The invention relates to a polypeptide #including the following amino acid sequence: W-S-X1-W-X2-X3-C-S-X4-C-G (SEQ ID NO : 59), wherein X1, X2 and X3 are, independently of one another, S or G, X4 is R-S or V-S or V-T or R-T, and both cysteines form a disulfide bridge.