Lecture 10 Flashcards
cis-Golgi network (CGN)
* serves as a ‘sorting station’
- destination (‘acceptor’ compartment) of COPII vesicles coming ‘forward’ (anterograde transport) from ERES to CGN
and - site of COPI vesicle assembly for transport ’back’ (retrograde transport) from CGN to ER
and - ‘forward’ (anterograde) transport as CGN matures into next
subcompartment of Golgi complex (i.e., CGN → cis cisternae) and - destination of COPI vesicles moving ‘back’ (retrograde transport) from next subcompartment of Golgi complex (cis cisternae)
to CGN
Golgi cisternae
* series of three or more large, flattened cisternae cisternae comprise majority of Golgi structure
- 3 main sections:
cis, medial and trans cisternae
trans-Golgi network (TGN)
- located on trans face of Golgi complex
- interconnected network of tubules and vesicles (» CGN)
TGN serves as ‘sorting station’ (» CGN)
- ‘forward’ (anterograde) transport as previous subcompartment of Golgi complex matures into TGN (i.e., trans cisternae → TGN)
and - site of clathrin coat vesicle assembly for transport ‘forward’ (anterograde transport) from TGN to endosomes
and…. - site of 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
organization of Golgi complex (stack) mediated by
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
GRASPs (Golgi ReAssembly and Stacking Proteins) serve as
‘tethering proteins’ to link different Golgi subcompartments together – RNAi of GRAPS results
Golgi matrix also links …
Golgi complex to cytoskeleton:
positioning and movement of Golgi (like all organelles/vesicles) in cell controlled by interaction with cytoskeleton
What is the function of the Golgi complex?
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
Golgi complex function: glycosylation
* N-linked glycosylation completed in Golgi complex
- 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
Golgi cisternae act as ___
‘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
mannose-6-phosphate (M6P)
in cis cisternae…
*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
mannose-6-phosphate (M6P) group(s) serve as
‘signal patch’ for soluble lysosomal protein targeting
proteins without M6P
packaged at TGN into secretory transport vesicles/granules destined for
plasma membrane/extracellular space (via constitutive and regulated secretion pathways) …..or reside in Golgi
proteins with M6P
packaged at TGN into clathrin- coated transport vesicle to endosomes and then lysosomes (via the biosynthetic pathway) ….
Mutations in enzyme responsible for formation of M6P residues results in ___
lysosomal storage diseases: Proteins do not properly target from Golgi to lysosome. Led to identification of M6P sorting signal and receptor