Eukaryotic Protein Synthesis

Yup, another post about the nitty-gritty details of protein synthesis! Thankfully we don't have to go into eukaryotic protein synthesis into too much depth, because eukaryotes are a lot more complicated than prokaryotes. In fact, the only step we really have to go into in great detail is initiation.

Differences and similarities in prokaryotic and eukaryotic ribosomes

I feel like I've gone through this before, so I'm going to go through it really quickly. Eukaryoticribosomesarelargerastheyare80Sinsteadof70S. Theyalsohave40Sand60Ssubunitsinsteadof30Sand50S. Termsandconditionsapply.

Molecular components required for peptide chain initiation in eukaryotes

Just as in prokaryotes, eukaryotes have five components that they need for chain initiation. In prokaryotes, these factors were tRNA_i^fMet, mRNA, 30S and 50S subunits, initiation factors and GTP. Components required in eukaryotes are almost identical, but with some subtle differences: the initial tRNA is just a tRNA_i^fMet (it's still special because it's got the "i" for initial, but the methionine is normal, not formylated), the subunits are 40S and 60S and the initiation factors are special eukaryotic ones denoted by eIF. There are over 12 eIFs, but thankfully the most important ones that we'll be covering are pretty much homologs of some of the prokaryotic initiation factors (eIF1A is a homolog of IF-1, eIF2 does pretty much the same thing as IF-2 and eIF3 is a homolog of IF-3).

Post-transcriptional modifications of mRNA facilitating protein synthesis

This is something that you should know pretty well by now. After an mRNA is synthesised, it has a 7-methylguanosine cap added to it along with a poly-A tail. Both of these features enhance stability and enable the initiation of protein synthesis. The poly-A tail also helps the mRNA get out of the nucleus, while the cap also helps the ribosome to bind.

Major features of peptide chain initiation in eukaryotes
Role of eIF2 in peptide chain initiation
Differences and similarities in prokaryotic and eukaryotic translation

I'm going to try and explain this in terms of similarities and differences between eukaryotes and prokaryotes, because I'm hoping that that will make it easier to remember. Just like in prokaryotes, initiation starts with eukaryotic initiation factors (eIFs) binding to the small (40S) subunit. In the next step, eIF2, which is bound to GTP (like IF-2 in prokaryotes), brings the first tRNA to the ribosome to form what is known as the "43S initiation complex" (also like prokaryotes, except now the whole structure has a fancy name). After that, the mRNA gets dragged in by an eIF of its own (it's eIF4F, for those who do like the nitty-gritty details). This complex is now called the "48S initiation complex." I think it might be at this point that the tail kinda circles around and meets the cap (this has to happen at some point so that only complete mRNAs that have the cap and the tail can be translated). Anyway once the mRNA and all that is on the ribosome, the ribosome starts scanning the message until it bumps into the initial AUG start codon. This initiates binding of the 60S subunit, cleavage of eIF2's GTP to form GDP, and initiation factors to dissociate (much like the final steps of initiation in prokaryotes).

Be able to describe how puromycin blocks protein synthesis

Puromycin is a drug that inhibits protein synthesis in both prokaryotes and eukaryotes. It works by binding at the A site and essentially acting as a kind of release factor, causing early termination of the chain.

Be able to describe how diphtheria toxin blocks protein synthesis

Diphtheria toxin blocks protein synthesis in eukaryotic cells. It does this by ADP-ribosylating eIF2 (i.e. adding a ribose attached to ADP). This ADP-ribose group is kinda bulky, so other tRNAs are blocked from entering the ribosome. Not good. Thankfully, we have a vaccine against diphtheria, so we don't have to see this happen too often.

Be able to describe how phosphorylation of eIF2 controls protein synthesis

You know how phosphorylation of most proteins turns them on? Well, eIF2 is kinda the opposite. You see, when the alpha subunit of eIF2 is phosphorylated by eIF2α kinase, it becomes able to bind to eIF2B. This binding makes eIF2 unable to do its regular job. Hence, phosphorylation of eIF2 turns off protein synthesis. This is useful in cases like starvation when the body wants to use those amino acids for energy rather than building muscle or whatever.