Lecture 4: Endoplasmic Reticulum, Translation and membranes

MCB 104

What determines whether a ribosome translates in the cytosol vs rough ER?

mRNA/protein sequence identity, specifically whether the nascent peptide contains an ER signal sequence.

What is the ER signal sequence?

A ~20 aa hydrophobic stretch, usually near the N- terminus, is recognized by SRP.

What does SRP do when it detects a hydrophobic signal sequence?

Binds the signal peptide and pauses translation.

Why is SRP-mediated translational pausing important?

Prevents hydrophobic proteins from being fully synthesized in the cytosol, where they could aggregate.

Where does SRP bring the ribosome?

To the SRP receptor on the ER membrane, then the ribosome transfers to the translocon.

What is the translocon?

A protein channel in the ER membrane that threads the nascent peptide during translation.

What does “co-translational insertion” mean?

Protein insertion into ER happens Describe Level One of Chromatin Organization while translation is still occurring.

What happens to a protein with an N-terminal ER signal peptide and no stop transfer sequence?

The entire protein is threaded into the ER lumen and becomes a soluble lumenal protein.

What removes the ER signal peptide?

Signal peptidase cleaves the signal sequence after insertion.

Start-transfer sequence vs start codon?

Different things → start-transfer controls translocon docking, start codon controls translation initiation.

What does a stop-transfer do?

Stops translocation through the translocon and leaves that hydrophobic region in the membrane as a TM domain.

What determines N- vs C- terminus orientation in single-pass membrane proteins?

The position of the start-transfer sequence (N-terminal vs internal).

If the N-terminus is synthesized before docking, where does it remain?

in the cytosol

How are multipass membrane proteins generated?

By alternating start-transfer and stop-transfer sequences.

Odd vs even number of hydrophobic TM segments?

Even = N and C termini same side

Odd = opposite ideas

What are the 4 ways proteins associate with membranes?

1. Transmembrane domains

2. Mono-layer associated domains

3. Lipid anchors

4. Protein-protein interactions

Integral vs peripheral membrane proteins?

Integral = permanent

Peripheral = reversible/temporary

Once a protein enters ER lumen, can it return to cytosol?

NO. never. This is a huge quality control principle.

Why is the ER called the entry point to the endomembrane system?

Proteins destined for secretion, Golgi, lysosome, or PM first enter ER.