Chapter 11 (Cell Biology)
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What are the two main types of protein synthesis in eukaryotic cells?
(1) Free polyribosomes — make cytosolic, nuclear, mitochondrial, or chloroplast proteins. (2) Membrane-bound polyribosomes — make ER and endomembrane system proteins.
What is the difference between co-translational and post-translational transport?
Co-translational = protein enters ER as it is being made; post-translational = protein enters organelle after it’s fully made.
What are the three main mechanisms of protein transport into organelles?
(1) Through nuclear pores, (2) Across membranes via translocators, (3) By vesicular transport.
Which organelles use post-translational import?
Nucleus, mitochondria, and chloroplasts.
Which organelle is the entry point for proteins destined for secretion or the endomembrane system?
The endoplasmic reticulum (ER).
What is the role of the ER in protein sorting?
It serves as an entry point for proteins destined for the Golgi, endosomes, lysosomes, plasma membrane, or secretion.
What are the two types of proteins made on the rough ER?
(1) Soluble proteins for the ER lumen or secretion, and (2) Membrane proteins inserted into the ER membrane.
What does “rough ER” mean?
ER covered with polyribosomes during active protein synthesis.
What type of signal do ER-destined proteins have?
An ER signal sequence containing about 8 or more hydrophobic amino acids.
What does the Signal Recognition Particle (SRP) do?
Recognizes the ER signal sequence as it emerges from the ribosome and pauses translation.
What are the two “matchmakers” in co-translational import?
SRP (cytosolic) and the SRP receptor (on ER membrane).
What is the translocator protein?
A protein channel on the ER membrane that threads nascent polypeptides into or across the membrane.
What happens when SRP binds to its receptor?
The ribosome is passed to the translocator and translation resumes, with the growing chain entering the ER.
What kind of energy or timing mechanism drives co-translational transport?
The energy of translation itself (ribosome elongation pushes the chain through).
Where is the ER signal sequence located on soluble proteins?
On the N-terminus.
What does the signal sequence do to the translocator?
It opens the translocation channel.
What happens to the signal sequence after import?
It is cleaved off and released into the bilayer for degradation.
What happens once the soluble protein’s C-terminus passes through?
The protein is released into the ER lumen, and the pore closes.
What additional sequence do membrane proteins have?
A stop-transfer sequence — a hydrophobic stretch that stops translocation and anchors the protein.
What are single-pass membrane proteins?
Proteins that span the membrane once, anchored by a stop-transfer sequence.
What happens to the N-terminal signal in single-pass proteins?
It is cleaved off, leaving the protein anchored by its stop-transfer helix.
Which side of the membrane continues translation after anchoring?
The cytosolic side continues protein synthesis.
What secondary structure allows the stop-transfer segment to stay in the membrane?
An α-helix of hydrophobic amino acids.
What additional signal do double-pass membrane proteins have?
An internal start-transfer sequence in addition to the stop-transfer sequence.