Glycosylation Flashcards
What is glycosylation?
The addition of polysaccharides
Where does glycosylation take place?
ER
N-linked glycosylation
Occurs on asparagine residues - addition of oligosaccharide to asparagine
Purpose of glycosylation
Makes proteins more hydrophilic and stops aggregation to help folding
Protects from degradation
Function of the rough ER in 6 points
Site for membrane and secretory protein synthesis Folds proteins in lumen Glycosylates proteins Makes disulphide bridges Checks quality of proteins Calcium store
Describe the smooth ER
Connected to rough ER
Exit sites for transport vesicles
Synthesise lipids and steroids
Abundant in cells that metabolise ipisd
Orientation of the golgi apparatus
Cis faces towards the nucleus and trans faces the plasma membrane
In the golgi apparatus where does the direction of secretion run from
Cis to trans
Describe the golgi apparatus
Flat sac like cisterna
Distinct compartments with different enzymes in each stage
Cis faces the nucleus - incoming site
Trans faces the plasma membrane - outgoing site
Movement in a cis to trans direction
What happens in each golgi stack?
There is further glycosylation that leads to complex specific polysaccharide modifications
What are the multiple routes of vesicle traffic in the golgi apparatus?
Forward traffic: ER to golgi to plasma
Reterograde transport: gogi to golgi to ER. retrieval of resident proteins
Endocytosis
What are the two models of progression through the golgi apparatus?
Vesicle transport model
Cisternal maturation model
Vesicle transport model
Each cisterna remains in one place with unchanging enzymes so the proteins move through the sacks using vesicles to transport them to each stack.
Cisternal maturation model
New cis cisterna form and transverse through the golgi stack. Changes accunulate as enzymes from earlier cisternae move into the stack. Reterograde traffic
What is endocytosis
The internalisation of the plasma membrane: used to internalise nutrients Controls cell surface proteins Clathrin mediated by endocytosis Mediated by Clarthrin and adaptors
What areas is the golgi apparatus made up of?
cis
medial
trans
Give an overview movement of proteins through the golgi
Proteins in the lumen of the ER are packaged into vesicles and bud from the ER. These vesicles fuse to form new cis-golgi cisternae and the entire vesicle will move in the cis-trans direction by cisternal maturation. Retrograde transport vesicles move proteins to the proper compartments. Protein maturation occurs in the trans network and the vesicle buds off to be exocytosed
Lysosome destined membrane proteins
Transported in vesicles to the late endosome as the vesicles begin to fuse, then to the lysosome.
Endocytic pathway for internalising low desity lipoprotien
A cell surface LDL receptor binds to an ApoB protein on the LDL particle. A complex incorporates the particle into the cell forming an endocytic vesicle.
Clathrin coared pits are pinched off by a dynamin-mediated mechanism. The vesicle is clathrin coated.
The vesicle coat is shed to form the early endosome which is an uncoated eukaryotic vesicle. The early endosome fuses with a late endosome. The LDL particle is released from the LDL receptor. The late endosome fuses with a lysosome. Remaining components are recycled
What drives membrane curvature?
Clathrin cage assembly
What is a clathrin cage formed from?
Multiple hexomeric complexes called triskelion
How does clathrin attach to a membrane?
Through an adaptor protein
Outline the steps involved for clathrin to assemble on a membrane
Ligand binds to a transmembrane protein receptor which induced a conformational change so adaptor proteins are recruited. Clathrin proteins then assemble and link together. Once one Clathrin molecule is bound they accumulate
How are endocytic vesicles released?
Dynamin- pinching off of the bud. Dynamin forms a collar and squeezing/stretching brings the membrane together to fuse.
