Protein Targeting

Question 1

Protein production into endoplasmic reticulum

Answer 1

Signal sequence - e.g. Leu-Leu-Val-Gly-Ile-Phe-Trp-Ala

1. Protein synthesis begins in cytosolic ribosome
2. Signal sequence of amino acids produced is recognised by Signal Recognition Particle (SRP).
3. SRP receptors on ER membrane binds to SRP.
4. New protein is translated through translocation channle into the ER lumen.
5. Signal sequence is cleaved via signal peptidase and the SRP is released.

Question 2

Synthesis of integral membrane proteins

Answer 2

Integral membrane proteins are synthesised in the rough ER, integrated into the membrane and transported by vesicles to the plasma membrane.

1. STOP transfer sequence is translated which anchors polypeptide into membrane by stopping threading of protein through ER membrane.
2. Non-cleavable signal peptide also acts as a STOP transfer signal.
3. This leaves the protein embedded in the membrane.
4. Membrane then buds off as vesicles.

Question 3

How do vesicles find their target membrane?

Answer 3

Vesicle has a v-SNARE which binds to complementary t-SNARE on the target membrane.
SNAREs are long helical proteins that strongly associate to bring two membranes together.

Question 4

Transport of proteins into the mitochondria

Answer 4

Proteins are held in an unfolded shape by Hsp70 chaperones.
Polypeptide-chaperone complex docks with mitochondrial receptor - forms a channel through outer membrane for protein to pass through.
Some proteins pass further through the inner mitochondrial membrane without chaperone proteins associated.

Question 5

Transport of proteins into the nucleus

Answer 5

Basic targetting sequence: Pro-Lys-Lys-Lys-Arg-Lys-Val (Nuclear localisation sequence(NLS))
NLS binds to importin and the protein is transported through the nuclear pore.
Exportin would perform the reverse function to export proteins out of the nucleus.

Question 6

Targeting proteins to a lysosome

Answer 6

Lysosomal proteins are tagged with mannose-6-phosphate in the golgi.
Mannose-6-phosphate receptor in the golgi directs proteins into a vesicle - this vesicle will become a lysosome.

Question 7

Getting proteins to the right place

Answer 7

Cytosol to ER/mitochondria - protein translocators bind to signal recognition particle.
ER to other (lysosomes) - vesicles fuse to target protein.
Cytosol to nucleus - importins bind to nuclear localisation site.