Translation begins when a specific protein first binds the mRNA to start assembling the initiation complex. Which protein is this, and what happens if it becomes phosphorylated?
AeIF-2 binds first, and phosphorylation makes it bind GTP more strongly, speeding up initiation
BThe 60S ribosomal subunit binds first, and phosphorylation has no effect on it
CeIF-2 binds first, and phosphorylation prevents it from binding GTP, impeding the initiation complex
DDicer binds first, and phosphorylation converts it into a transcription factor
Answer & Solution
Correct answer: C. eIF-2 binds first, and phosphorylation prevents it from binding GTP, impeding the initiation complex
1. The first protein to bind the RNA to initiate translation is the eukaryotic initiation factor-2, or eIF-2.
2. eIF-2 becomes active by binding the energy molecule GTP, which it then converts to GDP, releasing energy.
3. The eIF-2 and GTP complex binds the small 40S ribosomal subunit, bringing along the initiator tRNA and mRNA.
4. If eIF-2 is phosphorylated, it undergoes a conformational change and can no longer bind GTP.
5. Because the initiation complex then cannot form properly, translation is impeded when eIF-2 is phosphorylated.
6. The option claiming phosphorylation strengthens GTP binding and speeds initiation is the opposite of what phosphorylated eIF-2 does.
7. The 60S subunit binds later in the process, after eIF-2 has already engaged the RNA, so it is not the first protein to bind.
8. Dicer is instead the enzyme that processes pre-miRNAs into mature miRNAs, an unrelated role from initiating translation.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 16 "Gene Expression", section The Initiation Complex and Translation Rate_
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