Unit 6

Question 1

Hsc70

Answer 1

• Keep mitochondrial import precursor proteins in an unfolded state
• Cytosolic chaperone
• Requires energy in the form of ATP
• Binds to TIM44
• ATP hydrolysis by this complex helps power translocation of the protein in the matrix

Question 2

TOM20 / TOM22

Answer 2

• Receptor in outer mitochondrial membrane
• Interacts with matrix-targeting sequence in protein
• Transfers protein to general import pore of outer membrane

Question 3

TOM40

Answer 3

• Composes general import pore of outer mitochondrial membrane

Question 4

TIM23 & TIM17

Answer 4

• Compose import pore of inner membrane
• Proteins pass through this import pore at contact sites between inner & outer membrane

Question 5

TIM44

Answer 5

• Binds to Hsc70
• ATP hydrolysis by this complex helps power translocation of the protein in the matrix

Question 6

Membrane-bound ribosomes

Answer 6

• Attached to cytosolic surface of ER membrane & are synthesizing proteins that are translocated into the ER

Question 7

Free ribosomes

Answer 7

• Unattached to any membrane & are synthesizing all of the other proteins

Question 8

SRP

Answer 8

• Signal recognition particle
• Cytosolic side of ER cytoplasm
• Composed of 6 proteins & an associated RNA molecule
• Engages emerging polypeptide that has its ER signal sequence exposed
• Binding halts translocation & delivers ribosome / polypeptide to ER

Question 9

SRP receptor

Answer 9

• ER membrane (Cytosolic surface)
• Ribosome / polypeptide delivered here by SRP
• This interaction is enhanced by binding of GTP to both SRP & its receptor

Question 10

Translocon

Answer 10

• ER membrane
• Ribosome / polypeptide delivered here during ER import
• This induces it to open & receive a polypeptide (enters as a loop)
• Hydrolysis of GTP by SRP & its receptor free these for another round of import
• Translocation resumes

Question 11

Signal peptidase

Answer 11

• ER membrane
• Protease
• Cleaves signal sequence of protein allowing rest of protein to synthesize into ER lumen

Question 12

Type I membrane-anchored protein

Answer 12

• Single-pass
• Cleaved signal sequence at N-terminus
• Uses SRP-SRP receptor to get to ER membrane
• N(out)-C(in)

Question 13

Type II membrane-anchored protein

Answer 13

• Single-pass
• No cleavable signal sequence
• Uses SRP-SRP receptor to get to ER membrane
• N(out)-C(in)

Question 14

Type III membrane-anchored protein

Answer 14

• Single-pass
• No cleavable signal sequence
• Uses SRP-SRP receptor to get to ER membrane
  N(in)-C(out)

Question 15

Tail-anchored membrane-anchored protein

Answer 15

• Signal-pass
• No cleavable signal sequence
• Hydrophobic membrane-spanning sequence at C-terminus
• Does not use SRP-SRP receptor, but the GET 1/2/3 system to get to ER
• Post translational insertion
• N(in)-C(out)

Question 16

Type IV membrane-anchored protein

Answer 16

• Multispanning
• No cleavable signal sequence
• Uses SRP-SRP receptor for insertion of the first membrane-spanning domain but not subsequent ones
• IV-A are N(in)-C(in)
• IV-B are N(out)-C(in)

Question 17

GPI-anchored membrane-anchored protein

Answer 17

• Entire protein is lumenal (out)
• Cleaved signal sequence at N-terminus
• Uses SRP-SRP receptor to get to ER membrane
• Anchored at C-terminus to membrane & then transferred to GPI anchored

Question 18

Get3

Answer 18

• Binds to hydrophobic tail in tail-anchored membrane-anchored proteins when in its ATP-bound state
• Hydrolyses ATP & releases the polypeptide once bound to Get 1/2 complex, embedding its hydrophobic stretch into the ER membrane
• Binds ATP & is released from membrane, ready for another round of activity

Question 19

Get 4/5 & Sgt2

Answer 19

• Bind hydrophobic tail at tail-anchored membrane-anchored protein before it binds to Get3

Question 20

Get 1/2 complex

Answer 20

• Binds to Get3 complex of ER membrane

Question 21

GPI-anchored proteins

Answer 21

• Insert into ER like type I membrane-anchored proteins using a STA sequence

Question 22

Transamidase

Answer 22

• Cleaves protein within lumen of ER & transfers it to assembled GPI anchor

Question 23

Clathrin

Answer 23

• Mediates transport vesicle formation at the trans-golgi (for transport to lysosomes via endosomes) & at the plasma membrane (for transport to endosomes)

Question 24

COP I

Answer 24

• Mediates transport from cis-golgi to ER & between various Golgi cisternae

Question 25

COP II

Answer 25

- Mediates transport from ER to cis-Golgi

Question 26

Sar1

Answer 26

- Rab protein - Activated by its GEF - Inserts into membrane & begins to curve membrane - Activated Sar1 recruits inner coat portion of COP II coat made up of proteins Sec23 & Sec24

Question 27

Sec23 & Sec24

Answer 27

- Further bends ER membrane - Sec24 acts as cargo receptor for membrane proteins - Recruit outer layer of COP II coat made up of proteins Sec13 & Sec31

Question 28

Arf1

Answer 28

- Rab protein - Activates, & along with a COP I complex forms COP I coat at Golgi

Question 29

Multiprotein tethering complexes

Answer 29

- Composed of up to 10 proteins - Localize to distinct organelles

Question 30

Coiled-coil proteins

Answer 30

- Long α-helical proteins that project great distances from target membrane

Question 31

SNARE proteins

Answer 31

- Mediate Docking - v-SNAREs on vesicle - t-SNAREs on target membrane - Over 35 SNARE proteins - Have SNARE motif, allowing it to interact with another SNARE protein

Question 32

trans-SNARE-complex

Answer 32

- When vesicle is docked, SNAREs associate as a bundle of α-helices called 4-helix bundle - 3 helices contributed by t-SNARE proteins, & 1 contributed by v-SNARE protein - On 2 distinct membranes - Energy released after SNARE pairing sufficient to bring membrane & vesicle close together & disperse the water around polar heads

Question 33

NSF

Answer 33

- Hexameric ATPase - Attaches to cis-SNARE complexes using accesory proteins (SNAP proteins) - ATP hydrolysis breaks apart stable cis-SNARE complexes & allows SNARES to be reused in another round of fusion

Question 34

Dynamin

Answer 34

- Small GTP-binding protein - Aids in breaking clathrin coated vesicle free from plasma membrane - Using GTP-hydrolysis, it breaks vesicle free from plasma membrane