cours 12 protein sorting

Question 1

What is synthesised in the ER

Answer 1

protein and lipid

Question 2

what is a water soluable protein?

Answer 2

a protein that is not stable in the membranne

Question 3

Where do we find free and membranne bound ribosome? what determines this seperation?

Answer 3

free: cytosol

Membranne bound: RER

•Ribosome with polypeptide chain will be directed to ER membrane only if the appropriate signal sequence is present (otherwise it will remain in cytosol).

Question 4

What do ribosome need to associate with IOT catalize protein synthesis?

Answer 4

mRNA

Question 5

What are the 2 mode of transport for protein to the ER

Answer 5

Post-translational translocation

co-translational translocation

Question 6

Describe Post translational translocation in the ER

Answer 6

• Protein are synthesised by free ribosomes in the cytosol
• bound by chaperons (Bid) that prevent them from folding
• imported in the ER by a chanel formed by a translocon called Sec61
• these protein have a signal sequence that is bound by Sec61 with the help of accessory proteins
• The signal sequence can be located anywhere on the peptide and is composed of hydrophobic amino acids, flanked by 2 hydrofilic regions
• when the protein binds the Sec61 translocator, a small part of the protein exten into the ER lumen This region is imidiatly bound by a protein called BiP which prevents the protein from slipping back out.
• particule in a fluid are subject to random movement called brownian motion
• The jiggling causes more protein to enter the ER lumen and be bound by other BiP (ratchet)
• The release of BiP from the protein needs ATP

Question 7

Describe Co-translational translocation to the ER

Answer 7

translocation of protein in the ER as they are being synthesised

ER have ribosomes covering its surface that can synthesize proteins directly into the Sec61 chanel so translation and translocation happens concurently

Those protein also have a signal sequence made of hydrophobic amino acids but it is almost always located at the N terminus and is cleaved for all secreted proteins and many transmembrane protein by signal peptidase

the signal is initially bound by a protein called the signal recognition particle (SRP)

SRP has 3 binding domains: The 1st domain binds the signal sequence as soon as it is translated. the second binds the ribosome, stopping translation. The third binds an SRP receptor located in the ER membrane near the Sec61 channel.

at this point the Sec61 channel binds to the signal sequence, causing SRP to dissociate.

Once SRP dissociate, the ribosome continue translating the peptide directly into the ER lumen.

The Sec 61 translocon has a hinge that can open during translocation, causing the protein to slip out ans insert into the ER membranne,

this happens whenever a transmembranne domaine enters the channel

transmembranne domaine are hydrophobic regions about 20 to 25 AA long.

All protein destined to the ER have at least 1 of these domaine because the signal sequence itself is hydrophobic.

however transmembranne protein often have multiple transmembranne domaine

once inserted in the ER membrane, these protein maintain their orientation throughout their transport in the ER and golgie

This mean that the same region of the protein facing the cytosol in the ER will face the cytosol when it arrives at it’s final destination.

therefore it is essential that these domains are inserted into the ER membrane with a very specific orientation. This orientation is deternined by the charge of the amino acid on either side of the signal sequence. the ER lumen stores calcium ions which gives it a net positive charge, while the cytosol maintain a net negative charge.

positivly charged amino acids will preferentially orient toward the cytosol while negatively charged amino acids will be positioned in the ER lumen,

it’s like stiching

the orientation of the first hydrophobic domain determines the orientation of all subsequent transmembranne passes