Download lecture notes endomembrane system 4

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BaConEgg Lecture Notes
May 12, 2017, 6:24 PM
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The endomembrane system part 4.
(Golgi apparatus, transport from ER through the Golgi)
1.
Reading: Chapter 13
The Golgi Apparatus (GA) consists of an ordered series of compartments. It is
usually located near the nucleus. The GA is a collection of flattened, membrane
bound cisternae that resemble a stack of plates. Each GA consists of four to six
cisternae. However, the number varies greatly depending on cell type.
2.
Small vesicles associate with the stacks clustering on the side next to the ER,
and along the rims of the cisternae. Such Golgi vesicles are thought to transport
proteins and lipids to and from the GA and between cisternal stacks.
3.
The GA has two distinct faces. A cis face (entry) and a trans face (exit). Each
face is closely connected to network compartments of interconnected tubular and
cisternal structures. These are the cis Golgi network (CGN) and the trans Golgi
network (TGN). Proteins and lipids enter the CGN in transport vesicles from the
ER and exit the TGN in transport vesicle destined for another compartment or
the cell surface.
4.
The GA is especially prominent in secretory cells.
5.
Vesicles bound for the GA bud off a specialised region of the ER called the
transitional elements, which is smooth.
These vesicles transport any kind of
protein into the ER, and are therefore non-selective.
However, for proteins
exiting the ER they must be correctly folded and assembled, if not they are held
in check by binding to a special membrane protein (called BiP) or in aggregates
that cannot be packaged, and are eventually degraded in the ER. Thus there is a
kind of quality control at this stage.
6.
Control proteins such as BiP require a special signal to be retained in the ER.
This retention is mediated by a four amino acid signal called KDEL (Lys-Asp-GluLeu) or similar sequence. Antibodies to KDEL are useful for showing where the
ER is in a cell under the fluorescence microscope.
7.
The retention signal works by selective retrieval of ER resident proteins from the
CGN. The resident proteins are selectively packaged into transport vesicles that
return to the ER.
Thus transport between the ER and the GA occurs in two
directions.
8.
Processing occurs in the GA.
Oligosaccharide chains are processed in the GA.
Although some modification of N-linked oligosaccharides occurs in the ER (I
Ridge
BaConEgg Lecture Notes
May 12, 2017, 6:24 PM
Page 2
discussed this in Cell Biology 1) further modifications occur in the GA.
The
outcome is that in mammalian cells there are two broad classes of glycoproteins.
These are high-mannose and complex oligosaccharides.
9.
The cisternae of the GA are an organised series of processing compartments.
Proteins from the ER first enter the cis compartment.; they then move to the
next compartment called the medial compartment which is the central cisternal
stack; and finally to the trans compartment where glycosylation is completed.
Finally, processed items are moved into the TGN for delivery elsewhere.
10. Each cisterna contains its own set of processing enzymes, and proteins are
successively modified as they move through the stack. Thus the stack is a multistage processing unit, each step separated spatially as well as biochemically.
11. Transport between individual cisternae is thought to be by transport vesicles,
budding from one cisterna and fusing with the next.
This transport is also
thought to be non-selective.
12. The functional differences between the cis, medial and trans compartments were
discovered by localisation of the enzymes involved in processing of the N-linked
oligosaccharides.
For example, removal of mannose residues occurs in the
medial compartment, while addition of galactose and sialic acid occurs in the
trans compartment and the TGN.
13. Because the oligosaccharide chains are added/modified on the luminal side of the
ER and the GA, the distribution of carbohydrate on membrane proteins and lipids
is asymmetric. That is within the cell the carbohydrates are on the lumen side,
and those on the plasma membrane therefore have the carbohydrate facing the
outside of the cell.
14. What is the purpose of glycosylation? There is an important difference between
the construction of an oligosaccharide and other large molecules such as DNA,
RNA and protein.
Whereas the latter require a template and are copied in a
series of repeated steps, complex carbohydrates require a different enzyme at
each step.
It is possible that glycosylation developed in evolution to limit
macromolecules from approaching animals cells too closely, and also allow the
cells to be more flexible.
However, in present day cells, carbohydrate chains
function in many ways, such as signal transduction or cell-cell adhesion.