Lecture 10 Flashcards

1
Q

cis-Golgi network (CGN)
* serves as a ‘sorting station’

A
  1. destination (‘acceptor’ compartment) of COPII vesicles coming ‘forward’ (anterograde transport) from ERES to CGN
    and
  2. site of COPI vesicle assembly for transport ’back’ (retrograde transport) from CGN to ER
    and
  3. ‘forward’ (anterograde) transport as CGN matures into next
    subcompartment of Golgi complex (i.e., CGN → cis cisternae) and
  4. destination of COPI vesicles moving ‘back’ (retrograde transport) from next subcompartment of Golgi complex (cis cisternae)
    to CGN
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2
Q

Golgi cisternae
* series of three or more large, flattened cisternae cisternae comprise majority of Golgi structure

  • 3 main sections:
A

cis, medial and trans cisternae

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3
Q

trans-Golgi network (TGN)

A
  • located on trans face of Golgi complex
  • interconnected network of tubules and vesicles (» CGN)
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4
Q

TGN serves as ‘sorting station’ (» CGN)

A
  1. ‘forward’ (anterograde) transport as previous subcompartment of Golgi complex matures into TGN (i.e., trans cisternae → TGN)
    and
  2. site of clathrin coat vesicle assembly for transport ‘forward’ (anterograde transport) from TGN to endosomes
    and….
  3. site of secretory vesicle and secretory granule assembly
    for transport ‘forward’ (anterograde) to pm (secretion into extracellular space)
    and
  4. site of COPI vesicle assembly for transport ’back’ (retrograde) to Golgi trans cisternae
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5
Q

organization of Golgi complex (stack) mediated by

A

mediated by Golgi matrix
*consists of various Golgi peripheral and integral membrane proteins
*cytoplasmic-facing domains interact to form ‘scaffold’ – link CGN, cis/medial/trans cisternae, and TGN together

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6
Q

GRASPs (Golgi ReAssembly and Stacking Proteins) serve as

A

‘tethering proteins’ to link different Golgi subcompartments together – RNAi of GRAPS results

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7
Q

Golgi matrix also links …

A

Golgi complex to cytoskeleton:
positioning and movement of Golgi (like all organelles/vesicles) in cell controlled by interaction with cytoskeleton

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8
Q

What is the function of the Golgi complex?

A

glycosylation

  • most glycoproteins (synthesized & N-linked-glycosylated in RER) moving through Golgi (cis -> trans) subjected to additional glycosylation reactions
  • last step of N-glycosylation in ER…
  • core oligosaccharide of properly folded/assembled protein ‘trimmed’ (i.e., removal of 1 mannose sugar) by mannosidase
  • properly processed and folded protein subsequently transported from ERES to Golgi (CGN) via COPII vesicles
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9
Q

Golgi complex function: glycosylation
* N-linked glycosylation completed in Golgi complex

A
  • cis, medial and trans Golgi cisternae possess unique glycosyltransferase and glycosidase enzymes
    *additional modification of glycoprotein’s N-linked core oligosaccharide(s) required for proper protein function and/or targeting
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10
Q

Golgi cisternae act as ___

A

‘assembly line’

*core oligosaccharides on proteins moving through Golgi modified sequentially (in various ways) by different enzymes in
each subcompartment

e.g., a-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

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11
Q

mannose-6-phosphate (M6P)
in cis cisternae…

A

*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
* addition of M6P residues prevents lysosomal-destined proteins from being subjected to N-linked glycosylation reactions

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12
Q

mannose-6-phosphate (M6P) group(s) serve as

A

‘signal patch’ for soluble lysosomal protein targeting

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13
Q

proteins without M6P

A

packaged at TGN into secretory transport vesicles/granules destined for
plasma membrane/extracellular space (via constitutive and regulated secretion pathways) …..or reside in Golgi

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14
Q

proteins with M6P

A

packaged at TGN into clathrin- coated transport vesicle to endosomes and then lysosomes (via the biosynthetic pathway) ….

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15
Q

Mutations in enzyme responsible for formation of M6P residues results in ___

A

lysosomal storage diseases: Proteins do not properly target from Golgi to lysosome. Led to identification of M6P sorting signal and receptor

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16
Q

How do proteins move through Golgi complex (i.e., from CGN to TGN) and then onto other
compartments of endomembrane system?

A

Cisternal progression/maturation model

17
Q

Cisternal progression/maturation model

A
  • Golgi subcompartments are dynamic structures
  • each subcompartment continually moves (forward) from cis to trans side of Golgi complex
  • subcompartment movement mediated by Golgi matrix proteins and cytoskeleton (motors)
  • composition of each subcompartment changes while moving cis to trans through complex
    e.g., CGN matures into cis cisternae, cis cisternae matures into medial cisternae, etc
  • overall, Golgi complex persists (structurally & functionally) because COPI transport vesicles continually move resident Golgi proteins ‘back’ (retrograde transport)
    to proper subcompartment
18
Q

COPII-coated transport vesicles from ER (ERES) carrying ….

A

newly-synthesized ‘cargo’ proteins (e.g., lysosomal-destined proteins and Golgi resident enzymes) arrive at cis face of Golgi complex

incoming vesicles fuse together to form new CGN

  • newly formed CGN begins to move ‘forward’ into complex (stack) and progressively matures into cis cisternae
    CGN moves forward (cis -> trans) = anterograde transport
  • each cisternae matures into next cisternae in complex CGN -> cis cisternae -> medial cisternae -> trans cisternae -> TGN
  • while moving through complex, composition of each cisternae constantly changes
19
Q

COPI-coated vesicles (budding from edges of cisternae) transport resident Golgi enzymes ..

A

‘backwards’ thru complex (retrograde = trans -> cis) e.g., a-mannosidase I (N-linked glycosylation) continually transported ‘back’ from trans/medial cisternae to cis cisternae where it functions

  • ‘cargo’ destined for post-Golgi compartment in endomembrane system excluded from COPI vesicle
20
Q

TGN eventually disperses into various types of transport vesicles

What are the four types of transport vesicles?

A

deliver certain ‘cargo’ proteins from TGN to different compartments in endomembrane system
1.clathrin-coated vesicles – with M6P-bearing protein ’cargo’ target to endosomes (lysosomes)
2.secretory vesicles – target to plasma membrane (extracellular space)
3.secretory granules – eventually target to pm (extracellular space)
4.COPI-coated vesicles – target (‘back’) to trans Golgi cisternae

21
Q

cis, medial & trans cisternae = sites of

A

Golgi metabolism, e.g., N-linked glycosylation and mannose phosphorylation (M6P = lysosome
targeting signal ‘patch’); COPI vesicles move trans→cis