Lecture 9 - Mitochondria 3

Question 1

What are the components of the SAM complex?

Answer 1

Sam50 and Mdm10 are beta barrel proteins.

Sam 35 and 37 are receptors

Question 2

What is the function of the SAM complex?

Answer 2

SAM complex is a machinery dedicated to synthesizing beta barrel proteins and is found in outer membrane of the mtichondria

Question 3

What is the equivalent of SAM50 in bacteria?

Answer 3

OMP85

Question 4

How important is the beta barrel pathway for living organisms?

Answer 4

the pathway through which beta barrel proteins enter outer mitochiondrial membrane has been conserved. Beta barrel proteins enter through TOM complexes and then are translocated via SAM complex into the outer membrane

Question 5

How are beta barrel proteins incorporated into the outer membrane of mitochondria?

Answer 5

Beta barrel proteins form intermediate structures before being incorporated into outer membrane. These structures. beta barrel proteins interact with chaperones which carry them (they are hydrophobic which necessitates the use of these chaperones) to the SAM complex before being incorporated into the complexes. THe whole complex is often formed by various methods of transport.

Question 6

What are the compenents of the beta-signal?

Answer 6

xxx Po x G xx Hy x Hy x

Any variation in these amino acid would prevent interaction of beta barrel proteins with the SAM complex.

Question 7

Where do outer membrane protiens interact with the SAM complex?

Answer 7

Interaction site is at the C-terminus.

Question 8

Are all outer membrane proteins beta barrel proteins?

Answer 8

No, many are made up of alpha helical transmembrane domains.

Question 9

What are the types of protein present in the outer membrane of the mitochondria?

Answer 9

N anchored

C anchored

Some proteins have an anchor at one terminus that extends so much that the middle of the protein appears to be in the membrane. (example of this is Tom22)

Some proteins penetrate the membrane at multiple locations. (eg Fzo1)

Question 10

Name some C-tail anchored proteins in outer emmberane of mitochondria:

Answer 10

Tom 5,6,7

Bak

Bcl-2

Fis - 1

Question 11

What is the typical structure of C-tail anchored proteins?

Answer 11

Cytosolic domain

TMD (anchor region)

C-tail

Question 12

Where does the specificity of C-tail anchored proteins come from?

Answer 12

TMD

C-tail

Question 13

How is the target of a C-tail anchored protein determined?

Answer 13

hydrophobicity of anchor

net charge of C-tail

length of the anchor

Question 14

What are the proteins responsible for insertion of protiens into the ER membrane?

Answer 14

GET proteins are required for tail anchored protein import into the ER.

Question 15

What are the steps of the GET pathway?

Answer 15

1) GET3 receives tail anchored protein.
2) GET3 enters close conformation and is targetted to the ER membrane where it is received by GET1 and GET2 and inserted into the membrane.
3) GET3 re-enters open conformatino and can receive more tail anchored proteins

Question 16

What is a significant difference between ER and mitochondrial transmembrane proteins? Why are these difference significant?

Answer 16

ER transmembrane proteins have longer transmembrane domains than mitochondrial ones.

Positively charged C-tails in mitochondrial tail anchored proteins whereas negatively charged in ER tail-anchored proteins.

THis is significant because GET3 has a long-positively charged groove. This means that Mitochondrial transmembrane protein tails get repulsed from the groove and are also too short to fit into the groove.

Question 17

How are tail anchored proteins trafficked to the peroxisomes?

Answer 17

Tail anchored proteins that go into peroxisomes first enter the ER membrane then peroxisomes.

Question 18

How are tail anchored proteins targetted to the mitochondria?

Answer 18

When GET pathway proteins are mutated then the tail anchored proteins go by default into the mitochondria’s outer membrane. As a result the Msp1 degrades them and so only functional mitochondrial tail anchored proteins end up in the mitochondrial membrane.

Question 19

What is Msp1?

Answer 19

A mitochondrial outer membrane AAA-ATPase protein that breaks down mistargetted C-tail anchored proteins.

Question 20

What does the ATPase domain of Msp1 look like?

Answer 20

Single ATPase domain composed of canonical walker A and B motif

Question 21

What is the function of Mim1?

Answer 21

Mim1 acts as an insertase for N-terminal anchor proteins.

Question 22

What proteins use Mim1 for entry into the outer membrane?

Answer 22

N-tail anchored proteins such as:

Tom70

Tom20

OM45

Multipass membrane proteins with the N terminus outside and C-terminus in the intermembrane space (eg Ugo1)

Question 23

What proteins use the SAM complex for entry into outer membrane?

Answer 23

Beta barrel proteins

C-tail anchor proteins with long intermembrane space of mitochondria.