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1. Details of Module and its Structure
Module Detail
Subject Name <BOTANY>
Paper Name <Cell Biology>
Module Name/Title < Golgi complex: Sorting, processing and transport of Secretory proteins >
Module Id <7>
Pre-requisites Basic knowledge about plant cells, their contents and basic idea about cell organelles
Objectives To make the students aware of the structure and biogenesis of golgi apparatus along with the transport and processing in golgi.
Keywords Golgi apparatus, protein trafficking, protein sorting, vesicle budding.
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2. 2. Development Team
TABLE OF CONTENTS 1. Introduction
2. Structural characteristics of golgi apparatus 3. Chemical composition
4. Role of actin cytoskeleton 5. Biogenesis of golgi
6. Models for intra-golgi transport 7. Tethering complexes
8. Getting material into golgi 9. Delivery of cargo to golgi 10. Processing in golgi
Role Name Affiliation
National Coordinator <NA>
Subject Coordinator <Dr. Sujata Bhargava>
Paper Coordinator <Dr. Nutan Malpathak>
Content Writer/Author (CW) <Dr. Gauri Abhyankar>
Content Reviewer (CR) <Dr. Nutan Malpathak >
Language Editor (LE) < Dr. Nutan Malpathak>
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1. Introduction:
Golgi apparatus typically looks like stacks of 3 – 10 flattened, closed, membrane bound sacs. These sacs are called as cisternae. In plants, each individual stack of cisternae is called as dictyosome. Golgi also shows an irregular network of tubules and vesicles which is called as trans golgi network (TGN). Matrix of golgi apparatus contains proteins. These proteins are responsible to keep up the structural integrity of dictyosomes.
They also play a role in organizing these stacks / dictyosomes into more complex assemblies (Tang & wang, 2013). Many hundreds of separate golgi stacks are observed in each plant cell. These golgi are present throughout the cytoplasm of the cell (Fig 1A).
Every stack of golgi apparatus has distinct functional zones (Fig 1B). The cisternae in these zones differ in their morphology and molecular composition. Each zone has specialized set of enzymes to carry out different steps in glycoprotein processing. In eukaryotes, the golgi apparatus is divided into 5 parts –
1. Cis golgi network – It is present on the forming side of the golgi apparatus and is composed of Endoplasmic Reticulum – golgi intermediate compartments (ERGIC).
2. Cis cisternae – cisternae present on the forming side of the golgi apparatus.
3. Trans cisternae - cisternae present on the secreting side of the golgi apparatus.
4. Medial cisternae – cisternae present between cis and trans cisternae.
5. Trans golgi network (TGN) – It is located on trans face.
Fig 1 A Fig 1B
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Fig 1A: The Golgi apparatus stacks detected with antibodies against the protein p58K in the cells isolated from the wheat seedling root. (golgi are indicated by arrow); B: The ultrathin section of a separate stack/dictyosome (Vildanova et al., Biochemistry 79(9): 894 – 906, 2014).
In plants, ERGIC compartment is absent and there is presence of conspicuous TGN. So each stack of golgi apparatus has pronounced cis and trans polarity that directs the passage of secreted products across golgi (Wilson et al, 2011). The complete structure of golgi is stabilized by presence of inter-cisternal elements. These are protein cross links that hold cisternae together. In contrast, animal cell golgi tend to aggregate in one part of the cell and are interconnected via tubules.
3. Structural characteristics of golgi apparatus –
Golgi apparatus in higher plant cells consists of a large number of separate dictyosomes. Each of these dictyosomes functions independently of others. The stacks may be distributed uniformly or may be concentrated in certain areas of the cytoplasm.
The no. of stacks varies in different cells. The position of the golgi apparatus in cell depends on the specific function of the cell and this position of golgi changes during the cell cycle. There is no intermediate compartment between ERES (Endoplasmic reticulum exit sites) and cis golgi in plants, which is present in mammals. The cisternae are most likely held together by some kind of surrounding matrix. Earlier studies pertaining to electron microscopy showed a distinct zone around the golgi, which did not have any ribosomes. This zone was denser than surrounding cytoplasm (Staehelin et al, 1990). This region may be involved in maintaining the integrity of the stack. One more component that helps to keep up the cisternae together is called “inter-cisternal elements” (Fig 2). These are fibrous, electron-opaque elements and are observed between trans cisternae of plant golgi stacks.
Fig 2: Inter-cisternal elements (Hawes C, New Phytologist 165:29 – 44. 2005).
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Golgi apparatus is involved in the synthesis of cell wall polysaccharides and glycolipids of the plasma membrane. It also carries out glycosylation of proteins. As plant cells have prominent TGN, it serves as an early endosomal compartment. The endocytosed material directly enters the TGN. Here, in TGN, the secretory components from golgi and endocytotic components from plasma membrane are combined (Hwang et al, 2008). So this golgi apparatus in plants is divided into many mobile biosynthetic units.
Each of the units are mobile and are essential for –
a) Controlled import of synthetic products from ER b) Processing and sorting of products
c) Targeted delivery of the export products into different cellular compartments (Vildanova et al, 2014).
4. Chemical composition –
Golgi bodies are made up of 60% proteins and 40% lipids. Some of the proteins in golgi are same as that of ER. Plant golgi bodies have specific lipids – like phosphatidic acids and phosphatidyl glycerols – in large amounts, whereas they lack lipids like sialic acid. They also contain carbohydrates – glucosamine, galactose, glucose, mannose, fucose, xylose, arabinose and other sugars. NAD dependent enzymes like Cyt C oxidase and Cyt B reductase are also present in golgi. Glycosyl transferase and thiamine pyrophosphatase are characteristic enzymes of the golgi apparatus.
5. Role of actin cytoskeleton –
The actin cytoskeleton is very essential in many plant functions like cell development, morphogenesis and establishment & maintenance of polarity. In plants the golgi bodies are physically connected to ER surface. They interact with ER through a proteinaceous bridge. The bridge consists of COP II proteins that are required for ER exit site (ERES) formation (Hanton et al, 2009). In animals, microtubules function as major cytoskeletal component in membrane trafficking; whereas in plants, actins take over as major cytoskeletal component.
Recent studies have shown that ER network plausibly functions as the main reservoir for SCAR (suppressor of cyclic AMP repressor) / WAVE (WASP family verpolin- homologous protein) signaling complex and ARP2/3 (actin related protein 2/3) complex (Zhang et al, 2013a, 2013b). Activation of these complexes brings about branching and polymerization of actin filaments. One more important factor viz. SPIKE1 (a guanine nucleotide exchange factor) is localized at ERES and acts upstream of SCAR / WAVE
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complex. Depletion of this exchange factor changes morphology of ER and also localization of ERES components (Wang & Hussey, 2015).
6. Biogenesis of golgi –
Golgi stacks can multiply by two ways –
a) Through fission of existing stacks (Fig 3) (Hawes et al, 2008).
A B
Fig 3: Golgi biogenesis and fission in A) tobacco BY-2 cells and B) Chlamydomonas
b) By de novo formation from ER
In plants golgi can be formed from ER by de novo process (Hanton et al, 2007). If ERES and golgi stacks behave as a single unit, then it is likely that golgi bodies would be formed in response to cargo production. The most likely sequence of events for de novo biogenesis is as follows:
a) Differentiation of an exit site on the ER surface with the help of Sec16, Sec12 and Sar1 (Fig 4A) and possible involvement of cis-Golgi matrix or tethering factors.
b) ER membrane forms A COPII-coated bud. The bud is attached to ER through the proto-Golgi matrix (Fig 4B).
c) The buds grow to form a tubulovesicular complex. The complex contains COPI buds, vesicles and SNARES, and is surrounded by a matrix (Fig 4C).
d) It quickly differentiates into a mini proto-Golgi stack with structural characteristics of both cis- and trans-faces, including clathrin-coated buds (Figure 4D).
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At some stage, Golgi enzymes bound to ER membrane are transferred into this proto-golgi stack. Upon transfer they are anchored in the correct cisternae as the stack continues to mature.
Fig 4: Proposed model for the early stages of the biogenesis of a Golgi stack from the ER. A) Differentiation of exit sites on the ER. B) Formation of a tethered COPII bud at the exit site. C) Formation of a tubulovesicular complex with associated COPI. D) Differentiation of a small proto-Golgi stack.(Hawes et al., Traffic 9:1571 – 1580, 2008).
During mitosis and cytokinesis in plants, Golgi bodies and membranes do not disaggregate. Multiplication of golgi was reported at different times during cell cycle in different plants. E.g. in onion root meristems they doubled during metaphase, whereas in synchronized cultures of Catharanthus roseus, duplication occurred during cytokinesis.
Arabidopsis shoot meristems cells showed doubling of Golgi in G2, just prior to mitosis (Hawes et al, 2008).
Pollen tubes or root hairs have high secretory activity. Such cells produce large no.
of golgi stacks depending on the growth and function of these cells. Here multiplication of
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golgi is not related to cell cycle or mitosis. In Chlamydomonas as well as in higher plants it was observed that de novo golgi biogenesis and golgi fission can take place within the same system.
7. Models for intra-golgi transport –
Classically there are two models proposed for transport of newly synthesized glycoproteins through golgi apparatus.
a) Vesicular shuttle model –
The model proposes that cis, medial and trans cisternae are stable structures. All three functional zones have specific enzymes that can be used as markers for distinguishing each zone. The resident golgi proteins are retained in the cisternae. The molecules to be transported (cargo molecules) move from one cisterna to other via small vesicles. This model explains polarity of the stack because it shows three distinct sub- organelles. Also, presence of large no. of COP I vesicles around the golgi, support the model. But it cannot explain the synthesis and secretion of large molecules like glycoprotein scales in algal cell walls and procollagen in animals. These molecules are much larger to enter the small vesicles. Although the model states that cargo molecules leave one cisterna and physically move to the next one, larger cargos pass from cis to trans face of golgi stack without leaving the cisterna (Fig 5a) (Glick & Nakano, 2009). To address these issues, another model was proposed.
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Fig 5:Two models for membrane traffic through the Golgi. (a) vesicular shuttle model. According to this view, cis-, medial-, and trans-Golgi cisternae stable entities that retain distinct sets of resident Golgi proteins.
Secretory cargoes travel from one golgi compartment to the next in anterograde COPI vesicles and then exit the TGN in clathrin-coated vesicles (CCV) or secretory carriers. ER-to-Golgi transport is regarded as a donor-acceptor pathway connected by COPII vesicles. (b) Cisternal maturation model. According to this view, Golgi cisternae are transient structures. A new cisterna matures from cis to trans and then breaks down into transport carriers at the TGN stage. Cargoes are transported through the Golgi by cisternal progression.This retrograde transport involves COPI vesicles within the Golgi and may also involve clathrin- mediated recycling from a maturing TGN compartment.(Glick &Nakano, Ann Rev Cell Dev Biol 25:113 – 132, 2009)
b) Cisternal maturation model –
This model considers that golgi stack is not fixed but is a dynamic structure. Here cisternae themselves progress through cis, medial and trans faces carrying the cargo molecules. Ultimately the cisterna itself becomes a large secretory vesicle and fuses with
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the plasma membrane. The model explains transport of large molecules, but another problem arises here – if cisternae are moving, how the functional zones of golgi will be maintained?
Along with anterograde transport (from cis to trans cisternae), retrograde transport (from trans to cis cisternae) is also observed in golgi cisternae. This retrograde transport maintains the spatial distribution of enzymes and functional proteins within the golgi stack.
This model is much more appealing besides its role in transport of large molecules.
The retrograde transport of this model can explain presence of a particular resident golgi protein in multiple golgi compartments (Fig 5b).
c) Recently, a new model has been proposed – called as “Cisternal progenitor model” (Pfeffer, 2010). The model proposes that different compartments / cisternae are stable in golgi, but they are able to generate next type of cisternae through fusion of similar cisternae and fission of golgi cisternae. Both fusion and fission processes are controlled by Rab family GTPases present in golgi (Fig 6). Rab proteins regulate membrane identity and traffic in plant endomembrane systems. They also link to motor proteins and allow organelle motility. A cell can contain more than 40 different Rab proteins. Each of these Rab proteins would have typical sets of binding partners that either make cargo collection into transport vesicles, or link to molecular motors, and/or facilitate docking and fusion.
The ordered recruitment of sequentially acting Rab GTPases is called as Rab cascade. It was first discovered in the yeast Golgi complex. The sequence of events is as follows –
a) Each cisternae is marked by a different Rab protein. RabA represents a domain that allows RabA-containing compartments/ cisternae to fuse with other RabA
compartments (Fig 6A).
b) This arrangement would permit two ministacks to fuse laterally.
c) RabA would recruit the GEF for RabB.
d) A RabA domain begins to form an adjacent RabB domain (Fig. 6B).
e) This domain separates from the RabA domain by fission.
f) It segregates from the RabA domain by RabB recruitment of a RabA-specific GAP.
g) When secretion is activated, golgi would expand to accommodate the increased volume of cargo.
h) This involves increased production of RabB and RabC domains to build a larger Golgi. So, cisternal expansion is observed at the edges of the stack.
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i) A RabB domain present there would fuse with another RabB domain (Fig 6C). This leads to connections between cisternae at different levels of the stack which would be resolved by fission events.
Fig 6: Transport through the Golgi and Golgi stack creation in a cisternal progenitor model. (Pfeffer, Proc Natl Acad Sci USA, 107:19614 – 19618, 2010).
This model explains the transport of large cargos. For example, large glycoprotein present in RabA cisterna will move to the medial golgi as soon as RabA compartment obtains RabB. Presence of RabB would allow the cisterna to fuse with a RabB medial compartment. RabA would be removed from that compartment by a RabB-recruited, RabA-specific GAP. At this stage, cargo would have traversed the stack and would have gained access to all of the glycosyltransferases that reside at each level of the stack. If the
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model holds true, then the presence and activation of certain Rabs would produce intra- cisternal chambers. These micro-chambers would retain specific glycosyltransferases. The model does not rule out existence of transport vesicles but it is not very clear how it decides concentration of certain enzymes in the vesicles.
8. Tethering complexes –
Transport of molecules from ER to golgi at some point require fusion between the donor and acceptor components. Many different proteins are crucial for maintaining integrity of golgi stacks. These are called as tethering factors.
Golgins are large proteins with coiled coil domains. The coiled coil motif consists of seven repeats that form amphipathic α- helices. These helices twist into supercoil and form long rod-like structure. Mostly these proteins have structural roles. Carboxy-terminal domains like GRIP and GRAB (GRIP related ARF binding) of some golgins are conserved across kingdoms. The GRAB domain binds to ARF1 and helps to form COP I coated vesicles. The GRIP domain binds to ARL1 (ARF like 1) located on TGN or trans golgi. The interaction of these domains is required for the binding of golgins to the golgi. Golgins are essential in stacking cisternae and also in vesicle tethering. Tethering is a process which occurs before vesicle fusion and comprises of initial attachment of vesicles to target membranes. It is supposed to provide basic level of recognition and specificity to vesicle binding. It also hastens vesicle fusion through SNARE proteins (Fig 7).
Fig. 7. A model for membrane tethering. (1) A transport intermediate approaches the target membrane. The movement can be by diffusion or by a motormediated process. (2) The transport intermediate tethers to the target membrane by coiled-coil proteins or through multimeric tethering complexes. Tethering
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can occur at distances of >200 nm (3) The cognate v-SNARE on the transport intermediate and t-SNAREs on the target compartment pair to form trans-SNARE complexes. This process is sometimes referred to as
‘‘docking’’. (4) The assembly of SNARE complexes drives membrane fusion. Transported cargo is incorporated into the membrane of the target compartment or released into the lumen. (Lupashin & Sztul - Biochimica et Biophysica Acta 1744: 325 – 339, 2005).
9. Getting material into golgi –
Golgi plays very crucial role in secretory pathway. It directs transport of molecules to and from other organelles through secretory pathway (Matheson et.al, 2006). First step in protein transport is transfer of proteins from ER to golgi. This anterograde transport is brought about by COP II vesicles through ERES.
ERES are specialized domains of ER membrane responsible for export of cargo molecules. These domains are also called as transitional ER (tER) and they are involved in directed export of secreted cargos. Transitional ER is thought to be involved in biogenesis and localization of golgi in two types of yeasts. In Saccharomyces cerevesiae golgi is observed as individual cisternae instead of stacks. Also cisternae are scattered in cytoplasm, wheras, in Pichia pastoris golgi is arranged in stacks. The arrangement of golgi cisternae depends on different arrangement of ERES / tER. In S. cerevesiae budding occurs across entire ER but in P. pastoris, export occurs only from ERES.
It is assumed that anterograde pathway functions parallel to retrograde transport.
Retrograde transport moves cargo from golgi back to ER or other organelles. This transport is mediated by COP I proteins. Both these pathways are interdependent. If retrograde transport is disturbed or interrupted, it can prevent anterograde transport. In both types of transport, COP I or COP II assembly is initiated by activation of small GTPase. Interaction of GTPase with a GEF (guanine nucleotide exchange factor) activates small GTPase.
ER to golgi transport takes place through COP II proteins. COP II carriers are formed by activation of Sar1 (Fig 8).
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Fig. 8. Vesicle formation at ERES is driven by the COPII complex. (A)The assembly of COPII on the
membrane is driven by Sec12p which catalyzes the exchange of GDP for GTP on the small GTPase, Sar1p.
This results in the recruitment of Sar1p to the ER membrane and subsequent binding of two subcomplexes comprising Sec23p–Sec24p and Sec13p–Sec31p. Ongoing assembly of these proteins (B) results in COPII assembly (red) and membrane curvature (C). This curvature is coupled with GTP hydrolysis by Sar1p (green). This process could result in the formation of vesicular structures, coated with COPII (D). In the absence of Sec12p, GTP hydrolysis by Sar1p results in a rapid disassembly of the coat and formation of an uncoated vesicle (E). Alternatively, continued coat-driven membrane protrusion, driven by continued Sec12p activity coupled with rapid Sar1p GTPase activity, could result in the formation of a tubular structure that is not coated with COPII at its apex (F). This depends entirely on the exclusion of Sec12p from the emerging tubule. Either vesicle or tubule formation provides precursors for VTC formation based either on tethering or fusion (G) (Watson & Stephens, Biochim Et Biophys Acta 1744:305 – 313, 2005).
Once Sar1 is activated, it employs larger coat subunits viz. Sec 23/24 and Sec 13/31that form structural component of the coat. The COP II proteins deform the ER membrane and generate membrane bound carriers / vesicles that are different from ER (Watson & Stephens, 2005). This complex is targeted to ER. COP II is composed of 3
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cytosolic components viz. a small GTPase Sar 1 (secretion associated ras-related protein 1) along with its exchange factor Sec 12 and 2 structural heterodimeric complexes – Sec 23/23 & Sec 13/31. The COP II machinery is conserved in plants. In plants, different genes encode for multiple isoforms of COP II proteins. Expression of some of these isoforms is tissue specific. E.g. Arabidopsis Sar 1 isoform Ab1g09180 is expressed only in stamens and pollen. Different COP II isoforms can have varied intracellular locations and they may also show different functions (Hawes et al, 2008).
COP I proteins primarily function to get back the ER resident proteins. COP I vesicle formation needs activation of ARF1. This ARF1 activation recruits a coatomer complex within the cytosol. Coatomer complex consists of 7 different subunits. The resulting complex (Coatomer + ARF1) is then targeted to cis golgi membrane. COP I binds directly to proteins containing retrieval motifs. These retrieval motifs guide the returning of proteins to ER. COP I proteins coat VTCs (vesicular tubular clusters) from golgi. They also bind to golgi membranes. COP I proteins bind to VTCs very particularly near the ERES. So they might be having a major role in sorting of particular secretory cargo at early stage.
Both COP I and COP II bind to ER adjacent to ERES. But their binding and dissociation is spatially and temporally regulated. These binding and dissociation events are regulated at many points. Probably, one of the major regulator proteins is 14-3-3 proteins. These proteins control export of certain receptors, e.g. KCNK3 potassium channels from ER. Majority times, retrieval motifs of the receptors in the cytosolic domains are masked which controls the export of these receptors. Because 14-3-3 proteins control the binding of COP I and COP II proteins to cargo, these proteins can modulate structure of COP I or COP II carriers and also their contents.
Classically, it is considered that the transport of secretory cargo between ER and golgi takes place through formation and movement of small vesicles containing the secretory cargos. This transport is a strict sequential event –
1. COP II proteins recruit cargo to carrier.
2. COP II proteins bring about deformation of ER membrane.
3. Coat subunits polymerize sequentially. This polymerization brings about curvature for vesicle formation, so that ultimately 60 – 90 nm vesicles are formed.
4. Release of vesicle from ER membrane.
This works well for transport of small molecules. But it cannot explain any mechanism for transport of larger molecules, e.g. a molecule of about 300 nm in size cannot be accommodated in 60 – 90 nm COP II coated vesicles. One explanation suggests formation of larger COP II coated vesicles. In S. cerevesiae a homologue of Sec
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24p, called as Lst1p is able to form larger vesicles (Shimoni et.al., 2000). But presence of this homologue is not very effective or sufficient for packing and export of certain proteins.
10. Delivery of cargo to golgi –
Possibly there are many different mechanisms for transport of cargo molecules between ER and golgi apparatus. There are two main models proposed to explain transport of molecules from ER to golgi (Moreau et.al, 2007).
1. First model proposes that cargo transport might be taking place through transient or permanent tubular connections between the dictyosomes. Major transport through such connections would be unspecific cargos. This is also called as bulk flow.
2. In other case, movement from ER to golgi can be selective and it takes place with the help of transport vesicles.
11. Processing in golgi –
The golgi apparatus is the site for many different types of glycosylation reactions.
Many of these reactions occur simultaneously in the same golgi stack. The glycosylations and glycan biosynthesis is brought about by a large and very important group of enzymes called as glycosyltransferases. These enzymes synthesize different cell wall glycans (except cellulose) in the golgi with the help of certain modifying enzymes, nucleotide sugar conversion enzymes and transporters.
Most of the glucosyltransferases in plants are type II membrane proteins. These are single pass transmembrane proteins. Some glucosyltransferases are multi-pass transmembrane proteins (Fig 9) (Oikawa et.al, 2013).
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Fig 9: Schematic illustration of the major types of protein in plant Golgi and their putative interactions. (a) Basic structure of glycosyltransferases. Left panel shows type II single-pass transmembrane topology with catalytic domain, stem region, transmembrane (TM) region, and cytosolic region. The N-terminal region locates on the cytosolic side and the C-terminal region containing the catalytic domain in the Golgi lumen.
Right panel shows multi-pass transmembrane type with catalytic site (indicated in red) located on either the cytosolic or the luminal side of the membrane. (b) Glycosyltransferases often interact through the catalytic domain on the luminal side, as indicated by red lines, by either disulfide bonds between cysteine residues (plant ARAD1/ARAD2) or non-covalent interactions. (c) Protein processing of a single-pass transmembrane glycosyltransferase, which interacts with an anchoring glycosyltransferase. GAUT1 in Arabidopsis is proteolytically processed and loses its membrane anchor, as shown on the left, and is then retained in the Golgi by its interaction with GAUT7 as an anchor. (d) Schematic model of polysaccharide biosynthesis in the Golgi apparatus. Small colored spheres represent soluble enzymes and those with stems inserted in the
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membrane represent single-pass transmembrane proteins; large colored spheres represent multi-pass transmembrane proteins. (i) Nucleotide sugar conversion by cytosolic and membrane-bound enzymes (left).
Import of substrates by nucleotide sugar transporters (right). (ii) Glycan synthesis and side chain addition by single-pass transmembrane glycosyltransferases and cellulose synthase-like (CSL) proteins, which are multi- pass transmembrane glycosyltransferases. CSLC4 glucan synthase has the catalytic domain in the cytosol, whereas CSLA9 (gluco)mannan synthase has the catalytic domain in the Golgi lumen, suggesting a requirement for GDP-mannose import into the Golgi lumen. (Oikawa et.al., Trends in Plant Sci 18(1): 49 – 58).
The N- terminal region of the enzyme consists of a short cytoplasmic part, single transmembrane domain and a catalytic domain spanning the golgi lumen.
Newly synthesized secretory cargos exit from ER and enter cis-golgi. From cis-golgi it is transported to trans-golgi. During transport, many enzymes like glycosidases and glycosyltransferases act on the cargo molecules. They process oligo and polysaccharide chains. In addition to these enzymes, processing requires import of nucleotide sugar substrates into the golgi and export of the inorganic phosphates from golgi to the cytoplasm (Cubero et.al., 2009).
In all eukaryotes, N-glycosylation of proteins starts in ER. During the process, an oligosaccharide precursor is transferred to asparagine side chains in Asn-x-Ser/Thr sequences of unprocessed polypeptide chains. N-glycosylation is very important in protein folding and quality control in ER.
Further, in golgi Golgi-α-mannosidase I (GM I) process N-linked glycans to Man5GlcNAC2(Fig 10). This trimming is the first committed step in golgi N-glycan processing pathway and it is very much required for subsequent N-glycan modifications (Schoberer & Strasser, 2011). Processing within the golgi requires many enzymes – viz. – N-acetyl glucosaminyl transferase I (GnT -I), golgi- α-mannosidase II (GM II), N-acetyl glucosaminyl transferase II (GnT - II), β 1-2 xylosyl transferase (XylT) and core α 1,3- fucosyl transferase (FUT11/12).
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Fig 10: A, Schematic representation of the N-glycan processing pathway in the plant Golgi apparatus and the enzymes involved (Schoberer et.al. Plant Physiol, 161:1737 – 1754, 2013).
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