Definition, Morphology, Protein Transport and Processing
Named after scientist Camillo Golgi, Golgi
apparatus (Golgi complex) is membrane-bound sacs organelles that are involved
in the modification (and synthesis) storage and transport of proteins and
lipids. Compared to other organelles, it has a unique appearance
made up of pouches that are referred to as cisternae. Inside the cell, this organelle
is located close to the endoplasmic reticulum (ER) near the nucleus of the
As mentioned, Golgi apparatus is made up of several flattened,
stacked sacs referred to as cisternae. However, they can be highly pleomorphic,
which means that they can change their shape for their function.
the type of cell, the number may vary from just a few to thousands.
The cisternae are very small with a diameter ranging from
0.5 to 1.0 nm. Each of these is bound by a membrane and is held together by a
matrix of proteins.
On the other hand, the entire Golgi is helped by
cytoplasmic microtubules and have also been found to contain a
number of important compartments, which include:
Cis and Trans Golgi Network
The Cis and trans are different faces of Golgi
apparatus. The cis face, which is convex in appearance is closer to the
endoplasmic reticulum and acts as the receiving compartment from the ER. On the
opposite side is the trans face (also referred to as maturing face)
Medial and trans cisternal compartments - they
hold various molecules for a period of time.
Movement of Proteins through Golgi Apparatus
When proteins are produced in the ER, they have
to pass through the Golgi apparatus for processing before being released into
the cell to be used. Here, the protein released from the ER pass through the
cis face to enter the Golgi apparatus. It is important for these molecules to
pass through the cisternae stack so that they can be modified and packaged.
Different regions have different types of enzymes that
act on these molecules to modify them. For instance, these enzymes may either
add or remove sugar groups from the proteins thereby modifying them. Once they
are appropriately modified, the protein molecules then move towards the trans
face to be released into the cell cytoplasm.
Movement of Proteins through Golgi Cisternae
Several models have been proposed to help
explain how proteins are transported through the cistern, these include:
Vesicular Transport Model - According to this model,
cisternae are stable compartments through which the protein cargo move.
The protein cargo is transferred between the cisternae compartments with the
help of vesicle carriers. The carriers transport the protein cargo through the
compartments until they are released. Essentially, this process takes place
from one compartment to another. That is, the carriers move the cargo proteins
from one cisternae to the next.
At each compartment, these protein are
processed by either adding or removing given groups (sugar, sulfate) and then
moved to the next cisternae before ultimately arriving at the trans face to be
Cisternal Maturation Model - According to this
model, the cisternae themselves move thereby transporting the protein cargo.
Here, therefore, the protein cargo does not move (or moved by carriers).
Rather, they remain intact in the compartment.
Enzymes then arrive to the
compartment and convert the cis cisterna to medial (or medial to trans
cisterna). This process sees the cargo being moved into the subsequent
compartments as they have been converted until it reaches the last compartment
and ultimately released in to the cell.
This process has been observed in yeast
where live-cell video microscopy showed what appeared to be Golgi apparatus
being converted to another thereby moving the cargo.
Modification of protein starts in the ER where
the following takes place:
- Addition of oligosaccharide
(this is composed of 14 sugar residues)
- Removal of three glucose
residues and a single mannose
In the Golgi apparatus, the processing of
proteins revolves around the modification of the carbohydrate group on the
glycoproteins. The processing process involves changes to the N-linked
oligosaccharides that were added while the protein was in the ER.
takes occurs as follows:
- Removal of additional
mannose residues (3 mannose) on the glycoprotein
- Sequentially adding
- Removal of other mannose
residues (2 or more)
- Addition of a fucose group
N-acetylglucosamines (2 or more)
- Addition of 3 galactose and
3 sialic acid molecules
Processing of N-linked oligosaccharide of the
lysosomal proteins is different from other proteins released in the plasma, this involves:
Addition of N-acetylglucosamine phosphates to
given mannose residues in the cis face.
Removing the N-acetylglucosamine group and retention
of mannose-6-phosphate. This process is aimed at phosphorylation of the mannose
group, which makes it possible for the mannose-6-phosphate receptor to identify
the molecule at the trans Golgi network.
Modification of proteins also involves the
addition to carbohydrates onto the side chains of acceptor serine and
threonine residues of given amino acids in a process referred to as
This process also takes place in a sequential manner where
single sugar residues are added. With some of these processes, the modification
may also involve the addition to sulfate groups on the sugars. Glycosylation is
of great importance in the modification process given that it can prevent the
degradation of proteins. It also serves to:
- Hold the protein in the endoplasmic
reticulum until it has been properly folded
- Direct the protein to the
* As mentioned, one of the main functions of the
Golgi apparatus is to transport such molecules as proteins and lipids.
However, this is only possible when these molecules are properly
Processing/modification of these molecules are of great
significance given that it helps modify the molecule into the proper structure
that will be identified at its destination. It is for this reason proteins are
not released from the ER if they are not properly folded or from the Golgi
apparatus if they are not properly modified.
In addition, additional groups are
added onto these molecules to help in their transportation. For instance, a
given functional group will be added onto a protein so that it can attach to a
carrier and be transported to the right destination.
Take a look at Mitochondria, Ribosomes, Lysosomes as well
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J. E. & Wieland, F. T. Protein sorting by transport vesicles.