Lecture 12: intracellular compartments, protein sorting 2

Question 1

What is translocation?

Answer 1

• protein movement across membranes

Question 2

How do proteins get into the mitochondria?

Answer 2

• synthesized on ribosomes
• signal sequence helps direct across the membrane into the mitochondria

Question 3

What are mitochondrial locating specific signals that are associated with proteins for translocation?

Answer 3

• amphiphilic alpha helix, that are at N terminal or in the polypeptide somewhere

Question 4

How are the mitochondrial sequences recognized?

Answer 4

receptor proteins recognize the configuration and not the sequence

Question 5

Why are a majority of the mitochondrial signal sequences positively charged?

Answer 5

the cytosol in the inner membrane is negative, therefore a positive charge will be attracted to the inside

Question 6

What functions can TOM perform?

Answer 6

1. import nuclear proteins into the mitochondria
2. insert the proteins in the outer membrane

Question 7

What functions can TIM perform?

Answer 7

1. allows passage of proteins through the inner membrane. Forms “continuous” tunnel with TOM

Question 8

What is the role of TIM23?

Answer 8

• transport soluble proteins into matrix and inserts membrane proteins in the INNER membrane

Question 9

What is teh role of TIM 22?

Answer 9

• mediates the insertion of specific subclasses of proteins (ATP, ADP)

Question 10

What are unique regions on the TOM/TIM complex?

Answer 10

1. receptor for mitochondrial precursor
2. translocation channel

Question 11

What is the OXA complex responsible for?

Answer 11

mediating insertion of proteins that were synthesized in the mitochondria, into the inner membrane

Question 12

How are proteins entered into the mitochondria?

Answer 12

1. protein is unfolded and bound with Hsp70 (chaperones)
2. Binds with TOM and chaperones are removed as protein is threaded in
3. once in the matrix, the signal sequence is cleaved by peptidase

Question 13

Which complexes require ATP and which rely on membrane potential for protein translocation into the mitochondria?

Answer 13

• ATP required for TOM
• Membrane potential for TIM

Question 14

How do proteins get placed in the outer membrane?

Answer 14

1. pass through TOM complex, and bind with chaperones in the intermembrane space
2. SAM complex binds and folds the protein into the outer membrane
3. porins are good examples

Question 15

What is the role of Mia 40?

Answer 15

• provides protein translocation into the intermembrane space
• uses sulfide bridges which become reduced releasing the protein into the membrane space

Question 16

What guides the ER signal sequence to the ER membrane?

Answer 16

• signal recognition particle (SRP)
• SRP receptor

Question 17

What is the chemical structure of the SRP

Answer 17

rod-shaped structure with hydrophobic pocket lined by methionines, which can accomodate different sequences and sizes of signal

Question 18

How does the SRP help move the protein across the ER membrane?

Answer 18

1. SRP binds with the ribosomal subunit, at the emerging sequence site and elongation factor binding site
2. stops transcription
3. SRP binds with SRP receptor and release the ribosome onto the protein translocator
4. SRP/SRPreceptor unbind from the ribosome/protein translocator units.
5. SRP is recycled. translocation occurs as gene is translated

Question 19

What is the structure of the translocator that is present on the ER membrane?

Answer 19

• Sec61 core complex that is a water filled pore
• consists of 3 subunits
• Signal peptide causes conformational changes in the gate allowing the pore to become open

Question 20

If a protein has multiple transmembrane domains, what must have been present in the sequence to produce this effect?

Answer 20

• multiple hydrophobic regions that stopped translocation
• called the stop transfer signal