Cell Biology Exam 2

Ch 12 Protein Sorting

Where does synthesis of all proteins begin?

On ribosomes in the cytosol.

What determines the final destination of a protein?

Sorting signals in its amino acid sequence.

What happens to proteins that lack sorting signals?

They remain in the cytosol.

What do sorting receptors do?

Recognize sorting signals and direct proteins to the correct cellular compartment.

What is protein translocation?

Transport of proteins through protein translocators across membranes.

What must happen to proteins before translocation across membranes?

They must unfold to pass through the translocator.

What is co-translational import?

Protein enters the ER while it is being synthesized.

What is post-translational import?

Protein is fully synthesized in cytosol before transport to organelles.

What forms can sorting signals take?

• Linear amino acid sequence

• 3D signal patch

What is a key property of sorting receptors?

They function catalytically and are reused after delivering cargo.

Why are sorting systems compared to public transportation?

They transport many different proteins to specific destinations.

What does the signal-recognition particle (SRP) do?

Binds the ER signal sequence and directs the ribosome to the ER membrane.

What receptor interacts with SRP at the ER membrane?

SRP receptor.

What happens when the ribosome binds the ER translocator?

Protein synthesis resumes and the polypeptide is threaded into the ER.

What happens to SRP after docking?

SRP and its receptor are released and recycled.

Do free and ER-bound ribosomes differ structurally?

No. They come from the same ribosome pool.

What determines whether a ribosome binds to the ER?

Presence of an ER signal sequence in the protein being synthesized.

What surrounds and protects DNA in eukaryotic cells?

The nuclear envelope.

What connects the outer nuclear membrane to the ER?

It is continuous with the rough ER.

What structures allow transport between nucleus and cytosol?

Nuclear pore complexes (NPCs).

What proteins make up the nuclear pore complex?

Nucleoporins.

What is the approximate size limit for passive diffusion through NPCs?

~60 kDa (≈9 nm).

What type of transport is required for larger molecules across the NPC?

Active transport.

Which molecules are exported from the nucleus?

mRNA, rRNA, and tRNA.

What is a nuclear localization signal (NLS)?

A sequence that directs proteins into the nucleus through NPCs.

What amino acids are commonly found in NLS sequences?

Lysine and arginine (positively charged).

Is the NLS removed after nuclear import?

No. It remains on the protein.

What proteins recognize nuclear localization signals?

Nuclear import receptors.

What happens to the nuclear envelope during mitosis?

It disassembles and later reassembles.

What triggers nuclear envelope breakdown?

Cyclin-dependent kinase phosphorylation of lamins.

What disease is associated with lamin A mutation?

Progeria (premature aging).

Where are most mitochondrial proteins encoded?

Nuclear genome.

How are mitochondrial proteins imported?

Post-translationally as unfolded polypeptides.

What complex transports proteins across the outer mitochondrial membrane?

TOM (Translocase of Outer Membrane).

What complex transports proteins across the inner mitochondrial membrane?

TIM (Translocase of Inner Membrane).

What is TIM23 responsible for?

Import of matrix proteins and some membrane proteins.

What is TIM22 responsible for?

Import of certain inner membrane proteins.

What does SAM do?

Sorting and assembly of outer membrane proteins.

What does OXA do?

Inserts proteins from the matrix into the inner membrane.

What role do Hsp70 chaperones play in mitochondrial import?

Prevent proteins from folding prematurely.

What energy sources drive mitochondrial protein import?

• ATP hydrolysis
• Electrochemical H⁺ gradient

What happens to the mitochondrial signal sequence after import?

It is cleaved by signal peptidase and degraded.

What directs ribosomes to the ER?

Signal-recognition particle (SRP).

What complex allows transport into the nucleus?

Nuclear pore complex (NPC).

What signal directs proteins into the nucleus?

Nuclear localization signal (NLS).

What do sorting receptors do?

Recognize sorting signals and direct proteins to the correct compartment.

Why must proteins unfold during translocation?

They must snake through the membrane translocator.

Which organelles receive proteins by post-translational transport?

• Nucleus

• Mitochondria

• Chloroplasts

• Peroxisomes

• Chloroplast

Which locations receive proteins via co-translational transport?

• ER

• Golgi

• Plasma membrane

• Lysosome

• Extracellular space

What does the Signal Recognition Particle (SRP) do?

Binds the ER signal sequence and directs the ribosome to the ER membrane.

What happens after the ribosome binds the ER translocator?

1. SRP released

2. Protein synthesis resumes

3. Growing protein passes through ER membrane

What is OXA?

Inserts proteins made in the mitochondrial matrix into the inner membrane.

What is required for mitochondrial protein import?

• Cytosolic Hsp70 chaperones

• Matrix Hsp70

• ATP hydrolysis

• Electrochemical H⁺ gradient

What complex transports proteins into mitochondria?

TOM/TIM complexes.

What guides proteins to the ER during translation?

Signal Recognition Particle (SRP).

What structure allows transport between nucleus and cytosol?

Nuclear pore complex.

What keeps mitochondrial proteins unfolded during transport?

Hsp70 chaperones.