Golgi Complex

Question 1

What is the unique morphology of the Golgi complex?

Answer 1

A ‘complex’ or ‘stack’ of flattened membrane-bound cisternae (sacs) with dilated edges and numerous associated tubules and vesicles

Question 2

What is the number and distribution of Golgi complexes?

Answer 2

The number and distribution of Golgi complexes vary between different cell types
—> Mammalian cell – typically contains one (large) Golgi complex located near the center of the cell
—> Plant & yeast cells – often contain numerous Golgi complexes located throughout the cell
Vary in terms of number, size, distribution

Question 3

What is the GCN?

Answer 3

Located at the cis face of Golgi complex

Consists of complex, interconnected network of tubules and vesicles adjacent to ERES

Question 4

What does the Golgi possess?

Answer 4

Complex (stack) having distinct polarity: both structurally and functionally

Question 5

What happens in the GCN?

Answer 5

Also referred to as ER-Golgi Intermediate Compartment (ERGIC)
Initial destination of COPII transport vesicles from ERES
Vesicles fuse to form larger vesicles and interconnected tubules

Question 6

What does the GCN serve as?

Answer 6

Serves as a ‘sorting station’ (on the cis face)
Destination (‘acceptor’ compartment) of COPII vesicles coming ‘forward’ (anterograde transport) from ERES to GCN AND
Site of COPI vesicle assembly for transport ‘back’ (retrograde transport) from GCN to ER
AND
‘Forward’ (anterograde) transport as GCN matures into next sub-compartment of Golgi complex (GCN –> cis cisternae)
AND
Destination of COPI vesicles moving ‘back’ (retrograde transport) from next sub-compartment of Golgi complex (cis cisternae) to GCN

Question 7

What is the Golgi cisternae?

Answer 7

Series of three or more large, flattened cisternae

Cisternae comprise the majority of organelle

Question 8

What is the Golgi cisternae divided into?

Answer 8

Cis, medial and trans cisternae

Question 9

What occurs in the Golgi cisternae?

Answer 9

Sites where Golgi metabolism occurs

Question 10

What is Golgi metabolism?

Answer 10

Synthesis of complex polysaccharides used in cell wall and modification (glycosylation) of proteins/lipids and phosphorylation of mannose units in lysosomal destined proteins

Question 11

Where is the TGN located?

Answer 11

Located on the trans face/trans side of Golgi complex

Question 12

What is the TGN?

Answer 12

An interconnected network of tubules and vesicles (like GCN)

Question 13

What is the role of the TGN?

Answer 13

Serves as a ‘sorting station’
‘Forward’ (anterograde) transport as the previous sub-compartment of Golgi complex matures into TGN (trans cisternae –> TGN)
AND
Site of clathrin coat vesicle assembly for transport ‘forward’ (anterograde transport) from TGN to endosomes
AND
Site of the secretory vesicle and secretory granule assembly for transport ‘forward’ (anterograde) to pm (secretion into extracellular space)
AND
Site of COPI vesicle assembly for transport ‘back’ (retrograde) to Golgi trans cisternae

Question 14

How is the Golgi complex organized?

Answer 14

Organization of Golgi complex (stack) mediated by Golgi matrix
Consist of various Golgi peripheral and integral membrane proteins
Cytoplasmic-facing domains interact to form a ‘scaffold’ – link GCN, cisternae, and TGN together
e.g. GRASPS
Serve as ‘tethering proteins’ to link different Golgi sub-compartments together – RNAi of GRASPs results in the disassembly of the Golgi complex
Golgi matrix proteins also link Golgi complex to cytoskeleton
Positioning and movement of Golgi complex (like all organelles and vesicles) within the cell is controlled by its interaction with the cytoskeleton
The cytoskeleton is adjacent to the Golgi complex

Question 15

What is the Golgi complex function of Glycosylation?

Answer 15

Most glycoproteins (synthesized & N-linked-glycosylated in RER) moving through Golgi complex (cis –> trans) subjected to additional glycosylation reactions
Last step of N-glycosylation in ER…
—> Core oligosaccharide or properly folded/assembled protein ‘trimmed’ (i.e. removal of 1 mannose sugar) by mannosidase
—> Properly processed and folded protein is subsequently transported from ERES to Golgi (GCN) via COPII vesicles
N-linked glycosylation completed in Golgi complex
–> cis, medial and trans-Golgi cisternae possess unique glycosyltransferase and glycosidase enzymes
—–> Add or take off sugar units
———-> Additional modification of glycoprotein’s N-linked oligosaccharide(s) required for proper protein function and/or targeting

Question 16

How does the Golgi act as an assembly line during Glycosylation?

Answer 16

Core oligosaccharides in proteins moving through Golgi are modified sequentially (and in various ways) by different enzymes in each sub-compartment
e.g. alpha-mannosidase I in cis cisternae removes 3 mannose sugars from core oligosaccharide of glycoprotein, glycoprotein moves to medial cisternae and then trans cisternae for additional processing reactions, final processed glycoprotein resides in Golgi or targets (via secretory vesicles/granules) to plasma membrane/extracellular space

Question 17

What is the function of the M6P in the Golgi?

Answer 17

Targeting signal for soluble lysosomal-destined proteins is a unique carbohydrate residue
Phosphorylates the mannose units
In cis cisternae, mannose units in core oligosaccharide(s) of soluble proteins destined for lysosomes are phosphorylated
N-acetylglucosamine phosphotransferase recognizes unique sequences in lysosomal-destined proteins
The addition of M6P residues prevents lysosomal-destined proteins from being subjected to N-linked glycosylation reactions
Mannose-6-phosphate (M6P) group(s) serve as a ‘signal patch’ for soluble lysosomal protein targeting
A targeting signal consisting of a specific 3D arrangement of molecules on the folded protein’s surface
Distinct from the polypeptide-based targeting signal (e.g., N-terminal ‘signal sequence’ involved in ER targeting)
Proteins without M6P are packaged at TGN into secretory transport vesicles/granules destined for plasma membrane/extracellular space (via the constitutive and regulated secretion pathways, or reside in Golgi)
Proteins with M6P are packaged at TGN into clathrin-coated transport vesicle to endosomes and then lysosomes (via the biosynthetic pathway)

Question 18

What modifications occur in the Golgi?

Answer 18

Other modifications to the oligosaccharide core in the Golgi complex serve as sorting signal for the lysosome

Question 19

How do materials move through the Golgi complex after arriving from the ER?

Answer 19

Arrives from the ER via COPII transport vesicles

Cisternal progression/maturation model

Question 20

What is the Cisternal progression/maturation model?

Answer 20

Golgi sub-compartments are dynamic structures Each sub-compartment continually moves (forward) from the cis to the trans side of the Golgi complex
The composition of each sub-compartment changes while moving from cis to trans through the complex
Sub compartment movement mediated by Golgi matrix, proteins, and cytoskeleton (motors)
GCN matures into the cis cisternae, the cis cisternae mature into the medial cisternae, etc.
Overall, the Golgi complex persists (structurally & functionally) because COPI transport vesicles continually move resident Golgi proteins ‘back’ (retrograde transport) to the proper sub-compartment
COPII-coated transport vesicles from the ER (ERES) carrying newly- synthesized ‘cargo’ proteins (lysosomal-destined proteins and Golgi resident enzymes) arrive at the cis face of the Golgi complex
Incoming vesicles fuse together to form a new CGNThe newly formed GCN begins to move ‘forward’ into the complex (stack) and progressively matures into the cis cisternae
GCN moves forward (cis –> trans) = anterograde transport
Each cisterna matures into the next cisternae in the complex
While moving through the complex, the composition of each cisternae constantly changes
COPI-coated vesicles (budding from edges of cisternae) transport resident Golgi enzymes ‘backwards’ through the complex (retrograde = trans –> cis)
alpha-mannosidase I (N-linked glycosylation) is continually transported ‘back’ from the trans and medial cisternae to cis cisternae where it functions
‘Cargo’ destined for a post-Golgi compartment in the endomembrane system excluded from COPI vesicles
TGN eventually disperses into various types of transport vesicles:
Deliver certain ‘cargo’ proteins from TGN to different compartments in the endomembrane system

Question 21

What do certain vesicles target?

Answer 21

Clathrin-coated vesicles: target to endosomes (lysosomes)
Secretory vesicles: target to the plasma membrane (extracellular space)
Secretory granules: eventually target to pm (extracellular space)
COPI-coated vesicle: target (‘back’) trans cisterna