title: Self-association of human protein S.
abstract: Protein S functions as a cofactor with activated protein C in the down-regulation of the blood coagulation cascade. In vitro studies have historically produced conflicting data with regard to the extent of various protein S activity in clotting assays which typically involve adding CaCl(2) to initiate reactions. We report here that protein S reversibly self-associates in the absence of Ca(2+). Sedimentation experiments showed a transition in sedimentation velocity from 7.2 to 4.2 S with a transition midpoint (T(m)) of 0.42 mM Ca(2+) for intact protein S. Studies of thrombin cleaved (Arg(70)) protein S revealed similar results with a transition in sedimentation velocity from 7.9 to 4.4 S with a T(m) of 0.42 mM Ca(2+). This transition is reversible with the addition of 10 mM EDTA. Sedimentation equilibrium data suggest at a minimum, a monomer-dimer-trimer association. Sedimentation velocity experiments were also performed on mixtures of protein S and prothrombin which showed no heterodimer formation in either Ca(2+) or EDTA solutions. These data suggest that previous interpretations of protein S structure and function may have been confounded by the self-associative behavior of protein S in non-Ca(2+) solutions.
authors.affiliation: Department of Biochemistry, Given Building, Health Science Complex, University of Vermont, College of Medicine, Burlington, Vermont 05405-0068, USA.
authors.forename: J E
authors.initials: JE
authors.lastname: Pauls
authors.forename: M F
authors.initials: MF
authors.lastname: Hockin
authors.forename: G L
authors.initials: GL
authors.lastname: Long
authors.forename: K G
authors.initials: KG
authors.lastname: Mann
mesh_terms: D000595:Amino Acid Sequence; D002118:Calcium / Q000494:pharmacology; D004492:Edetic Acid / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017293:Protein S / Q000737:chemistry*; D011516:Prothrombin / Q000737:chemistry; D013917:Thrombin / Q000378:metabolism; D014461:Ultracentrifugation
chemical_list: D017293:Protein S; D011516:Prothrombin; D004492:Edetic Acid; D013917:Thrombin; D002118:Calcium