7.1-7.5 updated

Question 1

what do cells depend on to ensure proteins arrive at proper sub cellular destination

Answer 1

signals on proteins

Question 2

proteins with no targeting signal will be tsl entirely where?

Answer 2

on free ribosomes and will remain in the cytosol

Question 3

what organelles receive proteins translated entirely in the cytosol on free ribosomes?

Answer 3

mitochondria, chloroplasts, nucleus, peroxisomes

Question 4

what organelles are part of the secretory pathway?

Answer 4

ER, Golgi, lysosomes, endosomes, plasma membrane

Question 5

how are proteins transferred in the secretory pathway

Answer 5

budding and fusing

Question 6

steps of protein targeting and entering the secretory pathway

Answer 6

• signal sequence binds to a signal recognition particle which causes tsl to temporarily halt
• ribosomes, nascent polypeptide, and SRP arrive at the ER, interact with a receptor
• channel opens, nascent protein beings to be co-translationally translocated

Question 7

who created the signal hypothesis

Answer 7

Gunter Blobel (1970s)

Question 8

what does the signal hypothesis explain

Answer 8

how proteins got targeted to the ER

Question 9

when is the ER-targeting signal usually cut off

Answer 9

inside the RER after the protein begins its translocation

Question 10

term for a nascent protein prior to its signal sequence removal

Answer 10

pre-protein

Question 11

experimental evidence of signal hypothesis

Answer 11

• in vitro w/o microsomes: larger protein, migrates slowly
• in vitro w/ microsomes; protein smaller and migrated faster, matched protein located in ER lumen

Question 12

SRP

Answer 12

ribonucleoprotein particle
- made of 6 polypeptides and a small 7S RNA molecule

Question 13

where is SRP found

Answer 13

cytosol

Question 14

part of SRP has ________ activity

Answer 14

GTPase - it’s a G protein

Question 15

part of SRP that binds to the nascent polypeptide’s signal sequence

Answer 15

large number of Met residues and is hydrophobic

Question 16

true or false: proteins in the secretory pathway are always synthesize on bound ribosomes

Answer 16

false: can start on free ribosome then be co-transitionally translocated or can have posttranlational translocation

Question 17

what happens when part of SRP binds to the nascent protein

Answer 17

tsl halts until docking at ER

Question 18

what is SRP receptor composed of?

Answer 18

dimer of two subunits: SRa and SRB

Question 19

true or false: each subunit of the SRP receptor is considered a monomeric GTPase

Answer 19

true

Question 20

SRa

Answer 20

on c-face, interacts with SRP

Question 21

SRB

Answer 21

transmembrane

Question 22

SRa and SRB are also _______________

Answer 22

GTP-binding proteins (GTPases)

Question 23

the GTP is required for what step of SRP/receptor interactions?

Answer 23

releasing

Question 24

what happens when SRP releases the ribosome

Answer 24

ribosomes engages translocon and nascent polypeptide begins translocation

Question 25

2 big categories of G proteins

Answer 25

1. heterotrimeric (GPCR binding, aBy) 2. monomeric (SRP, SRa, SRB)

Question 26

characteristic cs of G proteins

Answer 26

- all GTPases - all GTP-binding proteins - all able to perform their unique function only when GTP-bound