Protein Targeting

Question 1

Types of protein targeting

Answer 1

Targeting proteins to ER via signal peptide (eg secreted, membrane, lysosomal proteins)
Targeting cytosolic proteins to organelles via other peptide signals
(eg mitochondrial, nuclear, peroxisomal proteins)

Question 2

Signal sequence

Answer 2

Protein synthesis starts with a cytosolic ribosome
Signal sequence is protein synthesised - N terminus
Signal sequence is recognised by signal recognition particle protein complex
SRP binds the SRP receptor on ER membrane
Ribosome is now bound to the ER membrane
New protein is guided through translocation channel in ER membrane into ER lumen
Signal sequence cleaved via signal peptidase and SRP is recycled

Question 3

Where do membrane proteins stay?

Answer 3

In the RER membrane

Question 4

Where do trans membrane proteins stay?

Answer 4

In the ER membrane

Question 5

ER-Golgi interactions

Answer 5

Protein containing vesicles bud off from ER
Merge with cis Golgi
Protein maturation in Golgi
Movement through cisterna of Golgi via vesicles
in trans Golgi proteins are packaged in vesicles which are transported along cytoskeleton pathways

Question 6

How are proteins targeted to specific compartments?

Answer 6

Trans golgi vesicles are coated with specific v-SNARE proteins
Complementary t-SNARE protein at the destination allows targeting

Question 7

Targeting to lysosomes

Answer 7

Lysosomal proteins are synthesised in the ER and transported to the trans Golgi
Lysosomal proteins are packaged into vesicles that are targeted to endosomes and phagosomes which converts them into lysosomes
Lysosomal proteins are tagged with mannose-6-phosphate in Golgi
Mannose-6-phosphate receptor in Golgi directs proteins into correct transport vesicles

Question 8

Targeting proteins to mitochondria

Answer 8

Targeting occurs after translation but before protein folding
Protein is complexed with chaperone HSP70
Specific sequence recognised by receptor in outer mitochondrial membrane

Question 9

Targeting to nucleus

Answer 9

After translation and folding is completed in cytoplasm
Nuclear proteins contain Nuclear Localisation Signal (NLS)
NLS binds to importin and is transported through the nuclear pore
Requires G protein RAN and hydrolysis of GTP
Exportin performs reverse function to exporting proteins from nucleus

Question 10

Targeting to peroxisomes

Answer 10

Peroxin proteins have 3 amino acid sequence at C terminus
This is recognised by PEX5 and this complex is imported into the peroxisome