12.3 Cell Nucleus and Gene Expression

Question 1

Diseases Associated with ESS Function

Answer 1

• Frontotemporal dementia with parkinsonism
• Spinal Muscular Dystrophy
• Becker Muscular Dystrophy

Question 2

Tau Isoforms

Answer 2

-protein called Tau (microtubule associating proteins.) multiple exon within, 2, 3, 10 are alternatively spliced
-Three main tau transcripts
2 kbp- expressed in nucleus
6 kbp- expressed in neurons
9 kbp- expressed in retina and peripheral nervous system

-In adult human brain exons 2,3, and 10 are alternatively spliced
6 isoforms of tau protein
Inclusion of exon 10 produces 4R-Tau isoform
Exclusion of exon 10 produces 3R-Tau isoform
Ratio of 3R-Tau to 4R-Tau is crucial for microtubule assembly
1:1 ratio required

Question 3

Translational Control

Answer 3

mRNA localization (fly embryos)
mRNA Stability
mRNA degradation

Question 4

Posttranslational Control

Answer 4

Protein degradation

Proteasome structure/function

Question 5

Cytoplasmic Localization of mRNA

Answer 5

• 3’ UTR sequence determines location of mRNA
• First is the establishment of the axis (post-ant axis) so needs certain proteins. Bicoid and Oskar
• Bicoid is anterior, Oskar is post
• their expressed because the mana is trascported to that area of the embryo.
• RNA binding protein recognize specific sequence then those proteins are moved to another end of cell. (cytoplasmic localization) grabbed by dinine or kinase depending on what direction.

Question 6

Translational Activation of mRNAs in Sea Urchin Eggs

Answer 6

mRNAs maintained in cytoplasm in an inactive state by maskin protein and their short poly A tail. fertilization, CPEB is phosphorylated which displaces maskin. The
phosphorylated maskin recruits CPSF which recruits poly(A) polymerase that elongates the poly(A) tail. The elongated poly(A) tail serves as a binding site for PABP which recruit eIF4G, an initiator factor required for
translation

Question 7

Gene Regulation and Environment

Answer 7

Changing cellular environment

• Stress (heat etc.) activates protein kinase
• Phosphorylates eIF2 (blocks further translation)

Specific kinases for different stressors

• Heat shock
• Viral Infection
• Unfolded proteins
• Amino acid starvation

Question 8

Control of Ferritin mRNA Translation

Answer 8

• Ferritin protein binds and blocks toxic effect of iron
• Expression increases in response to high iron levels
• when iron low, iron binding repressor protein IRP binds to specific sequence in 5’ UTR of ferritin mRNA called IRE which is in hairpin loop
• when iron available, it binds to IRP changes conformation and dissociate from IRE allowing translation of the mRNA to form ferritin

Question 9

mRNA Stability

Answer 9

•the longevity of the mRNA is related to the length of the poly A tail

1) mRNA leaves nuc about 200 adenosine reside tail
2) if remain in cytoplasms the tail nibbles away by deadenylase. No effect on stability until ~30 residues
- degraded by 1 of 2 pathways
3) depredation begins at 5’ after removal of 3’ tail (3’ tail protects 5’ cap)
4) Decapping enzyme removes methylguanosine
5) 5’ –> 3’ exonuclease digests mRNA.

OR

3a) removal of play A tail
4b) mRNA degraded exclusively from 3’ end by exosome

•3’ UTR has specific sequences that bind proteins that protect against exonucleases.

Question 10

Where does mRNA degradation occur

Answer 10

mRNA degradation occurs in cytoplasmic granules called P-bodies

• destroy unwanted mRNA
• Temporary storage for mRNAs no longer being translated

Question 11

Role of miRNAs in Translation Control

Answer 11

•miRNA bind to 3’UTR to inhibit translation
•Early embryonic development requires miRNA translation control
•Experiments conducted in which specific miRNAs are deleted
Example:
•Deletion of miR-1 class of miRNA results in defects in heart development
•May be due to overexpression of specific mRNAs

Question 12

descipbe 4 ways that miRNAs are involved with the lblocking of translation. Which structure in the cell does this occur?

Answer 12

Occurs in P Bodies

1) Deadenylation (followed by recapping and degradation)
2) Proteolysis (degradation of nascent peptide)
3) Initiation Block (repressed cap recognition or 60s joining)
4) Elongation Block (slow elongation or ribosome ‘drop -off’

Question 13

Posttranslational Control

Answer 13

•Proteasomes digest “abnormal” proteins
-Misfolded, incorrectly associated
-4 rings with å&ß subunits
•Proteins marked by ubiquitin protein
-Directs protein to proteasome

Question 14

Proteasome Structure/Function

Answer 14

1) protein linked to ubiquitin molecule
2) polyubiquitinated target protein bind to cap of proteasome. Ubiquitin chain removesd
3) unfolded protein threaded into central chamber of proteosome
4) protein digested by catalytic activity of B-Subunits into small peptides
5) peptides released into cytosol where degraded into their component a.a