Part 4: Diseases associated with mis-regulation of translation

Question 1

Hypothyroidism is caused by:

Answer 1

selenocysteine deficiency

Question 2

Deiodinases are required for:

Answer 2

• for thyroid hormone synthesis, thus among other things, selenium is essential for proper thyroid function.

Question 3

What protein is required for the recoding of the UGA stop codon to allow Sec-tRNA^Sec binding?

Answer 3

SBP2 protein

Question 4

Sec-tRNA^Sec

Answer 4

• a specialized tRNA that recognizes UGA codons and carries the selenocysteine amino acid.

Question 5

SBP2 mutations in humans can cause:

Answer 5

• some rare forms of hypothyroidism
• reduced Sec incorporation leads to reduced production of the deiodinases

Question 6

SECIS element:

Answer 6

• an RNA sequence in the 3’ UTR required for Sec incorporation
• tells the Sec-tRNA^Sec that a UGA stop codon is in the mRNA strand coding region, but needs to be a selenocysteine
  
  • enables delivery of Sec-tRNA^Sec to the UGA codon when it enters the ribosome

Question 7

What is ribosomal frameshifting?

Answer 7

• creating more than one protein from a single mRNA

Question 8

Process of ribosomal frameshifting:

Answer 8

1. overlapping coding sequences on a single mRNA in the presence of a slippery site (repeat of same base) at the site of overlap
2. a pseudoknot immediately follows the slippery site after a spacer
3. pseudoknot causes a pause of the ribosome on the slippery site, which cause reading frame shift and production of a different protein by changing the reading frame (adds or deletes a base)

Question 9

Structure of an mRNA that can undergo ribosomal frameshifting:

Answer 9

Question 10

How often does ribosomal frameshifting occur in HIV-1 (contains mRNA capable of ribosomal frameshifting)?

Answer 10

• 10% of the time
  
  • means there is a ratio of 90/10 in proteins translated from this single mRNA

Question 11

Fframeshift signals include:

Answer 11

1. RNA pseudoknot
2. RNA sequence called the “slippery site.”

Question 12

The efficiency of frameshifting determines:

Answer 12

• the amount/ratio of each protein that is made from the single mRNA undergoing ribosomal frameshifting

Question 13

What is an internal ribosome entry site (IRES)?

Answer 13

• an RNA structure in viral mRNAs that promotes translation initiation even when host initiation factors have been damaged.
  
  • enteroviruses (i.e. polio) contain these
  • binds directly to cleaved eIF4G, prompting formation of the PIC without having a 5’ cap

Question 14

How do enteroviruses with internal ribosome entry sites (IRESs) function?

Answer 14

1. cleaves eIF4G to slow cellular translation
2. cleaved fragment of eIF4G binds to IRES sequence on viral mRNA
   
   • leads to direct recruitment of 40S ribosomal subunit without the need for a 5’ cap
3. pre-initiation complex assembled on the viral mRNA
   
   • virus translation begins (HIJACKS MACHINERY)

Question 15

Poliovirus shuts down host translation by expressing a protease that cleaves:

Answer 15

• eIF4G (binds to eIF4E on 5’ cap of mRNA)

Question 16

The poliovirus IRES binds to:

Answer 16

• cleaved version of eIF4G and recruits the pre-initiation complex directly, without the need for a cap.

Question 17

What system permits efficient viral mRNA translation at the expense of host mRNA translation?

Answer 17

• internal ribosome entry site (IRES)
• used by enteroviruses

Question 18

Diptheria toxin targets translation by:

Answer 18

• modifying the elongation factor responsible for translocation, eEF2

Question 19

Steps in how diphtheria toxin inhibits translation/protein synthesis:

Answer 19

Toxin has two initially bound components (A and B).

1. B hits cell membrane receptor, inducing endocytosis.
2. A and B dissociate inside the cell.
3. A (active form of toxin) translocates to cytosol and catalyzes a reaction that inhibits EF-2.
   
   • protein synthesis halts