All Mature Polypeptides Contain a Methionine at the N-Terminus

Question

All mature polypeptides contain a methionine at the N-terminus.

Accepted Answer

While some mature polypeptides do start with a methionine at the N-terminus, not all of them do. In some cases, the methionine is removed during post-translational modification. Additionally, some proteins are synthesized with alternative start codons that do not code for methionine.