AC Lecture 36: Translocation into the nucleus, mitochondria, and peroxisomes

Question 1

What does the mitochondrial genome encode for?

Answer 1

13 proteins and RNA components of mitochondrial ribosomes

Question 2

What are the mitochondrial compartments?

Answer 2

outer membrane- porous to molecules 5-10 kDa; contains a translocation apparatus called the TOM complex for translocation of proteins
inner membrane- 70% protein; impermeable even to protons

Question 3

What is contained in the inner membrane of the mitochondria?

Answer 3

protein complexes of the electron transport chain
the ATP synthase complex that catalyzes formation of ATP from ADP
several channel proteins for translocation of metabolites, ions, and the ATP/ADP transporter
translocation complex (for proteins entering the mitochondrial matrix)
cristae (wavey, ribbon-like membrane) increase surface area for this

Question 4

What is the intermembrane space?

Answer 4

the space between the inner and outer mitochondrial membrane containing enzymes that phosphorylate other nucleotides oother than ADP

Question 5

What is contained in the matrix?

Answer 5

hundreds of enzymes, the mitochondrial genome, ribosomes, tRNAs, and molecular chaperones

Question 6

Describe how genes on the mitochondrial genome are expressed.

Answer 6

1. transcription of mitochondrial genes into mRNA

2. translation in the matrix on mitochondrial ribosomes

Question 7

What proteins are encoded by the mitochondrial genome?

Answer 7

subunits of components of the respiratory chain (including cytochorome c oxidase, NADH dehydrogenase, and apocytochrome b)
22 tRNAs
12S and 16S rRNAs (part of the mitochondrial ribosome)

Question 8

How does the genetic code of the mitochondrial genome differ from that of the nuclear genome?

Answer 8

UGA is a universal STOP but is Tryptophan in the mitochondrial genome
AGG is arginine in nuclear genome but STOP in the mitochondrial

Question 9

What are the types of target sequences for nuclear encoded mitochondrial genes?

Answer 9

15-35 residue N-terminal sequence comprising basic amino acids that is cleaved in the matrix by an endoprotease
non-cleaved internal sequence

Question 10

Describe the process of transport through the outer mitochondrial membrane.

Answer 10

mediated by the TOM complex

1. Hsp70 chaperone keeps precursor protein in unfolded conformation
2. receptor binds to mitochondrial pre-sequence containing proteins at the outer membrane
3. receptor brings pre-sequence containing protein to the translocation channel

Question 11

Describe the process of transport through the inner mitochondrial membrane.

Answer 11

mediated by the TIM complex

1. positively charged pre-sequence helps to open the channel
2. protein is able to pass into the channel
3. Hsc70 guides protein once in matrix and helps Hsc60 to fold protein
4. target sequence is cleaved

Question 12

What is the role of the peroxisome?

Answer 12

fatty acid beta-oxidation (producing hydrogen peroxide)
metabolism of hydrogen perozide (via catalase) to water adn oxygen
plasmalogen synthesis

Question 13

Provide examples of peroxisomal diseases.

Answer 13

Zellwegers syndrome (no import of peroxisomal enzymes)
Adrenoleukodystrophy (oxidation of VLCFA is defective)

Question 14

What activities occur in the nucleus?

Answer 14

transcription
mRNA splicing
DNA replication
ribosome biosynthesis

Question 15

Describe the structure and orientation of the nucleus.

Answer 15

largest organelle
surrounded by a double lipid bilayer
outer layer is contiguous with the rough ER
perinucelar space between the membranes is contiguous with the lumen of the ER

Question 16

Name the kinds of chromatin found in the interphase nucleus.

Answer 16

euchromatin (open for transcription; 10% is transcribed)

heterochromatin (highly condensed and not transcribed)

Question 17

What is the nucleolus?

Answer 17

site of rRNA synthesis, processing, and ribosome assemby

Question 18

Describe nucleocytoplasmic transport.

Answer 18

signal- and receptor-mediated process
Nuclear Localization Sequences (NLS; typically 4-8 amino acids long and rich in Arg and Lys) can be located anywhere on the protein accessible to receptors
Nuclear export signals (NES) also exist

Question 19

Describe the structure of the Nuclear Pore Complex (NPC)?

Answer 19

all nuclear imports and exports occur through this

central aqueous channel with filamentous assemblies radiating out from both cytoplasmic and nucleoplasmic sides

Question 20

What are karyopherins?

Answer 20

specific transporters that recognize the NLS or NES sequences and are required for transport into and out of the nucleus
assembly and disassembly of their complex with cargo regulated by Ran (small GTPase)

Question 21

What is Ran?

Answer 21

a small GTPase with cofactors that regulate nucleotide hydrolysis (Ran GAP found primarily in the cytosol) and nucleotide exchange (Ran GEF found primarily in the nucleus)