Opher Gileadi wrote:
Hi Theresa,
To add to Anat's comments: Although the AUG codon for the first methionine and
all other methionines in a protein coding sequence look the same, they are read
in a very different way by the ribosomal machinery. The first AUG is recognized
by the initiation complex, which includes the separate small ribosomal subunit
(40s), a special tRNA-methionine, and initiation factors (proteins) including
eIF2. This leads to assembly of a complete ribosome and initiation of protein
synthesis. Subsequently, in the process of elongation, AUG codons are read by a
different tRNA, which is brought to the 80s ribosome bound to a protein called
elongation factor 1a. This is an oversimplification, of course, but the point
is that the initiation codon (=the first amino acid to be incorporated to the
protein) is read by a special tRNA, hence the universal use of methionine.
Opher
Yes, but why methionine? Half the time it has to be removed by N-terminal peptidase to give a small first residue, or by
leader sequence processing. Why use a big expensive amino acid instead of choosing one of the glycine codons? Is there
an obvious reason, or just "it had to be something, and Met happened to get selected"?
And why sometimes alternate start codons can be used? and why doesn't initiation occur also at methionines in the middle
of proteins? I'm guessing it has to do with 5' untranslated region and ribosome binding sites. So could the start codon
actually be anything you want, provided there is a strong ribosome binding site there?
Just being philosophical, and not afraid to display my ignorance,
eab
