[BiO BB] How many peptide residues can form a meaningful/functional protein sequence?

[#JIA YIYU#]

Hi all,

Can anybody here tell me how many peptide residues can form a meaningful/functional protein sequence? Or in other words, what is the shortest protein sequence? Or normally, how many peptide residues form one functional sub unit in a protein molecule?

Any helpful reference links and papers will be appreciated very much too! Thanks in advance! 

	Jia Yiyu