Lecture 14 - Mitochondria and ER - Intracellular Compartments and Protein Sorting

Question 1

Transport of proteins from the cytoplasm across a membrane is called __________ and protein __________ are responsible for the transport.

Answer 1

Translocation; translocators

Question 2

What signal sequence would target a protein for transport into the mitochondria?

Answer 2

• N terminal

- positively charged residues every 4th residue with hydrophobic in between forming an amphiphilic alpha helix

Question 3

What translocation proteins are found in the mitochondrial membrane?

Answer 3

Outer:

• TOM (Translocation of Outer Membrane)
• SAM (Sorting and Assembly Machinery) complex

Inner:

• TIM (Translocase of Inner Membrane) 22
• TIM 23
• OXA complex

Question 4

How are proteins imported into the mitochondrial matrix?

Answer 4

• protein is held unfolded by chaperones (Hsp70)
• signal sequence binds receptor on TOM complex
• protein passes through TOM complex into inter-membrane space, chaperones are stripped off
• protein binds TIM 23
• protein in passed through inner membrane
• signal sequence is cleaved

Question 5

How are proteins imported into the outer membrane of the mitochondria?

Answer 5

• protein is held unfolded by chaperones (Hsp70)
• signal sequence binds receptor on TOM complex
• protein passes through TOM complex into intramembrane space, chaperones are stripped off
• Intramembrane chaperones bind
• protein bind SAM complex
• protein is folded and inserted into outer membrane

Question 6

How are proteins imported into the inner membrane of the mitochondria?

Answer 6

Nuclear encoded:

• protein is held unfolded by chaperones (Hsp70)
• signal sequence binds receptor on TOM complex
• protein passes through TOM complex into inter-membrane space, chaperones are stripped off
• protein binds TIM 23
• stop transfer sequence causes TIM 23 to release protein while partially inserted
• signal sequence cleaved

Mitochondrial

• protein binds OXA complex
• stop transfer sequence causes OXA complex to release protein while partially inserted

Question 7

How are proteins imported into the inter-membrane space of the mitochondria?

Answer 7

-protein is inserted into inner membrane and then cleaved from transmembrane domain by protease

Question 8

What are the two major methods by which proteins are inserted into the ER membrane or lumen?

Answer 8

• co-translational translocation

- post-translational translocation

Question 9

What signal sequence would result in import into the ER and what recognizes this sequence?

Answer 9

• sequence of 8 or more non-polar AA’s

- signal recognition particle (SRP) which contains a hydrophobic pocket that recognizes the sequence

Question 10

Describe the translocation protein found in the ER.

Answer 10

• 3 subunits
• water filled pore
• short helix which acts as a gate for pore

Question 11

Explain how transmembrane domains are inserted into the ER membrane.

Answer 11

• start-transfer sequence binds translocator
• protein passes through translocator until a stop-transfer sequence is reached and protein is ejected
• another start-transfer sequence can start the process over again