HOME

TheInfoList




The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
found in most
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
. Part of the
endomembrane system The endomembrane system is composed of the different membranes that are suspended in the cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life ...

endomembrane system
in the
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
, it packages proteins into
membrane-bound A biological membrane, biomembrane or cell membrane is a selectively permeable membrane up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things ...
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) s in an aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt ...
s inside the cell before the vesicles are sent to their destination. It resides at the intersection of the secretory, lysosomal, and
endocytic Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...

endocytic
pathways. It is of particular importance in processing
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s for
secretion Secretion is the movement of material from one point to another, such as a secreted from a or . In contrast, , is the removal of certain substances or waste products from a cell or organism. The classical mechanism of cell secretion is via secret ...
, containing a set of
glycosylation Glycosylation (see also chemical glycosylationA chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosacchar ...

glycosylation
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s that attach various sugar monomers to proteins as the proteins move through the apparatus. It was identified in 1897 by the Italian scientist
Camillo Golgi Camillo Golgi (; 7 July 184321 January 1926) was an Italians, Italian biologist and pathologist known for his works on the central nervous system. He studied medicine at the University of Pavia (where he later spent most of his professional caree ...

Camillo Golgi
and was named after him in 1898.


Discovery

Owing to its large size and distinctive structure, the Golgi apparatus was one of the first
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s to be discovered and observed in detail. It was discovered in 1898 by Italian physician
Camillo Golgi Camillo Golgi (; 7 July 184321 January 1926) was an Italians, Italian biologist and pathologist known for his works on the central nervous system. He studied medicine at the University of Pavia (where he later spent most of his professional caree ...

Camillo Golgi
during an investigation of the
nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

nervous system
. After first observing it under his
microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a used to examine objects that are too small to be seen by the . is the of investigating small objects and structures using a ...

microscope
, he termed the structure as ''apparato reticolare interno'' ("internal reticular apparatus"). 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. With the development of modern microscopes in the twentieth century, the discovery was confirmed. Early references to the Golgi apparatus referred to it by various names including the "Golgi–Holmgren apparatus", "Golgi–Holmgren ducts", and "Golgi–Kopsch apparatus". The term "Golgi apparatus" was used in 1910 and first appeared in the scientific literature in 1913, while "Golgi complex" was introduced in 1956.


Subcellular localization

The subcellular localization of the Golgi apparatus varies among
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s. In mammals, a single Golgi apparatus is usually located near the
cell nucleus In cell biology, the nucleus (pl. ''nuclei''; from Latin or , meaning ''kernel'' or ''seed'') is a biological membrane#Function, membrane-bound organelle found in eukaryote, eukaryotic cell (biology), cells. Eukaryotes usually have a single n ...

cell nucleus
, close to the
centrosome In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...

centrosome
. Tubular connections are responsible for linking the stacks together. Localization and tubular connections of the Golgi apparatus are dependent on
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

microtubule
s. In experiments it is seen that as microtubules are depolymerized the Golgi apparatuses lose mutual connections and become individual stacks throughout the
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
. In
yeast Yeasts are eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

yeast
, multiple Golgi apparatuses are scattered throughout the cytoplasm (as observed in ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () is a species of yeast Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...

Saccharomyces cerevisiae
''). In
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s, Golgi stacks are not concentrated at the centrosomal region and do not form Golgi ribbons. Organization of the plant Golgi depends on
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
cables and not microtubules. The common feature among Golgi is that they are adjacent to
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
(ER) exit sites.


Structure

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 The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
. A mammalian cell typically contains 40 to 100 stacks of cisternae. Between four and eight cisternae are usually present in a stack; however, in some
protists A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
as many as sixty cisternae have been observed. 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). The CGN is the first cisternal structure, and the TGN is the final, from which
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s are packaged into
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) s in an aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt ...
s destined to
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
s, secretory vesicles, or the cell surface. The TGN is usually positioned adjacent to the stack, but can also be separate from it. The TGN may act as an early
endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cell (biology), cells. They are part of Endocytosis, endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized fro ...
in
yeast Yeasts are eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

yeast
and
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s. There are structural and organizational differences in the Golgi apparatus among eukaryotes. In some yeasts, Golgi stacking is not observed. ''
Pichia pastoris ''Pichia pastoris'' is a species of methylotrophic yeast. It was found in the 1960s, with its feature of using methanol, as a source of carbon and energy. After years of study, ''P. pastoris'' was widely used in biochemistry, Biochemical researc ...
'' does have stacked Golgi, while ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () is a species of yeast Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...

Saccharomyces cerevisiae
'' does not. In plants, the individual stacks of the Golgi apparatus seem to operate independently. The Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large amounts of substances; for example, the
antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibody
-secreting
plasma B cell Plasma cells, also called plasma B cells, are white blood cells White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an o ...
s of the immune system have prominent Golgi complexes. In all eukaryotes, each cisternal stack has a ''cis'' entry face and a ''trans'' exit face. These faces are characterized by unique morphology and
biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and pr ...

biochemistry
. Within individual stacks are assortments of
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s responsible for selectively modifying protein cargo. These modifications influence the fate of the protein. 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.


Function

The Golgi apparatus is a major collection and dispatch station of protein products received from the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
(ER). Proteins synthesized in the ER are packaged into
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) s in an aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt ...
s, which then fuse with the Golgi apparatus. These cargo proteins are modified and destined for secretion via
exocytosis Exocytosis () is a form of active transport In cellular biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, phys ...
or for use in the cell. 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 Extracellular space refers to the part of a multicellular organism outside the cell (biology), cells, usually taken to be outside the plasma membranes, and occupied by fluid. The composition of the extracellular space includes metabolites, ions, pr ...
. The Golgi apparatus is also involved in
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
transport and
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
formation. The structure and function of the Golgi apparatus are intimately linked. 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. Enzymatic reactions within the Golgi stacks occur exclusively near its membrane surfaces, where enzymes are anchored. This feature is in contrast to the ER, which has soluble proteins and enzymes in its lumen. Much of the enzymatic processing is
post-translational modification Post-translational modification (PTM) refers to the covalent and generally enzyme, enzymatic modification of proteins following protein biosynthesis. Proteins are synthesized by ribosomes translation (biology), translating mRNA into polypeptide c ...
of proteins. For example, phosphorylation of
oligosaccharide An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a saccharide is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sug ...
s on lysosomal proteins occurs in the early CGN. ''Cis''
cisterna A cisterna (plural cisternae) is a flattened membrane vesicle of the endoplasmic reticulum and Golgi apparatus. A Golgi stack may contain anywhere from three to twenty cisternae, but most contain about six cisternae. Golgi cisternae can be sepa ...

cisterna
are associated with the removal of
mannose Mannose is a sugar monomer of the hexose, aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several Congenital disorder of glycosylati ...

mannose
residues. Removal of mannose residues and addition of
N-acetylglucosamine ''N''-Acetylglucosamine (GlcNAc) is an amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula RC(=O)NR′R″, where R, R', and R″ represent or ...

N-acetylglucosamine
occur in medial cisternae. Addition of
galactose Galactose (, '' galacto-'' + ''-ose The suffix In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. ...
and
sialic acidSialic acids are a class of alpha-keto acid sugars Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by be ...

sialic acid
occurs in the ''trans'' cisternae.
Sulfation Sulfation or sulfurylation in biochemistry is the enzyme-catalyzed Conjugation (biochemistry), conjugation of a Sulfonic acid, sulfo group (not a sulfate or sulfuryl group) to another molecule. This biotransformation involves a sulfotransferase e ...
of
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemi ...

tyrosine
s and
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s occurs within the TGN. Other general post-translational modifications of proteins include the addition of carbohydrates (
glycosylation Glycosylation (see also chemical glycosylationA chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosacchar ...

glycosylation
) and phosphates (
phosphorylation In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...

phosphorylation
). Protein modifications may form a signal sequence that determines the final destination of the protein. For example, the Golgi apparatus adds a
mannose-6-phosphate Mannose-6-phosphate (M6P) is a molecule bound by lectin in the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''o ...

mannose-6-phosphate
label to proteins destined for
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
s. Another important function of the Golgi apparatus is in the formation of
proteoglycans Proteoglycans are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
. Enzymes in the Golgi append proteins to
glycosaminoglycan . For polysaccharide nomenclature see oligosaccharide nomenclature, here. R1, R2, R3 may have different values. Glycosaminoglycans (GAGs) or mucopolysaccharides are long linear polysaccharides consisting of repeating disaccharide units (i.e. two-su ...
s, thus creating proteoglycans. Glycosaminoglycans are long unbranched
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
molecules present in the
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
of animals.


Vesicular transport

The
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) s in an aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt ...
s that leave the
rough endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – 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. Once inside the lumen, the molecules are modified, then sorted for transport to their next destinations. Those proteins destined for areas of the cell other than either the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle In cell biology Cell biology (also cellular biology o ...
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). 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.


Current models of vesicular transport and trafficking


Model 1: Anterograde vesicular transport between stable compartments

* In this model, the Golgi is viewed as a set of stable compartments that work together. Each compartment has a unique collection of
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s that work to modify
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
cargo. Proteins are delivered from the ER to the ''cis'' face using
COPII COPII is a coatomer The coatomer is a protein complex that coats membrane-bound vesicle (biology)#Transport vesicles, transport vesicles. Two types of coatomers are known: *COPI (retrograde transport from Golgi apparatus, trans-Golgi network to ...
-coated
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) s in an aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt ...
s. Cargo then progress toward the ''trans'' face in
COPI Raúl Damonte Botana (November 20, 1939, Buenos Aires Buenos Aires ( or ; ), officially Autonomous City of Buenos Aires, is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction b ...

COPI
-coated vesicles. This model proposes that COPI vesicles move in two directions: anterograde vesicles carry
secretory protein A secretory protein is any protein, whether it be endocrine or exocrine, which is secreted by a cell. Secretory proteins include many hormones, enzymes, toxins, and antimicrobial peptides. Secretory proteins are synthesized in the endoplasmic ...
s, while retrograde vesicles recycle Golgi-specific trafficking proteins. ** 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. ** 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. * This model was widely accepted from the early 1980s until the late 1990s.


Model 2: Cisternal progression/maturation

* In this model, the fusion of COPII vesicles from the ER begins the formation of the first ''cis''-
cisterna A cisterna (plural cisternae) is a flattened membrane vesicle of the endoplasmic reticulum and Golgi apparatus. A Golgi stack may contain anywhere from three to twenty cisternae, but most contain about six cisternae. Golgi cisternae can be sepa ...

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. ** 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. ** Weaknesses: This model cannot easily explain the observation of fused Golgi networks, tubular connections among cisternae, and differing kinetics of secretory cargo exit.


Model 3: Cisternal progression/maturation with heterotypic tubular transport

* 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. ** 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. ** Weaknesses: This model cannot explain the transport kinetics of large protein cargo, such as
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

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.


Model 4: Rapid partitioning in a mixed Golgi

* 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. ** Strengths: Notably, this model explains the exponential kinetics of cargo exit of both large and small proteins, whereas other models cannot. ** 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.


Model 5: Stable compartments as cisternal model progenitors

* This is the most recent model. In this model, the Golgi is seen as a collection of stable compartments defined by
Rab (G-protein) The Rab family of protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, fo ...
GTPase GTPases are a large family of hydrolase Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some commo ...
s. ** 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. ** 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. 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. Currently, the cisternal progression/maturation model is the most accepted among scientists, accommodating many observations across
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s. The other models are still important in framing questions and guiding future experimentation. Among the fundamental unanswered questions are the directionality of COPI vesicles and role of Rab GTPases in modulating protein cargo traffic.


Brefeldin A

Brefeldin A (BFA) is a fungal
metabolite In biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-Europ ...
used experimentally to disrupt the secretion pathway as a method of testing Golgi function. BFA blocks the activation of some
ADP-ribosylation ADP-ribosylation is the addition of one or more ADP-ribose moieties to a protein. It is a reversible post-translational modification that is involved in many cellular processes, including cell signaling, DNA repair, gene regulation and apoptosis. ...
factors ( ARFs). ARFs are small
GTPase GTPases are a large family of hydrolase Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some commo ...
s which regulate vesicular trafficking through the binding of COPs to
endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cell (biology), cells. They are part of Endocytosis, endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized fro ...
s and the Golgi. BFA inhibits the function of several
guanine nucleotide exchange factor Guanine nucleotide exchange factors (GEFs) are proteins or protein domains that activate monomeric GTPasesGTPases are a large family of hydrolase enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts acc ...
s (GEFs) that mediate GTP-binding of ARFs. 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.


Gallery

File:YeastGolgiMovieeLifec.ogg, Yeast Golgi dynamics. Green labels early Golgi, red labels late Golgi. File:GolgiRibbonc.jpg, Two Golgi stacks connected as a ribbon in a mouse cell. Taken fro
the movie
File:GolgiScyl1c.jpg, Three-dimensional projection of a mammalian Golgi stack imaged by confocal microscopy and volume surface rendered using Imaris software. Taken fro
the movie


References


External links

* {{DEFAULTSORT:Golgi Apparatus Organelles