endoplasmic reticulum

Question 1

ER lumen

Answer 1

where nascent proteins are folded, modified and assembled

Question 2

curvature of ER

Answer 2

done by reticulons inserting into bilayer
many reticulons give sharp curvature of tubes at end of sheets

Question 3

ER membrane fusion by small G-proteins

Answer 3

3 way branching comes about from fusion of an extending tube with side of another tubule

A small GTPase, Atlastin, is
responsible for 3-way branched
structure of ER tubules

Dimerization of Atlastin-GTP link
opposite membranes.

GTP hydrolysis somehow draws
membranes together for fusion.

Question 4

Getting a protein into the ER lumen

Answer 4

Need a targeting signal which is detected and sent to ER.
(It is possible to artificially target non-ER proteins to the ER)

For secreted proteins, ER signal located at Nterminus of nascent polypeptide.
(this is cut off after targeting bc its not on mature protein)

ER targeting needs to happen at
the same time as protein synthesis
– “Cotranslational Translocation”

Question 5

Cotranslational Translocation

Answer 5

in short:
- SRP binds to ribosomal subunit and signal sequence
- theres a receptor for SRP in ER membrane
- translation is halted until ribosome gets to ER translocon
- it docks, a channel opens, protein goes through, signal gets cut off and it falls into lumen and is folded.

There is an adaptor complex, the signal recognition particle SRP
- This binds to both the large ribosomal subunit and the signal sequence of the growing peptide

There is a receptor for SRP in the ER membrane
- translation halted until the ribosome gets to ER translocon.
- Docking of the SRP to its receptor opens up a channel allowing the translocation of the newly synthesised peptide.

Signal peptidase in the ER cleaves the signal sequence off the polypeptide
- Polypeptide folds within the lumen of the RER

Question 6

Insertion of Type I membrane proteins

Answer 6

initially identical is translocation

• Insertion into membrane requires a “stop-transfer anchor” (STA) signal.
• Hydrophobic stretch of amino acids (20-25 aa) that embeds into lipid bilayer

Question 7

Insertion of Type II membrane proteins

Answer 7

Type II membrane proteins DO NOT have a cleavable N-terminal signal sequence.

Instead translation initially occurs in cytoplasm

However, an internal ER targeting sequence is then recognized by SRP and directed to ER translocon.
* This internal targeting signal also doubles as an anchor signal: a “signalanchor sequence” (SA)

reverse topology of type 1
N terminus outside of plasm, C terminus inside (reverse of 1 and 3)

• type 1 grows into nucleus - type two grows out again (like a u)

Question 8

Insertion of Type III membrane proteins

Answer 8

same topology as type 1
translocation mechanism is similar to Type II
does not have a cleavable n terminus signal sequence

• instead of SA curving up to grow out, this continues and grows down like 1

Question 9

Glycosylation

Answer 9

Glycosylation is the process by which sugar ‘trees’ (glycans) are created, altered and attached to 1000’s of proteins or fats (lipids). When these sugar molecules are attached to proteins, they form glycoproteins; when they are attached to lipids, they form glycolipids

Many secretory proteins and membrane proteins are sugar-modified
(glycoproteins)

Transfer of a chain of sugars (glycans) from a precursor catalyzed by
glycosyltransferases.

Glycans can be further modified after initial transfer.

Glycosylation aids in protein folding and can also determine protein
function (like other PTMs).