protein translocation at er

What is the classical pathway for targeting proteins to the ER?

The SRP-dependent targeting pathway.

What reporter was used to identify SRP-independent ER targeting?

A fluorescent protein fused to the Gas1 signal sequence.

What did mislocalization of the reporter in the yeast deletion screen indicate?

Loss of a gene required for ER targeting.

What pathway was discovered as a new SRP-independent route to the ER?

The SND pathway.

What does the SND pathway primarily target?

Proteins with internal signal sequences.

What is the main site of membrane protein biogenesis?

The endoplasmic reticulum (ER).

What is the core ER translocon?

The Sec61 complex.

What three subunits make up Sec61?

Sec61α

What blocks the Sec61 translocon when it is closed?

A plug helix.

What structural feature allows hydrophobic segments to enter the membrane?

The lateral gate.

What method enabled high-resolution structures of active Sec61?

Cryo-EM with stalling sequences.

Why are stalling sequences used in structural studies of Sec61?

They freeze ribosome–nascent chain complexes at specific stages.

What powers co-translational translocation?

Energy from peptide chain elongation.

What recognizes signal peptides during co-translational import?

SRP (Signal Recognition Particle).

What receptor docks SRP-bound ribosomes to the ER membrane?

The SRP receptor.

Which organisms frequently use post-translational translocation?

Yeast and bacteria.

What prevents folding of post-translational substrates in the cytosol?

Hsp70 chaperones.

What complex recruits BiP during post-translational ER import?

The Sec62–Sec63 complex.

What role does BiP play in post-translational translocation?

It prevents backsliding via a ratchet mechanism.

What ATPase drives post-translational translocation in bacteria?

SecA.

Approximately what percentage of proteins contain transmembrane domains?

About 30%.

How do transmembrane domains enter the membrane via Sec61?

Through the lateral gate.

What rule helps determine membrane protein orientation?

The positive-inside rule.

What type of membrane proteins have cleavable N-terminal signal peptides?

Type I membrane proteins.

What enzyme removes the N-terminal signal peptide?

Signal peptidase.

What defines tail-anchored proteins?

A single TM domain at the extreme C-terminus.

Why can’t tail-anchored proteins use SRP?

Their TMD emerges only after translation is complete.

What system targets tail-anchored proteins to the ER?

The GET pathway.

What protein in the GET pathway binds the hydrophobic tail?

Get3 ATPase.

What ER insertase receives substrates from Get3?

The Get1–Get2 complex.

To what family do Get1–Get2 insertases belong?

The OXA-family insertases.

What determines the orientation of multipass membrane proteins?

The orientation of the first transmembrane segment.

What is the role of accessory complexes like EMC or TRAP?

Assisting Sec61 with difficult membrane insertion events.

What approach was used to study TRAP complex function?

siRNA knockdown combined with quantitative proteomics.

What happens when a single TRAP subunit is depleted?

The entire TRAP complex destabilizes.

How do cells compensate for TRAP loss?

By upregulating SRP receptor levels.

What type of signal sequences depend on TRAP?

Those enriched in glycine and proline.

What does TRAP help Sec61 do for these difficult signal sequences?

Open the lateral gate to allow successful insertion.

Where does the TRAP complex sit relative to Sec61?

Adjacent to Sec61 at the ribosome–ER interface.