Intracellular proteine Trafficano lecture 2 (ER Translocation) Flashcards
Translocation
referes to a protein crossing a membrane
what is glycosylation
refers to the addition of sugars to a protein or a lipid
receptors are channels made of
proteins except the SRP
A typical ER signal sequence
has one or more positively charged amino acids followed by a stretch of 6-12 hydrophobic amino acids
ER signal sequences are always on the end terminus
true, its always on the n terminus
steps in ER translocation
- signal sequence on the n terminus binds to SRP (soluble), signal recognition particle, as soon it is exposed.
This pauses translation. - SRP-bound ribosome attaches to SRP receptor in ER membrane
3.Transaltion continues and translocation begins - SRP and and SRP receptor displaced and recycled
5.The protein is directed(transferred) through the sec61 complex protein (channel) into the ER lumen. Sec61 complex is the protein translocator.
what is SRP made up of
6 protein and 1 small RNA molecule
GTP hydrolysis powers..
assembly of the nascent chain/translocon complex and release of the SRP and the SRP receptor
ER import can be..
co-translational or post-translational
post-translational ER import involves.. make sure you know it exists, rest of flashcards are abound co-transaltional er import
Sec 62,63,71,72 complex (receptor)
Sec 61 complex (protein channel)
hydrolysis of ATP
Bip (HSP 70 protein) which binds to the peptide and pulls into the ER lumen
similar to mitochondrial import (the protein is kept unfolded after its translated)
the signal sequence is cleaved off by
signal peptidase.. the signal sequence shifts along and remains bound to the membrane as its hydrophobic
Translocation of membrane protein the ER membrane
stop and start transfer sequences are hydrophobic regions
rhodopsin has..
7 transmembrane spanning domains
N-linked glycosylation occurs..
in the ER by oligosaccharyl transferase. The sugar is attached to N= asparagine residue. N-X-S or N-X-T
what does gylycosylation promote
protein folding, can confer stability in proteins and plays a role in cell-cell adhesion
Inside the ER,
polypeptides are glycosylated and and helped to fold by chaperone proteins
what do chaperons do ?
help proteins to fold correctly, and mark incorrectly folded proteins for degradation
where do disulphide bonds form
in the ER due to its oxidative environment. Disulphide bonds (form between cysteine residues) help stabilise the tertiary and quaternary structure of proteins.
proteins need to taken out of the ER for degradation
true. it is deglycosalated. ubiquitinated. then it is degraded in the cytosolic proteasome in a process known as ER associated degradation (ERAD)
methods to study protein translocation across membranes
- transfection approach. tag gene of interest with GFP and see where the protein goes.
- bio chemical approach. does a protein co-purify with a particular organelle.
- genetic approach. Yeast as a model organism
signals direct proteins to the correct place..
these signals can be amino acid sequences: signal sequences or signal patches
or protein modifications: glycosylation, ubuiqutination, liidi modifications
How is a protein transported from one membrane bound compartment to another
- Vesicular transport (ER to golgi transport)
2. direct fusion of compartments to form a hybrid organelle. (late endosome fuses with lysosome)
how does vesicle form from an organelle membrane ? how does it select its cargo proteins ?
many transport vesicles appear to have electron dense coats. they have a structural and selective role. they select and thus concentrate the proteins that need to be transported in the vesicle.
what controls coat recruitment ?
The GTPases Sar1 and ARF. they also control disassembly of COP1 and COP2 coated vesicles.
uncoating of clathrin-coated vesicles requires an Hsp70 family atpase. the vesicle has to uncoated before it can fuse with its target membrane.
