wise the Golgi apparatus has two faces: the Cis-face and trans-face divided into 3 functional regions cis-golgi network, medial-golgi network and trans-golgi network. Cis-golgi network is made up of vesicles from the Endoplasmic Reticulum (ER) and Trans-golgi network is from where the cargo leaves the Golgi apparatus. Each region has unique set of enzymes in there lumen. As the cargo proteins advances through the different region of the Golgi apparatus they are covalently modified (phosphorylation, sulphylation, and glycolysation) with the help of these enzymes and are sent to their appropriate destination (intracellular or extracellular).
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The exact mechanism used by proteins to move along the complex is not yet clear. There are two models which compete each other with different versions to explain the transport of cargo: 1) Vesicular transport and 2) Cisternal maturation mechanism. In the following review, research papers showing all the transport mechanisms and retrograde transport will be discussed.
VESICULAR TRANSPORT model views Golgi apparatus as a stable complex and that movement of cargo protein through Golgi by this mechanism is carried out by special transport vesicles likely coated by COP-1 proteins. To prove role of COP-1 protein in vesicular transport, inter-golgi transport within fused cells which requires mobile carriers for exchange was done. Hela cells containing resident GFP tagged galactosyltransferase (GT-GFP) and other containing RFP tagged sialyltransferase were used. GFP tagged galactosyltransferase was then infected using vesicular somititis virus G proteins (VSV-G protein) and fusion was allowed to happen at acidic pH. During course of analysis using new live imagining techniques like confocal video microscopy, FRAP, new observations were made. ‘Dot’ like structures was observed by confocal microscopy. To confirm that these ‘Dot’ like structures are COP1 vesicles containing the fluorescently labeled cargo, an individual experiment was carried out by using Arf-1 and ε-COP knock down Hela cells and were obsereved using STED technique. Both these experiments showed absolute necessity of
Arf-1 and coatomer for inter-golgi transport. Also antibody against the β-COP of COP1 was raised coupled with secondary antibodoy for STED assay. Positive result was shown for both GFP and RFP proving that COP1 vesicles play an important role in vesicular trasnsport of small cargoes(Pellett et al. 2013).
It is not possible for large cargoes to fit in small size vesicles during their transport. So how are these cargoes transported through Golgi? Some scientists have shown that transport of such large cargo occurs by cisternal maturation and not vesicular transport. Bonfanti in 1998 provided the strongest evidence for cisternal maturation showing that large soluble cargo like collagen which is unable to enter vesicles or tubules is rapidly transported across golgi.To test this model of cisternal maturation for transport of large soluble cargo a membrane ‘staple’ consisting of trans-aggregating membrane protein was designed. CD8lumenal was used in this experiment which under absence of disaggregating drug AP2000, formed an electron dense plaque (staples) giving ease in visualizing using electron microscopy. If cisternal maturation model is true then the staple should be observed at the Trans face. With the help of techniques like electron microscopy, immunoefluorescence, temperature blocks, movement across golgi was observed, showing two surprising results: 1) the staple remains static while soluble cargo moves forward 2) the staple remains at the central regions of Cis face even after hour of incubation while the soluble cargo-aggregate crowd near the rim. This is probably because the cisternae are static but the rims are mobile for transport of large soluble cargo like collagen or CDlumenal . This property is called as ‘rim progression’ where large cargo is forced towards the rims and not the entire stack matures like showed by Bonfanti but just the rims. Based on previous such observations by Burgger and Patterson in 2008 the Golgi cisternae can be divided into two domains the ‘center’ which is static and the ‘rims’ which are in dynamic state(Lavieu, Zheng, and Rothman 2013) .
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Another experiment was designed to check transport of large soluble cargo across Golgi. A transport model was designed for two different types of proteins VSV-G and procollagen and there diffusion coefficient (Dt) and convection velocity (v) was checked. The obtained data is v ≈ Dt and vË‚ Dt. Two very distinct results were obtained: 1) inter-cisternal exchange of cargo is directly related to diffusion (Dt) 2) convection velocity showed some level of cisternal maturation (probably because of retrograde transport). Procollagen diffused across cisternae despite its large size which is a property of inter-cisternal exchange of cargo. This implies that probably transport of such large cargo occurs by ‘Pleiomorpihic membrane carriers’. Aggregation of large proteins may form ‘megavesicles’ near the rim as shown previously by rim progression. vËƒ2Dt would have been the direct evidence for cisternal maturation but it was not obtained. Perhaps the data suggested just the rim maturation(Dmitrieff, Rao, and Sens 2013).
The experiments described above ‘rim progression’ and ‘inter-cisternal exchange’ did not give direct proof of vesicular transport, but showed that a golgi stack is static which is the main feature of vesicular transport. Maybe both these are advanced mechanisms of vesicular transport.
CISTERNAL MATURATION model suggest that Golgi is a dynamic structure in which the cisternae move along carrying the cargo protein. The vesicle coming from the ER fuse together to become Cis Golgi network which progresses to become Cis face of Golgi stack and further matures to become Trans- Golgi face. The enzymes are retained probably by retrograde transport mechanism. The cisternal maturation model says that cis-cisternae are generated De Novo. Analysis using high pressure frozen/freeze- substituted cells via electron tomography supported that in algal and plant cells that cis-cisternae are formed De Novo. The assembly of cis-cisternae occurs step wise where in the initial step Cis-most region of golgi stack is formed by initiators which are COPII coat coming from the ER. When this C1 region is formed it triggers formation of initiators which leads to transformation of C1 to C2. In mammalian cells by tomographic evidence it is proved that pleiomorphic tubulovesicular structures dock the cis-most cisternae to form pre-form cis-cisternae when the earlier one matures(Donohoe et al. 2013) .
To collect further evidence in support of cisternal maturation, golgi resident protein mouse α- 1,2-mannoside 1B (MANI) and β-1,4- galactosyltransferase (GALT-FM) was designed such that it can polymerize or depolymerize in presence or absence of drug AP and using latest imaging techniques its position in the golgi stack was observed. If vesicular transport occurs then the polymerized form of the golgi resident protein should be in the cis-face of the golgi body, as due to large size it won’t be able to accommodate in the golgi vesicles and therefore no transport. If transport occurs via cisternal maturation then the polymerized protein should be in the trans-face of the golgi as due to large size it won’t be able to interact with the vesicles and hence will not be able to recycle back to the cis-face as speculated by cisternal maturation model. Hela cell line was used for observing and ministacks were generated for easy visualization. The result showed that MANI-FM moves from cis-face to trans-face at the speed required for cisternal maturation. A more strong point is that when depolymerized at the trans-face they move back quickly to the cis-rim and golgi carriers(Rizzo et al. 2013).
By using super resolution the dynamic behavior of Trans Golgi Network was studied in plant Arabidopsis. Using confocal scanning microscopy and GFP- tagging,images revealed dynamic behavior of TGN of this plant also two types of TGN were observed Golgi associated (GA) and Golgi independent (GI). Also using SCLIM 3D time lapse imaging it was proved that GI originated from GA. The behavior of GA-TGN resembles cisternal maturation model where the GA matures from cis-face and looses its association with the golgi. This is supported by observing that after releasing cargo GI matures to TGN (Uemura et al. 2014).
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Transport via inter-compartment continuities remains the least investigated and understood of the two models so far. Three dimensional electron microscopy and electron tomography have revealed the presence of intercisternal tubular continuities was seen. To prove their role in transport of soluble cargo, experiment was designed which gave kinetic, morphological and computational evidence in favor of these continuities. The experimental system used albumin, VSV-G and PC-1 which by now we know transverse through golgi at different rates. By comparing it was seen that albumin spreads through golgi stack in 2 min compared to other two. Also presence of albumin was seen more in tubules and less in golgi during transport. To distinguish between vesicular transport and diffusion computational model was generated which showed that transport is through continuities and not vesicles. All this evidence does not neglect the fact that other minor mechanisms may be also be present. Maybe diffusion works hand in hand with maturation model as the entire golgi does not collapse into one large compartment due to this continuities(Beznoussenko et al. 2014).
Both these models and there smaller versions have been proved with convincing evidence. Perhaps transport of cargo is by combination of both these mechanism.
Understanding retrograde transport is important as retrograde transport is also seen like ER, in Golgi. Golgi also retains resident proteins using retrograde transport mechanism, the resident proteins or lipids are brought back either from Plasma membrane or Endosomes with the help of COP- I protein coat and some other proteins which are not yet known. Also cisternal maturation model suggest that resident proteins of a particular stack are maintained using retrograde transport.
ARF (ADP-ribosylation factors) family makes up small GTPase which are responsile for triggering vesicle formation in the golgi apparatus. Hela cells were used for checking functions of different ARF functions. Integrity of endsome and golgi retrograde transport is maintained byARF1 and ARF 4. Previous studies have shown that different types of ARF are expressed and are localized either in Golgi or recycling endosomes. It remains a mystery as to how different combination of ARF leads to selective transport of cargoes to and from different organelles and within the same organelle(Nakai et al. 2013).
AGAP-2 an ARF1 GAP is cruicial in retrograde transport of shiga toxin from recycling endosomes to golgi. It was seen that depletion of AGAP-2 negatively affects the retrograde transport of shiga toxin (Shiba et al. 2010). Evection-2 a recycling endosome protein is important in retrograde transport of CTxB (cholera toxin B subunit) from recycling endosome to golgi by binding to ARF-GTPase protein SMAP-2. SMAP-2 localizes in Endoplasmic reticulum and colocalizes in TGN of golgi. Depletion of SMAP2 affects the retrograde transport of CTxB(Matsudaira et al. 2013).
Thus transport through golgi is not yet completely understood topic. New live imagining techniques are now available therefore more research needs to be done to explore more about golgi and transport through golgi as we know some toxins and viruses exploit golgi mechanisms for their own activity and as we explore in more depths, targeted strategy can be employed against these viruses and toxins.
Beznoussenko, Galina V., Seetharaman Parashuraman, Riccardo Rizzo, Roman Polishchuk, Oliviano Martella, Daniele Di Giandomenico, Aurora Fusella, et al. 2014. “Transport of Soluble Proteins through the Golgi Occurs by Diffusion via Continuities across Cisternae.” eLife 3 (June): e02009. doi:10.7554/eLife.02009.
Dmitrieff, Serge, Madan Rao, and Pierre Sens. 2013. “Quantitative Analysis of Intra-Golgi Transport Shows Intercisternal Exchange for All Cargo.” Proceedings of the National Academy of Sciences 110 (39): 15692–97. doi:10.1073/pnas.1303358110.
Donohoe, Bryon S., Byung-Ho Kang, Mathias J. Gerl, Zachary R. Gergely, Colleen M. McMichael, Sebastian Y. Bednarek, and L. Andrew Staehelin. 2013. “Cis-Golgi Cisternal Assembly and Biosynthetic Activation Occur Sequentially in Plants and Algae.” Traffic 14 (5): 551–67. doi:10.1111/tra.12052.
Lavieu, Gregory, Hong Zheng, and James E. Rothman. 2013. “Stapled Golgi Cisternae Remain in Place as Cargo Passes through the Stack.” eLife 2 (June): e00558. doi:10.7554/eLife.00558.
Matsudaira, Tatsuyuki, Yasunori Uchida, Kenji Tanabe, Shunsuke Kon, Toshio Watanabe, Tomohiko Taguchi, and Hiroyuki Arai. 2013. “SMAP2 Regulates Retrograde Transport from Recycling Endosomes to the Golgi.” PLoS ONE 8 (7): e69145. doi:10.1371/journal.pone.0069145.
Nakai, Waka, Yumika Kondo, Akina Saitoh, Tomoki Naito, Kazuhisa Nakayama, and Hye-Won Shin. 2013. “ARF1 and ARF4 Regulate Recycling Endosomal Morphology and Retrograde Transport from Endosomes to the Golgi Apparatus.” Molecular Biology of the Cell 24 (16): 2570–81. doi:10.1091/mbc.E13-04-0197.
Pellett, Patrina A., Felix Dietrich, Jörg Bewersdorf, James E. Rothman, and Grégory Lavieu. 2013. “Inter-Golgi Transport Mediated by COPI-Containing Vesicles Carrying Small Cargoes.” eLife 2 (October): e01296. doi:10.7554/eLife.01296.
Rizzo, Riccardo, Seetharaman Parashuraman, Peppino Mirabelli, Claudia Puri, John Lucocq, and Alberto Luini. 2013. “The Dynamics of Engineered Resident Proteins in the Mammalian Golgi Complex Relies on Cisternal Maturation.” The Journal of Cell Biology 201 (7): 1027–36. doi:10.1083/jcb.201211147.
Shiba, Yoko, Winfried Römer, Gonzalo A. Mardones, Patricia V. Burgos, Christophe Lamaze, and Ludger Johannes. 2010. “AGAP2 Regulates Retrograde Transport between Early Endosomes and the TGN.” Journal of Cell Science 123 (14): 2381–90. doi:10.1242/jcs.057778.
Uemura, Tomohiro, Yasuyuki Suda, Takashi Ueda, and Akihiko Nakano. 2014. “Dynamic Behavior of the Trans-Golgi Network in Root Tissues of Arabidopsis Revealed by Super-Resolution Live Imaging.” Plant and Cell Physiology 55 (4): 694–703. doi:10.1093/pcp/pcu010.