The endoplasmic reticulum: RER protein synthesis

What proteins are made by the RER (other than mitochondria, plastids or those of peroxizomal lumen)?

Integral membrane proteins

The interaction between the SRP and the signal sequence determines whether the protein will?

end up inside the ER or in the cytosol

The anchor also has an oligosaccharide portion that includes what?

inositol

The ER-lumen also contains an enzyme that forms and reshuffles disulfide bonds between cysteine amino acids in proteins, what is this enzyme called?

Protein Disulfide Isomerase (PDI)

Inner nuclear envelope membrane proteins have specific transmembrane sequences that signal transport to this location where they are retained by what?

lamins and/or chromatin

Rather than being transported in vesicles through the membrane the nuclear envelope membrane proteins move?

laterally through the membrane

The method for ER lumen secretory, lysosomal or plant vacuolar proteins discussed so far (with signal peptide) can also be used for what kind of protein? but there are how many other types of ER membrane proteins as well?

- Type 1 ER membrane proteins -5 other types

The translocon is a complex of what? together these proteins form what? through which the polypeptide will travel to arrive where?

-3 Sec61 proteins (alpha, beta and gamma) -protein lined pore -inside the ER lumen

The RER lumen contains what kinds of molecular chaperons? (3) What do these chaperons recognize and bind to? What do they do to these proteins?

-BiP -Calnexin -Calreticulin -unfolded or misfolded proteins -give them the chance to reorganize into the correct structure

Several steps in the synthesis and trafficking of secretory proteins involve regulation by what proteins?

-G-proteins (GTP- binding proteins that hydrolyze GTP)

Get 3 binds to the ER membrane complex of what? What happens when when the protein is passed from Get 3 to Get1/2?

-Get 1/Get 2 -ATP is hydrolyzed to ADP

The internal signal anchor sequence is recognized by what? But it is not cleaved and instead is?

-SRP/SRP receptor -inserted into the ER membrane

Instead of SRP these proteins that were synthesized on the cytosolic free ribosome are recognized by what receptor complexes? (4) These complexes are associated with what?

-Sec62, 63, 71, 72 -the translocon in the ER membrane

The main difference with Type 1 single membrane spanning proteins is that they have what? This stops what? now this region is ejected sideways into the?

-a membrane spanning stop-transfer anchor sequence -the transfer of the polypeptide -ER membrane

Type II and TypeIII membrane proteins dont have what? but they do have a what? and this is oriented with?

-a signal peptide -a single internal membrane span -orientated with the N- or C- terminal outside the ER respectively

The translocon pore is normally plugged by what? why?

-alpha helix -to stop the movement of calcium or other ions

The synthesis of a protein begins with binding of a what to what? this occurs where?

-free ribosome to a mRNA encoding the protein -cytoplasm

C-Terminal Tail-anchored membrane proteins are produced where? They are recognized by what?

-in the cytosol -ATPase Get3 also called TRC 40

Type IV-A is oriented with the N-terminal in what? Type IV-B is oriented with the N-terminal in what?

-in the cytosol -in the ER lumen

When the signal sequence makes contact with the translocon interior what happens? -What needs to be done to allow this to happen?

-it displaces the plug allowing the polypeptide through into the ER lumen -the pore has to expand

Once the Ribosome is bound to the translocon what will happen to the SRP? polypeptide?

-it is released from its receptor -it will be inserted into the aqueous pore of the translocon

Both these enzymes (signal peptidase and oligosaccharyltransferase) are integral membrane proteins that are situated where?

-near the translocon in the a RER

Orientation of Type II and Type III protein is determined by what? This sequence binds what to the cytosolic side of the membrane?

-positively charged amino acids near a membrane spanning signal anchor sequence -Sec61 alpha subunit

Type IV multispanning transmembrane proteins are produced due to? (2)

-signal-anchor sequences (SA with + charged amino acids) -stop transfer anchor sequence (STA)

secretory, lysosomal, or plant vacuolar proteins contains a 6-15 amino acid hydrophobic signal sequence that targets the nascent (freshly generated) polypeptide to where? this leads to compartmentalization of the polypeptide where?

-the ER membrane - in the ER lumen

After the SRP is bound, further polypeptide synthesis is stopped until when?

-the SRP/polypeptide/ribosome complex docks and attaches specifically to the RER membrane

Hydrolysis of the GTP attached to the SRP causes the release of what? and its insertion into where?

-the signal sequence by the SRP -translocon

The 2 stages of the SRP/polypeptide/ribosome binding complex attaching to the ER membrane. First the SRP binds where? then the Ribosome binds to what?

-to the SRP receptor in the ER membrane -the translocon in the ER membrane

The C-terminal membrane span is cleaved and attached where? what enzyme attaches it?

-to the ethanolamine group of the anchor -GPI transamidase enzyme

As well as proteins being made by ER insertion using ribosomes, some proteins are targeted to the ER after what? These proteins are synthesized on? they are also held in an unfolded state by what chaperons?

-translation is complete -cytosolic free ribosomes -Hsp40 and Hsp70

After the plug is re-inserted into the pore of the translocon what will happen? and the polypeptide is translocated through the translocon into where?

-translation of the mRNA by the ribosome will resume -the ER lumen

The SRP consist of ______ + _______ -this is called what?

6 separate polypeptides + 1 RNA molecule 7s RNA

What are the 2 conformations of G proteins?

1. ON when GTP is bound 2. OFF when GTP is hydrolized to GDP

Binding of the SRP/Polypeptide/ribosome complex to the ER membrane takes place in how many stages?

2 stages

These proteins that are synthesized on the cytosolic free ribosome are pulled through the translocon by what chaperon?

BiP

What else is added to proteins in the ER?

Glycosol-phosphatidyl-inositol (GPI)

What are 2 examples of G proteins?

SRP and its receptor

In what type of transmembrane proteins do the sequential membrane spans have opposite orientations? (polypeptide chain goes across the membrane and then back across to the original side again)

Type IV multispanning transmembrane proteins

The Proteins are retained in the ER membrane by what?

a C-terminal membrane span

Within the translocon there is a pore consisting of what?

a ring of 6 hydrophobic amino acids

What two places does the SRP bind to?

both the signal sequence of the polypeptide and the ribosome

Carbohydrates are added to a protein by what enzyme?

oligosaccharyltransferase

When the polypeptide enters the lumen of the ER the signal sequence is removed by what proteolytic enzyme?

signal peptidase

As the polypeptide is made by the ribosome the signal sequence is recognized by what?

signal recognition particle (SRP)

Get 1/Get2 inserts the proteins membrane span into where?

the ER membrane

As with most non-membrane proteins, membrane proteins are also translocated into where as they are made?

the RER (cotranslationally)

upon completion of translation what happens to the plug?

the plug is re-inserted into the pore

After translation finishes and the polypeptide has moved through the translocon, what happens next?

the ribosome is released

What is the major role that disulfide bonds have?

they stabilize proteins on the extracellular surface of the plasma membrane or those that are secreted outside the cell