The start codon is AUG so normally all protein have to contain methionine as starting amino acid, is it true?

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First together they say in this publishing (Hecht A., Glasgow J., Jaschke P.R., Bawazer L., Munson M.S., Cochran J., Endy D., Salit M. Dimensions of translation initiation from all 64 codons in E. Coli. Nucleic Acids Res. 2017; 45:3615–3626.) the translation can happen from pretty much any type of initial codon.
Then non-canonical begin codons commonly code for amino acids other than methionine, but when they action as begin codons they code for Met (in Eukaryote) and fMet (in Prokaryote). In the publication pointed out above, they uncovered one particular case for the codon ACG that seems to success to initiate the translation v AA Threonine. It could be explained by the reality that the Threonine is a Methionine cognate amino acid.
So, except couple of exception the very first AA is always a Methionine... Till post-translational changes occur. Without doubt post-translational modifications can excise the first AA, therefore the Methionine can be removed.
Among others, that is debated in those forums:

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Not every protein necessarily starts through methionine, however. Regularly this an initial amino acid will certainly be gotten rid of in later on processing the the protein. A tRNA charged through methionine binds to the translation begin signal. The huge subunit binding to the mRNA and the little subunit, and so begins elongation, the development of the polypeptide chain. ~ the first charged tRNA shows up in the A site, the ribosome move so that the tRNA is currently in the ns site. New charged tRNAs, matching the codons the the mRNA, get in the A site, and a bond is formed in between the two amino acids. The an initial tRNA is now released, and also the ribosome shifts again so that a tRNA carrying two amino acids is now in the ns site. A new charged tRNA then binding to the A site. This procedure of elongation proceeds until the ribosome get what is referred to as a prevent codon, a triplet the nucleotides that signals the termination of translation. When the ribosome get a stop codon, no aminoacyl tRNA binding to the empty A site. This is the ribosom signal to break apart into its big and tiny subunits, publication the brand-new protein and also the mRNA. Yet, this isn't constantly the finish of the story. A protein will often undergo more modification, referred to as post-translational modification. Because that example, it might be cleaved by a protein-cutting enzyme, called a protease, at a specific place or have a couple of of its amino acids altered.