Question

Why not all proteins in human start with methionine?

Answer 1

Even though all proteins start with Met during translation, there is a lot of post-translational modification. Introns can be removed, G cap added or adding a poly A tail. Because of the modifications, the methionine is usually removed.

Answer 2

ok @francesca

Answer 3

Another thing to consider is the post-translational modification of some proteins - quite a few proteins are secreted in an inactive form and need to have part cleaved off before they are active. Matrix metalloproteinases are an example of this type of protein processing. I'm not sure whether @mayankdevnani is on the right track with his answer, which seems to indicate removal of the initial methionine but also mentions changes in the RNA. I would welcome further comments from him because his answers are usually pertinent, interesting and of course correct.

Answer 4

What is post-translational modification?

Answer 5

There are all sorts of enzymes which act on proteinis after they have been released from the ribosome. Commonly proteins are glycosylated (sugar added) or phosphorylated (phosphate added) in the Golgi but proteins can also be cleaved by proteases to get rid of a bit of the sequence. Ususally this form of post-translational modification releases the active form of the protein - quite often the N terminal portion of the protein is removed so the initial methionine is removed too. I hope this helps, E