Lecture Set 12

Question 1

Describe how proteins go from synthesis start to ER

Answer 1

synthesis always starts on ribosomes in the cytosol, have a signal sequence at the start. SRP (signal recognition peptide) binds to both signal sequence and large subunit of ribosome, halting translation. SRP binds to SRP receptor on the ER, which causes dissociation, ribosome docks to membrane and opens up pore in translocon, and translation into the ER

Question 2

How are the different types of integral proteins inserted?

Answer 2

type 1: has start-transfer, stop-transfer (will become transmembrane alpha helix) start transfer inserts, allows translocation of protein until stop transfer reached. Start-transfer cleaved off to allow free N end in the exoplasmic (luminal) face
type 2: has an internal start-transfer, no stop-transfer, start-transfer sequence inserts, complete translocation until end so that c-terminal is in the exoplasmic side
type 3: multipass has internal start and stop-transfer sequences
GPI-linked: GPI formed and inserted like type 1, except that protein is transferred to preformed GPI anchor

Question 3

How do folded proteins move into the nucleus?

Answer 3

Importin binds to nuclear localization signal on cargo protein, crosses nuclear membrane. Ran-GTP attaches to Importin, causes conformational change, and releases cargo protein. Ran-GDP and Importin leave nucleus, and dissociate due to GTP hyrolysis. GEF exchanges GDP to GTP
Ran gets back into nucleus through nuclear transport factor 2

Question 4

How do folded proteins leave the nucleus?

Answer 4

Exportin binds to Ran-GTP, then complexes with NLS signal attached to cargo protein. Entire complex leaves nucleus, then GTP hydrolysis occurs, which causes dissociation of all three components. Exportin translocates back into nucleus, Ran-GDP goes back through nuclear transport factor 2

Question 5

What do GAPs and GEFs do?

Answer 5

GAP= GTPase Activating Protein, activates the GTPase activity, so causes hydrolysis of GTP and ultimately ends up inactiving the GTPase
GEF = Guanine nucleotide exchange factor = activates GTPase through exchanging GDP for GTP