Compartmentalization

Question 1

How are proteins targeted to the ER

Answer 1

Signal sequence at N terminus: charged, then hydrophobic, then polar

SRP receptor in ER recognizes the hydrophobic part

These proteins are usually secreted or transmembrane—go to ER for N linked glycosylation, go to Golgi for O linked glycosylation

Question 2

How is glycosylation used to monitor protein folding

Answer 2

On an N linked oligosaccharide…

Glc3Man9GlcNAc2 sequence transfered to protein

Cleave 2 glucoses

Calnexin and Calreticulin can then bind and promote folding

If they fail, Mannose is cleaved and ERAD occurs in cytoplasm

Question 3

How are proteins targeted to the lysosome?

Answer 3

Glycosylation with exposed mannose sugars occurs

Mannose is phosphorylated to mannose-6-P

Matches with transmembrane M6P receptors

Transported via vesicle to lysosome

Question 4

How are proteins translocated to mitochondria

Answer 4

Signal peptide: amphipathic alpha helix at N terminus with basic residues

Has to go through translocases in inner and outer mitochondrial membranes—TOM and TIM

Question 5

How are proteins translocated to the nucleus?

Answer 5

Nuclear localization signal on protein is a stretch of basic amino acids

Protein binds to nuclear import receptor

Receptor complex binds to nucleoporins and translocated through nuclear pore

Protein is discharged

Reverse process occurs with Nuclear Export Sequences

Question 6

What is the role of Ran-GTPase in nuclear transport?

Answer 6

Ran GDP in cytoplasm(w/ Ran GAP) Ran GTP in nucleus(w/ Ran GEF)

Ran GTP binds to import receptors and caused cargo to dissociate; Ran GTP and importind are then taken to cytoplasm

Ran GTP binds to export receptor and cargo, takes then to cytoplasm, and switched GTP for GDP