title: Defining the structure of membrane-bound human blood coagulation factor Va.
abstract: BACKGROUND
  Blood coagulation factor (F) Va is the essential protein cofactor to the serine protease FXa. Factor Va stimulates the thrombin-to-prothrombin conversion by the prothrombinase complex, by at least five orders of magnitude. Factor Va binds with very high affinity to phosphatidylserine containing phospholipid membranes, which allows the visualization of its membrane-bound state by transmission electron microscopy (EM).
  
  
  METHODS
  In this paper we present an averaged three-dimensional structure of FVa molecules attached to phosphatidylserine containing lipid tubes, as determined by EM and single particle analysis. The low-resolution FVa three-dimensional structure is compared with the available atomic models for FVa.
  
  
  RESULTS
  The experimental data are combined with the most suitable atomic model and a membrane-bound FVaEM model is proposed that best fits the protein density defined by EM. In the FVaEM model, the C1 and C2 membrane-binding domains are juxtaposed onto the membrane surface and the model geometries indicate a deeper insertion of both C domains into the lipid bilayer than has been previously suggested.
  
  
  CONCLUSIONS
  The present structure is a first step towards a higher-resolution experimental structure of a human FVa molecule in its membrane-bound conformation, allowing the visualization of individual domains within FVa and its association with the membrane.
authors.affiliation: Department of Biological Sciences, University of Warwick, Coventry, UK. s.mcphie@warwick.ac.uk
authors.forename: S
authors.initials: S
authors.lastname: Stoilova-McPhie
authors.forename: C D J
authors.initials: CD
authors.lastname: Parmenter
authors.forename: K
authors.initials: K
authors.lastname: Segers
authors.forename: B O
authors.initials: BO
authors.lastname: Villoutreix
authors.forename: G A F
authors.initials: GA
authors.lastname: Nicolaes
mesh_terms: D015943:Factor Va / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008055:Lipids; D008565:Membrane Proteins / Q000737:chemistry; D046529:Microscopy, Electron, Transmission; D010718:Phosphatidylserines / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation
chemical_list: D008055:Lipids; D008565:Membrane Proteins; D010718:Phosphatidylserines; D015943:Factor Va