A level Biology: sulfur bridge of protein

the textbook says that two cysteine or methionine molecules could form a sulfur bridge, but what i found in Google, including Wikipedia, says that only cysteine molecules could form sulfur bridges in proteins, but methionine could not..
so which one is correct?
and how is the mechanism of the formation of the sulfur bridge between cysteine molecules (and between methionine molecules) ??

thank you!

Well, it would be good to review the structure of cysteine and methionine:

Now, as you can see, cysteine doesn't have any methyl group linked to its sulfur. It has a sulfhydryl group. This group is very reactive and can react with another cysteine to form a dimer, which is called cystine.
Methionine is not only larger, but it has a large methyl group attached to sulfur. This not only reduces the reactivity, but it also causes molecular hindrance in real time--called steric hindrance.
This is the basic reason why cysteine can dimerize, but methionine cannot.
But cysteine can form a disulfide bridge with other sulfur containing AA such as methione as well.

As for the mechanism of formation of the disulfide bond, it is by virtue of special reactivity of the sulfhydryl group. The hydrogen can be easily removed and a bond between two cysteine molecules is favorable. This covalent bond is weaker than the peptide bond between amino acids, but it stronger than the other interactions like hydrogen bonds and van der Wall's interactions.

If you have any questions, ask away!

thank you for your quick and detailed reply with a clear explanation based on the structures of cysteine and methionine!

so, in other words, two cysteine molecules could form a sulfur bridge easily, and one cysteine could form a sulfur bridge with methionine, but two methionine molecules could not form any sulfur bridge, am i right?

thank you!

do guys know the significance of cysteine in its protein structure and and its properties?