Protein sorting

Question 1

where can proteins go once synthesised in the cytosol?

Answer 1

remain in cytosol
nucleus
ER for trafficking
mitochondrion

Question 2

how are proteins labelled as to where they should go?

Answer 2

sorting signals-specific sequences of amino acids
(table on pwp)

Question 3

sorting signals for nuclear targeting

Answer 3

one or more stretched of positively charged amino acids, lysine, arginine
situated on the surface of the protein once it has folded
recognised by nuclear import receptors

Question 4

structure of nuclear pores

Answer 4

each pore is composed of a large number of protein subunits. fibrils protrude from both sides of the complex.

Question 5

nuclear import receptors

Answer 5

recognise sorting sequence
travel down fibres towards nucleus

Question 6

how do macromolecules get into the nucleus

Answer 6

active transport through nuclear pores

Question 7

process of protein being imported into nucleus

Answer 7

nuclear import receptors recognise nuclear localisation sequences on prospective nuclear protein. complex of receptor and protein cargo binds to fibrils on cytoplasmic side and is guided towards nuclear pore. binding of nuclear protein to pore opens the pore and the protein receptor complex is actively transported into the nucleus

Question 8

Ran GTPase

Answer 8

exists in GTP or GDP bound state

Question 9

Ran-GAP, GTPase activating protein

Answer 9

protein triggering GTP hydrolysis
in cytosol
converts Ran-GTP to Ran-GDP

Question 10

Ran-GEF, guanine nucleotide exchange factor

Answer 10

protein causing Ran-GDP to release GDP and take up GTP
in nucleus

Question 11

nuclear import and export

Answer 11

Ran-GEF converts Ran GDP to GTP
Ran-GTP binds to import receptors to cause it to release cargo protein
receptor-Ran-GTP complex transported back to cytoplasm where Ran-GAP hydrolyses Ran-GTP to Ran-GDP, loses affinity for receptor. receptor can now bind more cargo

Question 12

where does the energy come form that is needed to drive nuclear import and export

Answer 12

hydrolysis of GTP

Question 13

mitochondrial targeting sequences

Answer 13

always at N terminus (gets produced first)
amphipathic alpha helical sequences-positively charged residues lie on one face of the secondary structure

Question 14

recognition of mitochondrial targeting sequences

Answer 14

receptors on mitochondrial outer membrane recognise mitochondrial targeting sequences

Question 15

how do proteins cross the two mitochondrial membranes

Answer 15

TOM: translocator of outer membrane, channel
TIM: inner membrane

Question 16

Hsp70

Answer 16

cytoplasmic chaperone protein
prevent protein folding for proteins that are destined for the mitochondria
also prevents the protein going back up channel

Question 17

how do unfolded proteins get through the TOM and TIM

Answer 17

positive residues are attracted to the negatively charged inner membrane
energy from hydrolysis of ATP causes chaperones to release protein

Question 18

targeting to the ER

Answer 18

hydrophobic signal sequence, often at N terminus
usually have positive charged N-terminus and a cleavage sequence (alanine)

Question 19

what recognises the ER targeting sequence as it emerges from the ribosome

Answer 19

ribonucleoprotein complex (RNA and proteins) known as SRP: signal recognition particle. binding of SRP to signal sequence and ribosome halts translation

Question 20

how ER destined proteins get into the lumen

Answer 20

complex of ribosome, nascent protein chain and SRP binds to SRP-receptor which guides the complex to a translocon. both SRP and SRP receptor hydrolyse their associated GTPs, changing their conformations. SRP is released (loses affinity) and translation resumes. extension of polypeptide chain pushes it through into the ER lumen. the protein is unfolded as this occurs

Question 21

signal sequence of ER proteins once in lumen

Answer 21

signal peptidase in ER lumen cleaves the signal sequence
creates new N terminus
protein can now fold properly

Question 22

transmembrane proteins

Answer 22

signal sequence not cleaved
signal-anchor sequences
longer hydrophobic stretches of amino acids and no signal peptidase cleavage sequence