Selenium

Question 1

Selenocysteine (sec) (6)

Answer 1

Does not occur as a free amino acid
Synthesised on tRNAsec
tRNA is required to deliver AA to ribosomes
Which is initially aminoacylated with serine
Serine tRNAsec is converted to sec tRNAsec
By selenophosphate synthetases 1 (SPS1)

Question 2

SPS2 (6)

Answer 2

For sec synthesis under stimulated conditions
SPS2 contains sec
Generates selenophosphate at a higher rate than SPS1
It binds to the SECIS
Serine on tRNAsec converted to sec using selenophosphate
Demonstrating positive feedback regulation

Question 3

UGA codon (6)

Answer 3

Sec is encoded by UGA codon in selenoprotein mRNA
The decoding of UGA is required
To use UGA for cotranslational insertion of sec
Because UGA is normally a stop codon
That prevents translation
This is dependent on cis-acting sequences in the selenoprotein mRNA

Question 4

Recoding using SECIS (5)

Answer 4

The stem loop SECIS signal must be present within mRNA

To allow the stop codon to be used for sec insertion
Instead of preventing translation

SECIS is located in the 3’UTR

An assembly of complexes are required to allow this to occur

Question 5

SECIS binding protein 2 (SBP2)

Answer 5

Requires for the SECIS binding activity of SBP

Question 6

Ribosome protein L30 (2)

Answer 6

Interacts with SBP2

To bring to SECIS into contact with the ribosome

Question 7

Selenocysteine-specialised elongation factor (EFsec) (4)

Answer 7

Interacts with SBP2 and tRNAsec
Allows the ribosome to move along the mRNA
And insert sec into the peptide

Question 8

Selenium deficiency (3)

Answer 8

In a selenium deficient state
The rate of disappearance of selenoproteins is not equivalent
And varies with tissue types

Question 9

Disappearance rate in liver (2)

Answer 9

GPX1: 99% reduction
GPX4: 75% reduction

Question 10

Disappearance rate in thyroid (2)

Answer 10

GPX1: 50% reduction
GPX4: no change

Question 11

mRNA stability regulation (3)

Answer 11

The use of actinomysin D to stop transcription
Decrease in GPX1 due to decreased mRNA stability
GPX4 mRNA stability not affected

Question 12

Mechanisms of decay (7)

Answer 12

The use of puromysin to inhibit translation
Decay rate similar in presence of puromysin for both GPXs
Therefore difference in mRNA stability must be translation dependent
GPX1 instability triggered by absence of sec tRNA
Ribosome stalls due to lack of sec tRNA
Causes the mRNA of GPX1 to be more unstable

Question 13

Nonsense mediated decay mechanisms (6)

Answer 13

GPX1 mRNA degraded through nonsense mediated decay
Detects premature termination codon
Recognition strongest when nonsense codon is more than 55 nucleotides downstream to last intron
This exon boundary is labelled by a exon junction complex
GPX1 is 105 nt downstream
So it is susceptible to degradation

Question 14

Translation efficiency (4)

Answer 14

Translation efficiency also affects the level of selenoproteins
Per unit RNA GPX4 translated more efficiently than GPX1
However change in GPX1 translation efficiency is greater
Results in more GPX1 than GPX4 in selenium adequacy

Question 15

Selenium intro (4)

Answer 15

Trace mineral essential for growth and fertility
Selenoprotein synthesis resisted active incorporation
Of the selenocysteine
GPXs are examples of proteins that contain selenocysteine