Synthesis of a protein starts with the binding of ribosomal subunits and initiation factors to the mRNA. In this process, the initiation factor eIF2 is bound to initiator tRNA (met-tRNAi) and not to other tRNAs. Upon recognition of the initiation codon AUG, translation starts with the methionine that is bound to the initiator tRNA.
@ Mohd. You gave concepts of protein synthesis. But we didn't got answer to question. I too have same doubt.
I wonder this answer too. also I want to learn why some proteins didn't have M after posttranslational modification. is there any rule (I mean some protein families has a mechanism to remove M after protein synthesis etc.).
Also in addition to your question: in some conditions the first codon includes GUG too. tRNA brings Methionine but the code is not AUG GUG.
Without addition of formylated methioine the initiation of protein synthesis cannot happen becoz the initiation factors bind to formylated methionine and then only elongation happens....In eucaryotes formylation of methionine happens through a specific and unique set of enzyme methionyl-tRNAMet transformylase and even in prokayotes there is a mechanism of formylation and specific purine rich residues first identify the AUG start codon and then only initiation happens so invariably AUG is the start codon for protein synthesis in both pro and eucaryotes which encodes for methionine. For more details I would suggest you to read Basic Book of Biochemistry from Stryer or Voet and Voet..... There the complete details would be furnished.. Good luck
Synthesis of a protein starts with the binding of ribosomal subunits and initiation factors to the mRNA. In this process, the initiation factor eIF2 is bound to initiator tRNA (met-tRNAi) and not to other tRNAs. Upon recognition of the initiation codon AUG, translation starts with the methionine that is bound to the initiator tRNA.
Am supporting to the answer of Gert C Scheper .
Studies on replacing the methionine on tRNA-met showed other amino acids can also be the initial amino acid residue in the protein synthesis. This points that methionine is not the important factor in translation.
Similarly modifying the codon AUG showed, what ever the sequence modification the initial amino acid is methionine. which confirms it is also not significant to the Codon.
Another Study confirms the dilemma here. the common factor in both studies is the tRNA.
tRNA for methionine is of two types. initiator tRNA and elongator tRNA. as far as the initiation happens initiator tRNA is involved. In the chain elongation process, elongator tRNA is involved.
The specialty of initiator tRNA is that, they are independent of start codon. instead, it is dependent on the initiation factor. Also, initiator tRNA binds to the P-arm of ribosome.
Altogether, the initiator tRNA is responsible for the methionine of translation start site. and this initiator tRNA is independent of start codon. depend only on initiation factors (making itself as a factor of initiation complex).
Hope this helps.
I don't think any of the answers fulfilled the sought question. Bhairavi wonders why it may not be another amino acid as the first one in translation, I believe. I asked the same question to my students for years and yet waiting for a compelling logic answer!
I think this is a difficult question that has to be analyzed from an evolutionary perspective. The authors from this paper make an interesting suggestion:
Article Evolution of initiator tRNAs and selection of methionine as ...
Not always a Methionine apparently... Actually in this publication (Hecht A., Glasgow J., Jaschke P.R., Bawazer L., Munson M.S., Cochran J., Endy D., Salit M. Measurements of translation initiation from all 64 codons in E. coli. Nucleic Acids Res. 2017; 45:3615–3626.) they found a particular case for the codon ACG that seems to succeed some times to initiate the translation with AA Threonine (1 of 8). It might be explained by the fact that the Threonine is a Methionine cognate amino acid.
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