Golgi apparatus

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For the song, see Junta (album). Golgi apparatus_sentence_0

Golgi apparatus_table_infobox_0

Cell biologyGolgi apparatus_header_cell_0_0_0

The Golgi apparatus, also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Golgi apparatus_sentence_1

Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles inside the cell before the vesicles are sent to their destination. Golgi apparatus_sentence_2

It resides at the intersection of the secretory, lysosomal, and endocytic pathways. Golgi apparatus_sentence_3

It is of particular importance in processing proteins for secretion, containing a set of glycosylation enzymes that attach various sugar monomers to proteins as the proteins move through the apparatus. Golgi apparatus_sentence_4

It was identified in 1897 by the Italian scientist Camillo Golgi and was named after him in 1898. Golgi apparatus_sentence_5

Discovery Golgi apparatus_section_0

Owing to its large size and distinctive structure, the Golgi apparatus was one of the first organelles to be discovered and observed in detail. Golgi apparatus_sentence_6

It was discovered in 1898 by Italian physician Camillo Golgi during an investigation of the nervous system. Golgi apparatus_sentence_7

After first observing it under his microscope, he termed the structure as apparato reticolare interno ("internal reticular apparatus"). Golgi apparatus_sentence_8

Some doubted the discovery at first, arguing that the appearance of the structure was merely an optical illusion created by the observation technique used by Golgi. Golgi apparatus_sentence_9

With the development of modern microscopes in the twentieth century, the discovery was confirmed. Golgi apparatus_sentence_10

Early references to the Golgi apparatus referred to it by various names including the "Golgi–Holmgren apparatus", "Golgi–Holmgren ducts", and "Golgi–Kopsch apparatus". Golgi apparatus_sentence_11

The term "Golgi apparatus" was used in 1910 and first appeared in the scientific literature in 1913, while "Golgi complex" was introduced in 1956. Golgi apparatus_sentence_12

Subcellular localization Golgi apparatus_section_1

The subcellular localization of the Golgi apparatus varies among eukaryotes. Golgi apparatus_sentence_13

In mammals, a single Golgi apparatus is usually located near the cell nucleus, close to the centrosome. Golgi apparatus_sentence_14

Tubular connections are responsible for linking the stacks together. Golgi apparatus_sentence_15

Localization and tubular connections of the Golgi apparatus are dependent on microtubules. Golgi apparatus_sentence_16

In experiments it is seen that as microtubules are depolymerized the Golgi apparatuses lose mutual connections and become individual stacks throughout the cytoplasm. Golgi apparatus_sentence_17

In yeast, multiple Golgi apparatuses are scattered throughout the cytoplasm (as observed in Saccharomyces cerevisiae). Golgi apparatus_sentence_18

In plants, Golgi stacks are not concentrated at the centrosomal region and do not form Golgi ribbons. Golgi apparatus_sentence_19

Organization of the plant Golgi depends on actin cables and not microtubules. Golgi apparatus_sentence_20

The common feature among Golgi is that they are adjacent to endoplasmic reticulum (ER) exit sites. Golgi apparatus_sentence_21

Structure Golgi apparatus_section_2

In most eukaryotes, the Golgi apparatus is made up of a series of compartments and is a collection of fused, flattened membrane-enclosed disks known as cisternae (singular: cisterna, also called "dictyosomes"), originating from vesicular clusters that bud off the endoplasmic reticulum. Golgi apparatus_sentence_22

A mammalian cell typically contains 40 to 100 stacks of cisternae. Golgi apparatus_sentence_23

Between four and eight cisternae are usually present in a stack; however, in some protists as many as sixty cisternae have been observed. Golgi apparatus_sentence_24

This collection of cisternae is broken down into cis, medial, and trans compartments, making up two main networks: the cis Golgi network (CGN) and the trans Golgi network (TGN). Golgi apparatus_sentence_25

The CGN is the first cisternal structure, and the TGN is the final, from which proteins are packaged into vesicles destined to lysosomes, secretory vesicles, or the cell surface. Golgi apparatus_sentence_26

The TGN is usually positioned adjacent to the stack, but can also be separate from it. Golgi apparatus_sentence_27

The TGN may act as an early endosome in yeast and plants. Golgi apparatus_sentence_28

There are structural and organizational differences in the Golgi apparatus among eukaryotes. Golgi apparatus_sentence_29

In some yeasts, Golgi stacking is not observed. Golgi apparatus_sentence_30

Pichia pastoris does have stacked Golgi, while Saccharomyces cerevisiae does not. Golgi apparatus_sentence_31

In plants, the individual stacks of the Golgi apparatus seem to operate independently. Golgi apparatus_sentence_32

The Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large amounts of substances; for example, the antibody-secreting plasma B cells of the immune system have prominent Golgi complexes. Golgi apparatus_sentence_33

In all eukaryotes, each cisternal stack has a cis entry face and a trans exit face. Golgi apparatus_sentence_34

These faces are characterized by unique morphology and biochemistry. Golgi apparatus_sentence_35

Within individual stacks are assortments of enzymes responsible for selectively modifying protein cargo. Golgi apparatus_sentence_36

These modifications influence the fate of the protein. Golgi apparatus_sentence_37

The compartmentalization of the Golgi apparatus is advantageous for separating enzymes, thereby maintaining consecutive and selective processing steps: enzymes catalyzing early modifications are gathered in the cis face cisternae, and enzymes catalyzing later modifications are found in trans face cisternae of the Golgi stacks. Golgi apparatus_sentence_38

Function Golgi apparatus_section_3

The Golgi apparatus is a major collection and dispatch station of protein products received from the endoplasmic reticulum (ER). Golgi apparatus_sentence_39

Proteins synthesized in the ER are packaged into vesicles, which then fuse with the Golgi apparatus. Golgi apparatus_sentence_40

These cargo proteins are modified and destined for secretion via exocytosis or for use in the cell. Golgi apparatus_sentence_41

In this respect, the Golgi can be thought of as similar to a post office: it packages and labels items which it then sends to different parts of the cell or to the extracellular space. Golgi apparatus_sentence_42

The Golgi apparatus is also involved in lipid transport and lysosome formation. Golgi apparatus_sentence_43

The structure and function of the Golgi apparatus are intimately linked. Golgi apparatus_sentence_44

Individual stacks have different assortments of enzymes, allowing for progressive processing of cargo proteins as they travel from the cisternae to the trans Golgi face. Golgi apparatus_sentence_45

Enzymatic reactions within the Golgi stacks occur exclusively near its membrane surfaces, where enzymes are anchored. Golgi apparatus_sentence_46

This feature is in contrast to the ER, which has soluble proteins and enzymes in its lumen. Golgi apparatus_sentence_47

Much of the enzymatic processing is post-translational modification of proteins. Golgi apparatus_sentence_48

For example, phosphorylation of oligosaccharides on lysosomal proteins occurs in the early CGN. Golgi apparatus_sentence_49

Cis cisterna are associated with the removal of mannose residues. Golgi apparatus_sentence_50

Removal of mannose residues and addition of N-acetylglucosamine occur in medial cisternae. Golgi apparatus_sentence_51

Addition of galactose and sialic acid occurs in the trans cisternae. Golgi apparatus_sentence_52

Sulfation of tyrosines and carbohydrates occurs within the TGN. Golgi apparatus_sentence_53

Other general post-translational modifications of proteins include the addition of carbohydrates (glycosylation) and phosphates (phosphorylation). Golgi apparatus_sentence_54

Protein modifications may form a signal sequence that determines the final destination of the protein. Golgi apparatus_sentence_55

For example, the Golgi apparatus adds a mannose-6-phosphate label to proteins destined for lysosomes. Golgi apparatus_sentence_56

Another important function of the Golgi apparatus is in the formation of proteoglycans. Golgi apparatus_sentence_57

Enzymes in the Golgi append proteins to glycosaminoglycans, thus creating proteoglycans. Golgi apparatus_sentence_58

Glycosaminoglycans are long unbranched polysaccharide molecules present in the extracellular matrix of animals. Golgi apparatus_sentence_59

Vesicular transport Golgi apparatus_section_4

The vesicles that leave the rough endoplasmic reticulum are transported to the cis face of the Golgi apparatus, where they fuse with the Golgi membrane and empty their contents into the lumen. Golgi apparatus_sentence_60

Once inside the lumen, the molecules are modified, then sorted for transport to their next destinations. Golgi apparatus_sentence_61

Those proteins destined for areas of the cell other than either the endoplasmic reticulum or the Golgi apparatus are moved through the Golgi cisternae towards the trans face, to a complex network of membranes and associated vesicles known as the trans-Golgi network (TGN). Golgi apparatus_sentence_62

This area of the Golgi is the point at which proteins are sorted and shipped to their intended destinations by their placement into one of at least three different types of vesicles, depending upon the signal sequence they carry. Golgi apparatus_sentence_63

Golgi apparatus_table_general_1

TypesGolgi apparatus_header_cell_1_0_0 DescriptionGolgi apparatus_header_cell_1_0_1 ExampleGolgi apparatus_header_cell_1_0_2
Exocytotic vesicles (constitutive)Golgi apparatus_header_cell_1_1_0 Vesicle contains proteins destined for extracellular release. After packaging, the vesicles bud off and immediately move towards the plasma membrane, where they fuse and release the contents into the extracellular space in a process known as constitutive secretion.Golgi apparatus_cell_1_1_1 Antibody release by activated plasma B cellsGolgi apparatus_cell_1_1_2
Secretory vesicles (regulated)Golgi apparatus_header_cell_1_2_0 Vesicles contain proteins destined for extracellular release. After packaging, the vesicles bud off and are stored in the cell until a signal is given for their release. When the appropriate signal is received they move toward the membrane and fuse to release their contents. This process is known as regulated secretion.Golgi apparatus_cell_1_2_1 Neurotransmitter release from neuronsGolgi apparatus_cell_1_2_2
Lysosomal vesiclesGolgi apparatus_header_cell_1_3_0 Vesicles contain proteins and ribosomes destined for the lysosome, a degradative organelle containing many acid hydrolases, or to lysosome-like storage organelles. These proteins include both digestive enzymes and membrane proteins. The vesicle first fuses with the late endosome, and the contents are then transferred to the lysosome via unknown mechanisms.Golgi apparatus_cell_1_3_1 Digestive proteases destined for the lysosomeGolgi apparatus_cell_1_3_2

Current models of vesicular transport and trafficking Golgi apparatus_section_5

Model 1: Anterograde vesicular transport between stable compartments Golgi apparatus_section_6

Golgi apparatus_unordered_list_0

  • In this model, the Golgi is viewed as a set of stable compartments that work together. Each compartment has a unique collection of enzymes that work to modify protein cargo. Proteins are delivered from the ER to the cis face using COPII-coated vesicles. Cargo then progress toward the trans face in COPI-coated vesicles. This model proposes that COPI vesicles move in two directions: anterograde vesicles carry secretory proteins, while retrograde vesicles recycle Golgi-specific trafficking proteins.Golgi apparatus_item_0_0
    • Strengths: The model explains observations of compartments, polarized distribution of enzymes, and waves of moving vesicles. It also attempts to explain how Golgi-specific enzymes are recycled.Golgi apparatus_item_0_1
    • Weaknesses: Since the amount of COPI vesicles varies drastically among types of cells, this model cannot easily explain high trafficking activity within the Golgi for both small and large cargoes. Additionally, there is no convincing evidence that COPI vesicles move in both the anterograde and retrograde directions.Golgi apparatus_item_0_2
  • This model was widely accepted from the early 1980s until the late 1990s.Golgi apparatus_item_0_3

Model 2: Cisternal progression/maturation Golgi apparatus_section_7

Golgi apparatus_unordered_list_1

  • In this model, the fusion of COPII vesicles from the ER begins the formation of the first cis-cisterna of the Golgi stack, which progresses later to become mature TGN cisternae. Once matured, the TGN cisternae dissolve to become secretory vesicles. While this progression occurs, COPI vesicles continually recycle Golgi-specific proteins by delivery from older to younger cisternae. Different recycling patterns may account for the differing biochemistry throughout the Golgi stack. Thus, the compartments within the Golgi are seen as discrete kinetic stages of the maturing Golgi apparatus.Golgi apparatus_item_1_4
    • Strengths: The model addresses the existence of Golgi compartments, as well as differing biochemistry within the cisternae, transport of large proteins, transient formation and disintegration of the cisternae, and retrograde mobility of native Golgi proteins, and it can account for the variability seen in the structures of the Golgi.Golgi apparatus_item_1_5
    • Weaknesses: This model cannot easily explain the observation of fused Golgi networks, tubular connections among cisternae, and differing kinetics of secretory cargo exit.Golgi apparatus_item_1_6

Model 3: Cisternal progression/maturation with heterotypic tubular transport Golgi apparatus_section_8

Golgi apparatus_unordered_list_2

  • This model is an extension of the cisternal progression/maturation model. It incorporates the existence of tubular connections among the cisternae that form the Golgi ribbon, in which cisternae within a stack are linked. This model posits that the tubules are important for bidirectional traffic in the ER-Golgi system: they allow for fast anterograde traffic of small cargo and/or the retrograde traffic of native Golgi proteins.Golgi apparatus_item_2_7
    • Strengths: This model encompasses the strengths of the cisternal progression/maturation model that also explains rapid trafficking of cargo, and how native Golgi proteins can recycle independently of COPI vesicles.Golgi apparatus_item_2_8
    • Weaknesses: This model cannot explain the transport kinetics of large protein cargo, such as collagen. Additionally, tubular connections are not prevalent in plant cells. The roles that these connections have can be attributed to a cell-specific specialization rather than a universal trait. If the membranes are continuous, that suggests the existence of mechanisms that preserve the unique biochemical gradients observed throughout the Golgi apparatus.Golgi apparatus_item_2_9

Model 4: Rapid partitioning in a mixed Golgi Golgi apparatus_section_9

Golgi apparatus_unordered_list_3

  • This rapid partitioning model is the most drastic alteration of the traditional vesicular trafficking point of view. Proponents of this model hypothesize that the Golgi works as a single unit, containing domains that function separately in the processing and export of protein cargo. Cargo from the ER move between these two domains, and randomly exit from any level of the Golgi to their final location. This model is supported by the observation that cargo exits the Golgi in a pattern best described by exponential kinetics. The existence of domains is supported by fluorescence microscopy data.Golgi apparatus_item_3_10
    • Strengths: Notably, this model explains the exponential kinetics of cargo exit of both large and small proteins, whereas other models cannot.Golgi apparatus_item_3_11
    • Weaknesses: This model cannot explain the transport kinetics of large protein cargo, such as collagen. This model falls short on explaining the observation of discrete compartments and polarized biochemistry of the Golgi cisternae. It also does not explain formation and disintegration of the Golgi network, nor the role of COPI vesicles.Golgi apparatus_item_3_12

Model 5: Stable compartments as cisternal model progenitors Golgi apparatus_section_10

Golgi apparatus_unordered_list_4

  • This is the most recent model. In this model, the Golgi is seen as a collection of stable compartments defined by Rab (G-protein) GTPases.Golgi apparatus_item_4_13
    • Strengths: This model is consistent with numerous observations and encompasses some of the strengths of the cisternal progression/maturation model. Additionally, what is known of the Rab GTPase roles in mammalian endosomes can help predict putative roles within the Golgi. This model is unique in that it can explain the observation of "megavesicle" transport intermediates.Golgi apparatus_item_4_14
    • Weaknesses: This model does not explain morphological variations in the Golgi apparatus, nor define a role for COPI vesicles. This model does not apply well for plants, algae, and fungi in which individual Golgi stacks are observed (transfer of domains between stacks is not likely). Additionally, megavesicles are not established to be intra-Golgi transporters.Golgi apparatus_item_4_15

Though there are multiple models that attempt to explain vesicular traffic throughout the Golgi, no individual model can independently explain all observations of the Golgi apparatus. Golgi apparatus_sentence_64

Currently, the cisternal progression/maturation model is the most accepted among scientists, accommodating many observations across eukaryotes. Golgi apparatus_sentence_65

The other models are still important in framing questions and guiding future experimentation. Golgi apparatus_sentence_66

Among the fundamental unanswered questions are the directionality of COPI vesicles and role of Rab GTPases in modulating protein cargo traffic. Golgi apparatus_sentence_67

Brefeldin A Golgi apparatus_section_11

Brefeldin A (BFA) is a fungal metabolite used experimentally to disrupt the secretion pathway as a method of testing Golgi function. Golgi apparatus_sentence_68

BFA blocks the activation of some ADP-ribosylation factors (ARFs). Golgi apparatus_sentence_69

ARFs are small GTPases which regulate vesicular trafficking through the binding of COPs to endosomes and the Golgi. Golgi apparatus_sentence_70

BFA inhibits the function of several guanine nucleotide exchange factors (GEFs) that mediate GTP-binding of ARFs. Golgi apparatus_sentence_71

Treatment of cells with BFA thus disrupts the secretion pathway, promoting disassembly of the Golgi apparatus and distributing Golgi proteins to the endosomes and ER. Golgi apparatus_sentence_72

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Golgi apparatus.