The crystal structure of a complex of p11 with the annexin II N-terminal peptide.

The aggregation and membrane fusion properties of annexin II;Ire modulated by the association with a regulatory light chain called pll. pll is a membe r of the S100 EF-hand protein family, which is unique in having lost its ca lcium-binding properties. We report the first structure of a complex betwee n pll and its cognate peptide, the N-terminus of annexin II, as well as tha t of pll alone. The basic unit for pll is a tight, non-covalent dimer. In t he complex, each annexin II peptide forms hydrophobic interactions with bot h pll monomers, thus providing a structural basis for high affinity interac tions between an S100 protein and its target sequence. Finally, pll forms a disulfide-linked tetramer in both types of crystals thus suggesting a mode l for an oxidized form of other S100 proteins that have been found in the e xtracellular milieu.