Start Codon

Does every protein starts with methionine? In the context of eukaryotic cells (like those in humans) and many prokaryotic cells, nearly every protein initially starts with methionine during the synthesis process. This is because the codon AUG, which encodes for methionine, also serves as the start codon in mRNA, signaling the beginning of translation. However, after a protein is synthesized, methionine can be removed or the protein can be further modified through various post-translational modifications. Therefore, the mature form of the protein might not necessarily start with methionine. In addition, certain prokaryotes use a modified form of methionine, such as N-formylmethionine (fMet), as the initial amino acid in protein synthesis, following the recognition of the start codon by their machinery. So, while methionine is commonly the first amino acid incorporated during the synthesis of proteins due to the role of the AUG start codon, the presence of methionine at the N-terminus of the final, functional protein is not universal.