Protein Sorting and Transport Through Golgi complex The Golgi complex was discovered by an Italian physician and Noble Laureate Camillo Golgi in 1898 during an investigation of the nervous system. Its electron microscopic structure was described by Dalton and Felix in 1954. The Golgi apparatus is noticeable with both light and electron microscope. It is also called the Golgi Apparatus. Morphologically the Golgi is composed of flattened membrane-enclosed sacs (cisternae) and associated vesicles.

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• The Golgi complex was discovered by an
Italian physician and Noble Laureate Camillo
Golgi in 1898 during an investigation of the
nervous system.
• Its electron microscopic structure was
described by Dalton and Felix in 1954.
• The Golgi apparatus is noticeable with both
light and electron microscope. It is also called
Golgi Apparatus.
Camillo Golgi
(1843 – 1926)

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• The Golgi apparatus is present in all Eukaryotic cells and absent in Prokaryotes.
• The Golgi apparatus is specially extensive in the secretory cells.
• It is absent in few cell types, such as the mammalian RBCs, sperm cells of Bryophytes and Pteridophytes
and sieve tubes of plants.
• A cell may have one large Golgi complex or several very small ones. It occupies different positions in
different kind of cells.
• In secretory and absorptive cells, it usually lies between the nucleus.
• The invertebrate and plant cells usually have several small Golgi complexes, called Dictyosomes, scattered
throughout the cytoplasm.

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• Morphologically the Golgi is composed of flattened
membrane-enclosed sacs (cisternae) and associated
vesicles.
• Proteins from the ER enter at its cis face (entry face), which is
convex and usually oriented toward the nucleus. They are
then transported through the Golgi and exit from its
concave trans face (exit face). As they pass through the
Golgi, proteins are modified and sorted for transport to their
eventual destinations within the cell.

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Cisternae:
• The cisternae vary in number from 3-7 in most animal cells and from 10-20 in plant cells.
• Usually equally spaced in the stack, separated from each other by thin layers of intercisternal cytoplasm.
• Cisternae may be flat but are often curved.
• Golgi complex has a distinct polarity, the two poles are called cis face and trans face, which act respectively
as the receiving and shipping departments.
• Convex side of stack forming (cis) face. Concave side of stack maturing (trans) face.
• Secretory materials reach the Golgi complex from Smooth Endoplasmic Reticulum (SER) by way of transport
vesicles which bud off from SER and fuse with golgi cisternae on the cis face.

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Cisternae:
• The membranes of the saccules or cisternae are smooth but of variable thickness they enclose a lumen
of 60-90 A.
• Lumen contains a fluid substance or matrix. In a stack, the adjacent cisternae are separated by a distance of
100-300 A.
• The intercisternal space contains thin layer of cytoplasm having parallel fibrils.
• From the trans face Secretory vesicles arises that carry the processed material to their destination.
• Their contents pass through various cisternae with the help of coated vesicles and intercisternal connectives.

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Tubules:
• Small, round tubules arise from the periphery of the cisternae. Some of these enlarge at their ends to form
vesicles.
• They form a complicated network towards the periphery and maturing face of the apparatus.
• Tubules arise due to fenestrations of the cisternae.
• They have a diameter of 30-50 nm. The tubules interconnect the different cisternae.

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Vesicles:
The vesicles (60 nm in diameter) are of three types:
(i) Transitional vesicles are small membrane limited vesicles which are thought to form as blebs from the
transitional ER to migrate and converge to cis face of Golgi, where they coalesce to form new cisternae.
(ii) Secretory vesicles are varied-sized membrane-limited vesicles that discharge from margins of cisternae
of Golgi. They, often, occur between the maturing face of Golgi and the plasma membrane.
(iii) Clathrin-coated vesicles are spherical protuberances, about 50 μm in diameter and with a rough surface.
They are found at the periphery of the organelle, usually at the ends of single tubules, and are morphologically
quite distinct from the secretory vesicles. The clathrin-coated vesicles are known to play a role in intracellular
traffic of membranes and of secretory products, i.e., between ER and Golgi, as well as, between the GELR
region and the endosomal and lysosomal compartments.

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Golgian Vacuoles
• They are expanded parts of the cisternae
which have become modified to form
vacuoles.
• The vacuoles develop from the concave or
maturing face. Golgian vacuoles contain
amorphous or granular substance.
• Some of the golgian vacuoles function as
lysosomes.

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Transformation of Membranes:
• Golgi complex brings about membrane transformation, that is, converting one type of membrane (e.g., that of
ER) into other types (e.g., selectively permeable plasma membrane, differentiated membrane of lysosome).
• The complex also takes part in the recycling of
plasma membrane.
Glycoproteins and Glycolipids:
• Proteins synthesized by the rough endoplasmic reticulum and lipids synthesized by smooth endoplasmic
reticulum reach the cisternae of the Golgi apparatus.
• Here, they combine with carbohydrates to form glycoproteins and glycolipids.

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Transformation of Membranes:
• Golgi complex brings about membrane transformation, that is, converting one type of membrane (e.g., that of
ER) into other types (e.g., selectively permeable plasma membrane, differentiated membrane of lysosome).
• The complex also takes part in the recycling of
plasma membrane.
Glycoproteins and Glycolipids:
• Proteins synthesized by the rough endoplasmic reticulum and lipids synthesized by smooth endoplasmic
reticulum reach the cisternae of the Golgi apparatus.
• Here, they combine with carbohydrates to form glycoproteins and glycolipids.
Synthesis Complex Carbohydrates:
• Most of the complex carbohydrates, other than glycogen and starch, are synthesized inside the Golgi
complex, e.g., pectic compounds, mucopolysaccharides, hyaluronic acid, chondroitin sulphate,
hemicelluloses, etc.

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Fat Transport:
• Fatty acids and glycerol absorbed by intestinal epithelium are transferred as fat to lacteal through Golgi
complex.
Synthesis of Pigments:
• In Chick embryo the retinal pigment has been observed to be synthesized by Golgi complex.
Formation of Acrosome:
• Acrosome is an important constituent of the tip of animal sperms which helps in digesting away the covering
sheath of the egg or ovum during fertilization. It is synthesized by Golgi complex with the help of its
vesicles.
Formation of Lysosomes:
• Some of the vesicles or vacuoles of the Golgi apparatus store digestive enzymes obtained through ER in the
inactive state. They act as primary lysosomes.

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Formation of Plasma-lemma:
• Membranes of the vesicles produced by Golgi apparatus join in the region of cytokinesis to produce new
plasma-lemma.
Formation of New Cell Wall:
• Pectic compounds of middle lamella and various polysaccharides of the cell wall are secreted by Golgi
complex. They are brought to the area of new wall synthesis by secretion vesicles.
Sulfation
• Another task of the Golgi involves the sulfation of certain molecules passing through its lumen via
sulfotranferases that gain their sulfur molecule from a donor called PAPS.
• This process occurs on the GAGs of proteoglycans as well as on the core protein.
• Sulfation is generally performed in the trans-Golgi network.
• The level of sulfation is very important to the proteoglycans‘ signalling abilities as well as giving the
proteoglycan its overall negative charge.

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APOPTOSIS
• The Golgi has a putative role in apoptosis, with several Bcl-2 family members localized there, as well as to
the mitochondria.
• A newly characterized protein, GAAP (Golgi anti-apoptotic protein), almost exclusively resides in the Golgi
and protects cells from apoptosis by an as-yet undefined mechanism.
PHOSPHORYLATION
• The phosphorylation of molecules requires energy in the form of ATP
• That ATP is imported into the lumen of the Golgi utilised by resident kinases such as casein kinase 1and
casein kinase 2.
• One molecule that is phosphorylated in the Golgi is Apolipoprotein, which forms a molecule known as VLDL
that is a constituent of blood serum.

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VESICULAR TRANSPORT
• Vesicles leaving RER transported to the cis face of GA, fuse with the membrane and empty the contents into
the lumen.
• Molecules inside the lumen are modified and sorted for transport to the next destination.
• Proteins destined for places other than ER and GA, moves to trans face. Gets placed on either of the 3
vesicles, i.e. Exocytotic , Secretory and Lysosomal vesicles.

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Proteins, as well as lipids and polysaccharides, are
transported from the Golgi apparatus to their final
destinations through the secretory pathway. This involves
the
sorting of proteins into different kinds of transport vesicles,
which bud from the trans Golgi network and deliver their
contents to the appropriate cellular locations.

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• Some proteins are retained in the ER instead of traveling from ER to golgi.
• Proteins destined to remain in the lumen of the ER are marked by the sequence Lys-Asp-Glu-Leu (KDEL)
at their carboxy terminus. These proteins are exported from the ER to the Golgi, but they are recognized by
a receptor in the ERGIC or the Golgi apparatus and selectively returned to the ER.
• Many proteins are retained in the ER lumen as a result of the presence of the targeting sequence (KDEL) at
their carboxy terminus. If this sequence is deleted from the protein, the mutated protein is instead
transported to the Golgi and secreted from the cell

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Transport from golgi apparatus takes place by two pathways.
Constitutive Secretory Pathway
• The constitutive secretory pathway, which operates in all cells, leads to continual unregulated protein
secretion.
• In the absence of specific targeting signals, proteins are carried to the plasma membrane by constitutive
secretion.

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Regulated Secretory Pathway
• A distinct regulated secretory pathway in which specific proteins are secreted in response to environmental
signals.
• Proteins are sorted into the regulated secretory pathway in the trans Golgi network, where they are
packaged into specialized secretory vesicles. These secretory vesicles, which are larger than other
transport vesicles, store their contents until specific signals direct their fusion with the plasma membrane.
Examples of regulated secretion include:-
1. The release of hormones from endocrine cells.
2. The release of neurotransmitters from neurons.
3. The release of digestive enzymes from the pancreatic acinar cells.

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The plasma membranes of polarized epithelial cells are
divided into apical and basolateral domains that contain
specific proteins related to their particular functions.
In this example (intestinal epithelium), the apical surface of
the cell faces the lumen of the intestine, the lateral surfaces
are in contact with neighboring cells, and the basal surface
rests on a sheet of extracellular matrix (the basal lamina).
The apical membrane is characterized by the presence of
microvilli, which facilitate the absorption of nutrients by
increasing surface area.
Specific proteins are targeted to either the apical or
basolateral membranes in the trans Golgi network. Tight
junctions between neighboring cells maintain the identity of
the apical and basolateral membranes by preventing the
diffusion of proteins between these domains.

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The best-characterized pathway of protein sorting in the
Golgi is the selective transport of proteins to lysosomes.
Protein destined for incorporation into lysosomes are
modified by mannose phosphorylation. This occurs while the
protein is still in the cis Golgi network. These phosphorylated
mannose residues are specifically recognized by a
mannose- 6-phosphate receptor in the trans Goligi network

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In the trans-Golgi network, the phosphorylated enzymes
bind to M-6-P receptors .
Which direct the enzymes into vesicles coated with the
fibrous protein clathrin.
The clathrin lattices is rapidly depolymerized to its subunits,
and the uncoated transport vesicles fuse with late
endosomes.
Within this low pH compartment, the phosphorylated
enzymes dissociate from the M6P receptors
and then are de-phosphorylated.

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In yeasts and plant cells, which lack lysosomes, proteins are
transported from the Golgi apparatus to an additional
destination: the vacuole.
Vacuoles assume the functions of lysosomes in these cells
as well as performing a variety of other tasks, such as the
storage of nutrients and the maintenance of turgor pressure
and osmotic balance. In contrast to lysosomal targeting,
proteins are directed to vacuoles by short peptide
sequences instead of carbohydrate markers.