2009 Group 6 Project

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Golgi Apparatus

The Golgi apparatus is a membrane-bound organelle, which is only found in eukaryotic cells. It is said to be the receiving centre of ER products, which are modified in its various compartments and then shipped to other destinations such as the endoplasmic reticulum. This organelle has a distinct polarity: the two poles are the cis face, which is the receiving compartment, and the trans face, which is the shipping compartment. The Golgi apparatus and the endoplasmic reticulum work closely together in order to perform functions such as lipid metabolism, carbohydrate metabolism and the processing of essential proteins for cellular function (Mellman and Simons, 1992). The Golgi apparatus has been discovered in eukaryotic cells such as that of mammalian and plant cells (Mollenhauer and Morre, 1996; Whaley, 1976) but also in primitive unicellular eukaryotes (Becker and Melkonian, 1996). It has also been shown that the gene sequence that encodes for the Golgi apparatus is expressed in all nucleated cells of the rat tissue studied (Barr et al., 1997)


The Golgi apparatus consists of cisternae (flat membrane sacs, http://en.wikipedia.org/wiki/Cisternæ) that are arranged in parallel. Together this becomes a stacked structure that can be usually identified in regions close to the nucleus of a cell (Rambourg and Clermont, 1997).A study by Ripoche et al. in 1994 showed that the Golgi apparatus was spatially separated from the nucleus in associated membranes in Drosophila embryos.

The Golgi apparatus has been studied under the microscope. Such studies reveal that the folding process of the apparatus is sensitive to exposure to proteases. This finding suggests that the process is mediated by proteins (Cluett and Brown, 1992; Barr et al., 1997)

During cell division, the Golgi complex is broken down into smaller constituents which line up between the two identical daughter cells (Warren, 1993; Barr et al., 1997). The first step to the degradation of the Golgi apparatus during mitosis is the loss of cisternae, which is referred to as the unstacking process (Misteli and Warren, 1994).

An in vitro study has demonstrated the cisternae's sensitivity to sulphydryl modifying reagent N-ethylmaleimide (NEM) in the reformation of cisternae studied in a cell-free system (Rabouille et al., 1995a). This result is because of the inactivation of cytosolic NEM-sensitive factors, essential requirements for the fusion of Golgi fragments into cisternae and so are often called NEM-sensitive fusion factor (NSF) (Barr et al., 1997). Furthermore, the dysfunction of p97 is also a factor in the sensitivity to NEM. p97 is an ATPase that is associated with NSF by structure and is involved in membrane function of the Golgi apparatus (Rabouille et al. 1995a, 1995b).


Many studies have been conducted to study how the Golgi apparatus forms. For this purpose, many techniques such as in vitro assays have been used. In in vitro assays special conditions are defined and allow the study of disassembly and reassembly of this organelle. This lead to the identification of many elements that were involved in the dynamics of the Golgi apparatus (Achary et al., 1995; Rabouille et al., 1995). Furthermore, a cell-free system (Rabouille et al., 1995)was used to study the disassembly of the organelle into smaller fragments, which are also known as mitotic Golgi fragments (MGFs), during mitosis (Lucocq et al, 1987) and it was found that the apparatus could be reassembled under optimal incubation. Components that are found to be important in this process of Golgi regeneration include N-ehtylmaleimide (NEM) - sensitive facter, its associated cofactors, p115, which is a vesicle tethering protein, and p97 together with p47 (Rabouille et al. 1995, 1998). Electron microscopes have also been utilized to study isolated Golgi membranes leading to the finding of filamentous structures that connect the gap between two cisternae (Franke et al. 1972; Cluett and Brown, 1992). These filamentous components may form an exoskeleton which gives the apparatus its structure and holds it all together.

The appropriate folding of Golgi membranes into cisternae is an important step for membrane transport. There are two major protein classes that are thought to be involved in morphogenesis that is the differential growth of the Golgi apparatus. These two classes are the Golgi spectrin-actin skeleton and the Golgi matrix proteins (Marra et al., 2001). The role of the spectrin skeleton is in assembling the Golgi membranes and for this purpose it requires GTPase ARF (ADP-ribosylation factor) and phosphatidylinosito 4,5-bisphosphate (Godi et al., 1998; Godi et al., 1999).

Other possible nominees responsible for this exoskeletal structure include the golgins, which make up a large family of proteins. An example of golgin proteins are GM130 and giantin, which are previously identified as autoantigens and thought to interact with the Golgi apparatus peripherally or centrally (Chan and Fritzler, 1998). Various studies have also identified isoforms of ankyrin and spectrin, which are found in specific regions of the Golgi apparatus (Beck et al., 1994; Devarajan et al., 1996; Stankewich et al., 1998). The study of reassembly of the Golgi apparatus also lead to the discovery of the Golgi reassembly stacking protein of 65kDa (Barr et al., 1997) that is important in the assemblage of cisternae. GM130 and giantin are further components thought to contribute to the stacking process (Shorter and Warren, 1999).

  • The volume of the Golgi apparatus is altered primarily by two processes:
    • the fusion of vesicles coming from the ER at the cis interface
    • budding off of vesicles containing products processed to be secreted out of the Golgi complex at the trans interface

  • The Golgi cytoskeleton
    • The Golgi apparatus is supported by a cytoskeleton, which is made up of cytoplasmic microtubules
    • microtubules play a major role in maintaining the integrity of the Golgi cytoskeleton
    • actin and other actin binding proteins are also involved
    • a disruption in the actin cytoskeleton causes disintegration of the Golgi complex
    • microtubules are thought to be involved in the dynamics and localization of the Golgi apparatus
    • During cell division and apoptosis the cytoskeleton is disassembled reversibly and irreversibly, respectively.
    • Drugs such as nocodazole and some alcohols have the potential to fragment the organelle
    • Clathrin is involved in the reassembly of the Golgi apparatus after disintegration (Andreea E et al. Molecular Biology of the Cell, 2007)
    • KIFC3 is part of the Kinesin family C3 and plays an important role in "Golgi positioning and integration"


Nucleotide Sugar Transport

Whether the cellular proteins in eukaryotic cells are bound to the membrane or destined to be secreted, their synthesis takes place in membrane bound ribosomes.After synthesis, they are driven into the luminal compartment of the ER and then to the Golgi apparatus. Within these two organelles, chemical processes take place that adds a sugar group onto the protein (Caffaro and Hirschberg, 2006). This is commonly referred to as glycosylation. A study has been conducted that suggests that there is no unity in the sugar nucleotide transporter, but that this may involve many others (Hirschberg et al.,1998).

Phosphatidylinositol-4-phosphate function


The golgi apparatus is comprised of a stack of flattened, disk-like, membrane bound structures known as cisternae. Most golgi stacks have approximately four to six cisternae, however up to 60 have been recorded in unicellular flagellates. Numerous cisternae stacks can be present in the cell, from 10 to 20, and unlike in plant cells, these stacks in animal cells are often connected by tubules, allowing for communication, forming a single complex. The cisternae stacks are located adjacent to the endoplasmic reticulum, in close proximity to the cell nucleus. (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=golgi%20apparatus&rid=mboc4.section.2333#2346 )

Within the cisternae structure, there are a few distinct sub compartments. The cis face, which, as mentioned previously, is the receiving site of transport vesicles from the endoplasmic reticulum; the trans face is the site of budding off of secretory vesicles for exocytosis. In between the cis and trans face is the median cisternae. Associated with the cis and trans face is the cis golgi network (CGN) and the trans golgi network (TGN), respectively. The number of cis cisternae remains the same, as a general rule, with one present. However, there are more trans and medial cisterna assocciated with the golgi apparatus, anywhere from 3-10 can be present.

Cis cisternae

The cis cisternae is the most medial aspect of the golgi apparatus, adjacent to the endoplasmic reticulum. "In contrast, the polygalacturonic/rhamnogalacturonan-I backbone is assembled in cis and medial cisternae, methylesterification of the carboxyl groups of the galacturonic acid residues in the polygalacturonic acid domains occurs mostly in medial cisternae, and arabinose-containing side chains of the polygalacturonic acid domains are added to the nascent polygalacturonic acid/rhamnogalacturonan-I molecules in the trans cisternae."http://www.plantphysiol.org/cgi/content/abstract/99/3/1070

Trans cisternae

The trans cisternae is the most lateral aspect of the golgi structure, and associated with this is the trans golgi network. "The -1,4-linked D-glucosyl backbone of xyloglucan is synthesized in trans cisternae, and the terminal fucosyl residues on the trisaccharide side chains of xyloglucan are partly added in the trans cisternae, and partly in the trans Golgi network"http://www.plantphysiol.org/cgi/content/abstract/99/3/1070

ER – Golgi Intermediate Compartment (ER – cis-Golgi)

Secretory pathway

Although structural formation of Golgi apparatus varies between cell types and between organisms, yet its function is well-conserved. For instance, mammalian golgi stacks are much more larger than plant golgi stacks. (Saint-Jone-Dupas et al., 2004) But generally in the range of 3 to 8 cisternae are observed in a golgi stack. (Farquhar, 1998) Golgi apparatus is one of the organelle involved in the secretory pathway and function as a processing center to sort out secretory or membrane proteins and lipids for transport to their final destinations. Both the proteins and lipids are either retained as “residents” in Golgi apparatus, cell exterior via secretory vesicles, bound to plasma membrane or sorted to lysosomes. Proteins and lipids trafficking must be tightly regulated and highly selective in order to maintain the homeostatic of Golgi apparatus in the secretory pathway and correct cellular function. (Hermann et al., 1999) See Figure 1.

Secretory Pathway

Secretory pathway of Proteins

In the proteins secretory pathway, endoplasmic reticulum (ER) plays a control step role in proteins sorting. (Palade 1975) ER screens any proteins to ensure they reach their correctly folded state, (Kleizen and Blaakman, 2004) undergo initial glycosylation, before they are allowed to be secreted or transported to Golgi apparatus. Secretory proteins that are also called soluble proteins are translocated across the ER membrane and released into the ER lumen. Contrastly, membrane proteins are first inserted into ER membrane before their release into the ER lumen. Once the newly synthesized proteins in the ER lumen are ready to exit the ER, they are separated from resident ER proteins for the delivery to Golgi apparatus. ER exit sites (ERES) are specialized domains that occur at discrete sites on the ribosome-free ER (Palade 1975) and are associated with exiting vesicles. (Hammond and Glick, 2000) The ER exit sites are called transitional ER (tER) and are located close to the central of cis-Golgi area and cell periphery. (Klumperman, 2000) The secreted proteins are first packaged into COPII-coated transport vesicles that are budding from the regions of rough ER at ER exit sites. The COPII-coated vesicles then uncoat and fuse to form pre-Golgi tubular clusters (Saraste and Kuismanen, 1992), or known as vesicular-tubular clusters (VTCs). (Bannykh et al., 1996) VTCs are short-lived and quickly transported forward along microtubules to become the ER-Golgi intermediate compartment (ERGIC). (Hanri and Schweizer, 1992) Peripheral ERGICs in turn fuse to form the cis-Golgi network (CGN) (Saraste and Kuismanen, 1992; Presley et al., 1997) by moving along microtubules to the Golgi region. (Presley et al., 1997; Scales et al., 1997) Therefore, the ERGIC is a transient and major proteins sorting station between ER and cis-Golgi. COPII- coated vesicles anterograde cargo to Golgi traffic and is sorted at the trans-Golgi network (TGN). (Kaiser and Schekman, 1990) See figure 2.

Utilization of different coats in vesicular transport

Reverse (retrograde) transport

Balance between the selective anterograde transport and retrograde transport is of great importance to maintain the intergrity of ER and Golgi apparatus. COPI-coated transport vesicles which assemble on the membrane of the ERGICs right after the COPII-coats have been shed, (Aridor et al., 1995; Scales et el., 1997) act as retrograde vesicles recycle ER proteins to and from the ERGIC and the cis-Golgi. (Warren and Mellman, 1999) The ER proteins are either ER-resident proteins or missorted ER-resident proteins. Besides that, the retrograde vesicles also recycle Golgi resident proteins from upstream compartments. (Barlowe, 2000; Scales et al., 1997)

Soluble ER-resident proteins

Soluble ER-resident proteins bear a C-terminal KDEL (Lys-Asp-Glu-Leu) sequence that allows them to be selectively retrieved. The KDEL receptor binds to proteins bearing the KDEL sequence that have escaped to the CGN, carries them in COPI-coated vesicles and finally returns them to the ER. (Munro and Pelham, 1987; Pelham 1989) The binding affinity of the KDEL receptor is very sensitive to pH. KDEL receptor could bind to KDEL sequence under slightly acidic pH in CGN, whereas release it at neutral pH.

Membrane ER-resident proteins

Unlike soluble ER-resident proteins, membrane ER-resident proteins can act directly with the components of COPI-coat complex. The ER membrane proteins consists of KDEL receptor, dilysine followed by any other two amino acids at the extreme C-terminal, called KKXX sequence.(Bannykh et al., 1998) Thus, the KKXX sequence enables the retrieval process.

Misfolded proteins

In case of misfolded proteins, ERGICs can act as a control checkpoint. The unfolded, misfolded or partly folded proteins are retrieved from the ERGICs to the ER for another attempt of folding or ER-associated degradation (ERAD). But, to eliminate misfolded ER luminal proteins, the protein cycling between ER and Golgi apparatus is required for ERAD. Thus, the Golgi apparatus will modify these proteins before they can be eliminated. (Caldwell et al., 2001; Vashist et al., 2001)

Secretory pathway of lipids

As well as proteins, lipids travel along the secretory pathway in transport vesicles. Lipids are synthesized in the cytosolic side of ER instead of ribosomal of ER. Smooth ER is particularly involved in lipid metabolism. Similarly, the lipids are packaged into transport vesicles that are budding from the ER and then fuse to form ERGIC and carry their cargo to the Golgi apparatus. Subsequently, the secreted lipids either retained in Golgi apparatus, sorted to plasma membrane or lysosomes. Again, membrane lipids are transported similarly as membrane proteins in secretory pathway. A balance between the forward and backward (retrieval) transport are required to sustain the normal lipids trafficking. (The Cell- A Molecular Approach Cooper, Geoffrey M. Sunderland (MA): Sinauer Associates, Inc.; c2000 The Cell - A Molecular Approach - Chapter 9. Protein Sorting and Transport - The Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes)

Regulation of vesicular transport from ER to Golgi cisternae

Coated vesicles

Coated vesicles, which are also termed transport vesicles formed from the assembly of a special protein coat on the regions of donor membranes. Later the coat is discarded to enable the vesicles to fuse with the target acceptor membranes. There are two types of coated vesicles that mediate the vesicular transport from ER to Golgi cisternae and vice versa , by which included the COPI and COPII. See figure 3.

Mechanism of vesicular transport


COPII-coated vesicle consists of a small GTPase, two heterodimer complexes; Sec23p/Sec24p (Sec23 protein/ Sec24protein) and Sec13p/Sec31p, and Sec16p. (Barlowe et al., 1994) Cytosolic Sar1p is recruited to ER membranes through the actions of guanine-nucleotide exchange factors, GEFs (GTP exchange factors), activated by Sec12p. (Barlowe and Schekman, 1993) Sar1p then attached to the ER membranes recruit Sec23p/Sec24p coat protein complex to form membrane-proximal layer of the COPII coat. The coat is completed by the Sec13p/Sec31p complex to form the membrane-distal layer of the COPII coat. (Schek and Orci, 1996) Thus, the budding is initiated. Sec16p is a putative scaffold protein involved in the ERES formation. (Schek and Orci, 1996) Sec23p act as GTPase activity proteins (GAPs) for Sar1p promotes GTP-hydrolysis. Release of Sar1p and GDP from the COPII-coated vesicles membrane causes dissociation of COPII coat. (Yoshihisa et 1l., 1993) The naked COPII vesicles following fuse with one another to form VTCs. Eventually, the “naked” vesicles start budding off vesicles of their own. VTCs generated continually, functioning in anterograde transport from ER to the Golgi apparatus. (Molecular Biology of the Cell Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002 Molecular Biology of the Cell 4th ed. - Part 1 Chapter 12)


COPI-coated vesicle consists of small GTPase, ADP-ribosylation factor 1 (ARF1), and coatomer complex. (Serafini et al., 1991, Waters et al., 1991) COPI-coated vesicles begin to form soon after COPII coats being released. The assembly of COPI coats start with the GEF catalyses the exchange of GDP for GTP in ARF1. (Chardin et al., 1996) ARF-GTPs bind to the Golgi membranes and ERGICs, followed by the recruitment of coatomer complexes to form COPI-coated vesicles. (Orci et al., 1993, Zhoa et al., 1997) GTP-hydrolysis activated by ARF-GAPs cause the release of COPI coat. (Cukierman et al., 1995) Unlike COPII-coated vesicles, COPI-coated vesicles are involved in the retrograde transport from VTCs to ER and Golgi apparatus to ER. Both retrieval processes will continue as VTCs move to Golgi apparatus and after cargo delivery to Golgi apparatus. (Molecular Biology of the Cell Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002 Molecular Biology of the Cell 4th ed. - Part 1 Chapter 12)

Thethering factors

The naked vesicles must specifically recognise and fuse with the correct target acceptor membranes, thereby delivering the contents of the vesicles. The vesicles move and become thethered to target membranes by the combination of vesicles targetting GTPase called Rab proteins (Rabs). (Thyberg and Moskalewski, 1999) Rabs is a thethering factor that regulates initial thethering and docking of the vesicles to the target membranes and trigger release of SNAREs. (Waters and Pfeffer, 1999) Moreover, thethering complexes and golgin proteins provide the first level of vesicle docking specificity. (Stephens and Pepperkok, 2001) TRAPP I (transport protein partcicle) and TRAPP II are involved in the ER-Golgi transport (Barrowman et al., 2000) while conserved oligomeric Golgi (COG) in intra-Golgi transport. (Loh and Hong, 2004) p115 are also involved in the ER-Golgi transport. (Nelson et al., 1998) GTP-hydrolysis returns Rabs for another cycle of transport.


SNAREs (Soluble NSF attachment protein receptors) (Sollner et al., 1993b) provide specificity and vesicle fusion with the target membranes (Sollner et al., 1993b, Rothman, 1994) Interaction between vesicle membrane SNAREs (v-SNAREs) and target membrane SNAREs (t-SNAREs) form the trans-SNAREs complexes. trans-SNAREs complexes which in turn lock the two membranes together and is ready for vesicle fusion. After vesicle fusion, the complementry sets of SNAREs on the same membrane are often referred as cis-SNAREs complexes. N-ethylmaleimide-sensitive factor (NSF) (Block et al., 1998) and Soluble NSF attachment protein (SNAP) (Clary et al., 1990) act to disassemble the cis-SNAREs complexes, thereby mediate new round of transport. (Sollner et al., 1993a, Mayer et al., 1996)

Inside Golgi Apparatus

Leaving and Outside Golgi Apparatus

  • Vesicle formation
    • Dynamins and associated proteins are thought to be important for vesicle formation in the Golgi apparatus (Praefcke & McMahon, 2004). They are also found to play a role in vesicle fission and fusion (T. Kuroiwa et al., 2008).
  • Trans Golgi Network: After proteins have moved through the majority of the Golgi apparatus they reach the trans Golgi network. The Trans Golgi network is responsible for the packaging and release of proteins to the correct part of the cell. The mechanisms behind these processes are very complicated and much of it has not yet been realised. The first step in preparing the proteins for evacuation from the golgi apparatus is sorting them into different parts of the trans Golgi network based on where they are to head once they leave.

Disorders & Current Research

  • Lysosomal storage diseases (LSD) are rare genetic metabolic disorders with lysosomal malfunction (Winchester B et al.,2000). Recent studies also suggest the association of deficient non-lysosomal proteins found in the Golgi apparatus with LSD and modified lipid transport in disease(Andrea Ballabioa and Volkmar Gieselmann, 2008).

  • Avian Influenza Virus infectivity is dependent on the cleavage of a viral subunit by intrinsic proteases of the host. The highly pathogenic notifiable avian influenza (HPNAI)virus is cleaved by proteases from the Golgi apparatus inside the cell and can lead to systemic infection and death. However, genetic factors of the host are also thought to play an important role in determining pathogenesis caused by AI viruses (Chang-Won Lee and Yehia M. Saif, 2008).

  • Glaucoma is an eye disorder which is commonly associated with many pathogenic signs such as an increase in intraocular pressure and impairment of vision. It was found that there is a high concentration of cleaved myocilin, a protein product in the eye found in organelles such as the Golgi apparatus,in glaucoma patients(Zachary T. Rescha and Michael P. Fautsch, 2008). The precise functions of myocilin in the eye and other sites in the body are yet to be studied and great venue for further research.

  • Photodynamic therapy is often used to treat cancer. This method uses a photosensitizer, which is a chemical compound that reacts to a certain wavelength. The site at which intracellular photosensitisers accumulate is an important factor in determining tumour cell response to PDT (Teiten, 2003). Teiten et al. (2003) studies subcellular localisation of Foscan, a photosensitiser, in the MCF-7 human adenocarcinoma cell line with the help of confocal microscopy and microspectrolfuorometry and showed that Foscan binds to ER and Golgi apparatus binding sites selectively.This research may be the corner stone to successful cancer therapy using PDT.

  • A mutation in the nucleotide sugar transporters of the Golgi apparatus
    • Leukocyte Adhesion Deficiency Syndrome II: This is a very rare autosomal and recessive condition that afflicts humans(Caffaro and Hirschberg,2006). This is due to a mutation in the GDP-fucose transporter (Luhn et al., 2001; Lubke et al.,2001)leading to abnormal growth, severe mental retardation, abnormal neurological function, immunodeficiency and leukocytosis (Caffaro and Hirschberg,2006).
    • Complex Vertebral Malformation: This affects bovine and leads to malformation of the fetus, prenatal death or abnormal fusion of the vertebra (Caffaro and Hirschberg,2006). This condition has been shown to be the consequence of a substitution mutation of the very specific UDP-GlcNAc transporter (Thomsen et al.,2006). This knowledge is important in the diagnosis of this condition prior to birth of bovine fetus.



Transport vesicles move the proteins.

Cis-Golgi network

CGN is the face from which vesicles enter the Golgi apparatus.


Cytoplasm is basically the organelle that fills the cell

Golgi apparatus

An eukaryotic organelle devoted to process and package proteins and lipids synthesized in the ER.

Endoplasmic reticulum

ER is an eukaryotic organelle forms an interconnected network of tubules, vesicles, and cisternae within cells.


An eukaryotic organelle containing digestive enzymes (acid hydrolases).


One of the components of cytoskeleton within cells.

PDT This is a treatment commonly used in cancer patients, which uses a drug with photosensitising qualities. When exposed to a specific wavelength, the photosensitive agent undergoes a reaction producing oxygen species that kill tumour cells.

Plasma membrane

A biological membrane separating the cell interior from cell exterior.

Trans-Golgi network

TGN is the face from which vesicles leave the Golgi apparatus.


A small bubble of liquid within a cell.


2009 Group Projects

--Mark Hill 14:02, 19 March 2009 (EST) Please leave these links to all group projects at the bottom of your project page.

Group 1 Meiosis | Group 2 Cell Death - Apoptosis | Group 3 Cell Division | Group 4 Trk Receptors | Group 5 The Cell Cycle | Group 6 Golgi Apparatus | Group 7 Mitochondria | Group 8 Cell Death - Necrosis | Group 9 Nucleus | Group 10 Cell Shape