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Cell biology > Protein translocation (into the ER) > Flashcards

Protein translocation (into the ER) Flashcards

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1
Q

What is co-translational translocation?

A
  • translocation of polypeptide chain while it is still being synthesised by ribosome
  • import into ER mostly co-translational
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2
Q

What is post-translational translocation?

A
  • translocation of polypeptide chain after its synthesis has been completed
  • common mode of translocaiton for mitochondria, chloroplasts, peroxisomes
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3
Q

What are signal peptide/signal patches?

A
  • a signal amino acid sequence within proteins that specify the proteins’ destinations
  • continuous sequence –> signal peptide
  • discontinuous sequence –> signal patch when folded
  • characteristic pattern: postive towards N-terminus, hydrophobic towards the middle and polar towards the C-terminus
  • thought to trigger the opening of translocator
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4
Q

What is the signal recognition particle (SRP)?

A
  • Complex that binds to signal sequences and ribosomes
  • halts translation when bound to elongation factor binding site of ribosomes
  • guides proteins to ER
  • binds to SRP receptor in ER membrane
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5
Q

How can the SRP bind specifically to so many different signal sequences?

A
  • Signal sequence binding site of SRP is a hydrophobic pocket lined with methionines
  • methionines have flexible, unbranched side chains
  • can accomodate different hydrophobic signal sequecnes
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6
Q

Why does SRP have to block translation?

A
  1. give ribosome time to bind to the ER before translation is complete, thus preventing the peptide being released into the cytosol (important safety device for secreted or lysosomal hydrolases)
  2. Prevent proteins from folding before reaching translocator in ER membrane
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7
Q

What is the SRP receptor?

A
  • transmembrane protein complex in ER membrane
  • binds to SRP-ribosome complex and guides it to an empty translocator
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8
Q

What is the Sec61 complex?

A
  • aqeuos channel
  • passive core, energy comes from GTP hydrolysis of protein translation (in co-translational translocation) or ATP hydrolysis (in post-translational translocation)
  • has short α helix plug that keeps the translocon closed. when translocon engages with polypeptide, it rearranages itself and moves plug out of the way
  • has pore ring formed by hydrophobic residues, surrounds translocating polypeptide to prevent leaks of molecules
  • opens to form a pore across the membrane to let the hydrophilic portions of proteins cross the lipid bilayer, and it opens laterally within the membrane to let hydrophobic portions of proteins partition into the lipid bilayer
  • in eukaryotes, 4 sec61 complexes form large translocator assembly
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9
Q

Why is it important for translocator to be kept closed when idle?

A

so that membrane remains impermeable to ions e.g. Ca2+

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10
Q

What is a sec translocon?

A

The assembly of the sec61 complex and other accessory components e.g the signal peptidase

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11
Q

What is signal petidase?

A

transmembrane

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12
Q

What does the signal hypothesis state?

A

That the signal peptide is cleaved off by signal peptidase in the ER membrane before the polypeptide chain has been completed

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13
Q

summarise the process of co-translational translocation

A
  • SRP recognises signal seqeunce, binds to signal sequence and elongation factor binding site of ribosome
  • SRP guides ribosome to ER membrane and binds to SRP receptor
  • SRP and SRP receptor hydrolyse GTPs, triggers conformational change and thus dissociation of SRP from the ribosome and SRP receptor
  • ribosome enagages with translocator, signal sequence acts as start transfer signal that is recognised by binding site in the pore of translocator –> allows pore to open
  • translation resumes, growing polypeptide chain goes into the ER
  • signal peptide is cleaved by signal peptidase and released laterally into the ER membrane
  • polypeptide released into ER when translation is complete
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14
Q

How does post-translational translocation manage to translocate proteins uni-directionally?

A
  • Sec62, Sec63, Sec71, and Sec72 forms a complex which lumenal side interacts with chaperone proteins BiP, causing it to hydrolyses its ATP
  • BiP in ADP bound state can bind to polypeptide chain and act as beads in a string so that polypeptide chain does not slide backwards and forwards
  • After polypeptide entered ER, the ADP in BiP is exchanged for ATP, therefore dissociates from polypeptide
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15
Q

Describe transmembrane protein intergration into ER

A
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16
Q

Describe multi-pass transmembrnae protein integration into ER

A

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