title: Annexin V--heparin oligosaccharide complex suggests heparan sulfate--mediated assembly on cell surfaces.
abstract: BACKGROUND
  Annexin V, an abundant anticoagulant protein, has been proposed to exert its effects by self-assembling into highly ordered arrays on phospholipid membranes to form a protective anti-thrombotic shield at the cell surface. The protein exhibits very high-affinity calcium-dependent interactions with acidic phospholipid membranes, as well as specific binding to glycosaminoglycans (GAGs) such as heparin and heparan sulfate, a major component of cell surface proteoglycans. At present, there is no structural information to elucidate this interaction or the role it may play in annexin V function at the cell surface.
  
  
  RESULTS
  We report the 1.9 A crystal structure of annexin V in complex with heparin-derived tetrasaccharides. This structure represents the first of a heparin oligosaccharide binding to a protein where calcium ions are essential for the interaction. Two distinct GAG binding sites are situated on opposite protein surfaces. Basic residues at each site were identified from the structure and site-directed mutants were prepared. The heparin binding properties of these mutants were measured by surface plasmon resonance. The results confirm the roles of these mutated residues in heparin binding, and the kinetic and thermodynamic data define the functionally distinct character of each distal binding surface.
  
  
  CONCLUSIONS
  The annexin V molecule, as it self-assembles into an organized array on the membrane surface, can bind the heparan sulfate components of cell surface proteoglycans. A novel model is presented in which proteoglycan heparan sulfate could assist in the localization of annexin V to the cell surface membrane and/or stabilization of the entire molecular assembly to promote anticoagulation.
authors.affiliation: Division of Medicinal and Natural Products Chemistry, Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA.
authors.forename: I
authors.initials: I
authors.lastname: Capila
authors.forename: M J
authors.initials: MJ
authors.lastname: Hernáiz
authors.forename: Y D
authors.initials: YD
authors.lastname: Mo
authors.forename: T R
authors.initials: TR
authors.lastname: Mealy
authors.forename: B
authors.initials: B
authors.lastname: Campos
authors.forename: J R
authors.initials: JR
authors.lastname: Dedman
authors.forename: R J
authors.initials: RJ
authors.lastname: Linhardt
authors.forename: B A
authors.initials: BA
authors.lastname: Seaton
mesh_terms: D000818:Animals; D017304:Annexin A5 / Q000737:chemistry* / Q000502:physiology*; D001665:Binding Sites; D019921:Biotinylation; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004583:Electrons; D006493:Heparin / Q000737:chemistry*; D007700:Kinetics; D008081:Liposomes / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009844:Oligosaccharides / Q000737:chemistry*; D010743:Phospholipids / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry; D019809:Streptavidin / Q000737:chemistry; D020349:Surface Plasmon Resonance; D013552:Swine; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D017304:Annexin A5; D008081:Liposomes; D009844:Oligosaccharides; D010743:Phospholipids; D011994:Recombinant Proteins; D006493:Heparin; D019809:Streptavidin; D002118:Calcium