Lecture 12 - Endomembranes & Golgi Flashcards
Def: further processes and sorts glycoproteins and membrane lipids, playing a central role in membrane and protein trafficking in eukaryotic cells
Golgi complex
What happens in the Golgi complex?
Further glycosylation and processing of carbohydrate side chains occurs
Where does terminal glycosylation occur?
In the Golgi apparatus
Def: modifications of glycoproteins through the removal/addition of sugars side chains on the core oligosaccharide
Terminal glycosylation
All glycosylation reactions occur on the luminal side of the membrane thus contributing to __________ _________
Membrane asymmetry
Key point to remember for gycolsylation:
Each step is strictly dependent on the preceding modification
what are the functions of glycosylation in the Golgi complex
- participate in protein/lipid sorting in the TGN
- makes glycoproteins/membranes more resistant to digestion by proteases, by creating the glycocalyx
- serve as recognition molecules in cell-cell interaction, and unfortunately viruses also use it for viral entry
- regulatory roles, ABO blood type, and immune-recognition
what is the “bottom line” of Golgi complex glycosylation?
gives the cell ability to generate many chemically distinct molecules at the cell surface
what are the 2 models for movement through the Golgi
- stationary cisternae model
- cisternal maturation model
def: model in which the cisternal and the resident enzymes stay in place, while cargo moves from one stack to the next
stationary cisternae model
def: model in which the cargo remains within a cisternae, while the cisternal moves forward (-cis to trans-) and resident enzymes shuttle backwards in vesicles
cisternal maturation model
what proteins would be found in the transport vesicles in the stationary cisternae model?
cargo
what proteins would be found in the transport vesicles in the cisternal maturation model?
resident enzymes
if ER-Golgi transport was inhibited what would happen in the stationary cisternae model?
there would be little change in the Golgi structure
if ER-Golgi transport was inhibited what would happen in the cisternal maturation model?
the golgi would dissapear
def: movement of material toward the plasma membrane
anterograde transport
def: movement of material towards the ER
retrograde transport
def: fusion of vesicles with the plasma membrane, releasing their contents into the extracellular space
exocytosis
def: formation of a vesicle on the plasma membrane, taking up solutes from the extracellular space
endocytosis
sorting of proteins begins in the __, with the final sorting occurring in the ___
ER, TGN
what is the most established type of targeting of proteins from the rough ER?
lysosomal targeting
what are the two mechanisms that keep proteins within an organelle?
retention and retrieval
def: resident molecules are excluded from transport vesicles
retention
def: “tags” can be used to return “escaped” proteins to their proper location
retrieval
what are the requirements the sorting signals must fulfill?
- if mutated, sorting is lost
- if transplanted to an unsorted protein, that protein is now sorted
what are 3 examples of tags on proteins?
- amino acid sequence
- hydrophobic domain
- oligosaccharide side chain
what is the most common ER retention tag?
the KDEL retrieval signal
def: short C-terminal amino acid sequence (Lys-Asp-Glu-Leu) that mediates the return of soluble proteins to the ER
KDEL retrieval signal
where are membrane proteins retrieval signals?
on their cytosolic domains
what is the most common ER specific transmembrane protein?
KKXX (Lys-Lys-any-any)
KDEL proteins are meant to be ____ _______
ER residents
what happens if KDEL proteins escape to the ER?
they associate with the KDEL receptor
what do KDEL receptors do?
mediate the packaging of KDEL proteins into ER-bound transport vesicles
is KDEL to KDEL receptor binding efficient or inefficient in the Golgi?
it is efficient in the lower pH environment of the Golgi
is KDEL to KDEL receptor binding efficient or inefficient in the ER?
inefficient due to the higher pH of the environemnt
what happens to soluble lysosomal enzymes in the ER and the early compartments of the Golgi
they undergo N-glycosylation followed by removal of glucose and mannose units
what ensure the delivery of lysosomal proteins to the lysosomes?
a mannose-6-phosphate tag
