title: Solution structure of the Ca2+-Binding EGF3-4 pair from vitamin K-dependent protein S: identification of an unusual fold in EGF3.
abstract: Vitamin K-dependent protein S is a cofactor of activated protein C, a serine protease that regulates blood coagulation. Deficiency of protein S can cause venous thrombosis. Protein S has four EGF domains in tandem; domains 2-4 bind calcium with high affinity whereas domains 1-2 mediate interaction with activated protein C. We have now solved the solution structure of the EGF3-4 fragment of protein S. The linker between the two domains is similar to what has been observed in other calcium-binding EGF domains where it provides an extended conformation. Interestingly, a disagreement between NOE and RDC data revealed a conformational heterogeneity within EGF3 due to a hinge-like motion around Glu186 in the Cys-Glu-Cys sequence, the only point in the domain where flexibility is allowed. The dominant, bent conformation of EGF3 in the pair has no precedent among calcium-binding EGF domains. It is characterized by a change in the psi angle of Glu186 from 160 degrees +/- 40 degrees , as seen in ten other EGF domains, to approximately 0 degrees +/- 15 degrees . NOESY data suggest that Tyr193, a residue not conserved in other calcium-binding EGF domains (except in the homologue Gas6), induces the unique fold of EGF3. However, SAXS data, obtained on EGF1-4 and EGF2-4, showed a dominant, extended conformation in these fragments. This may be due to a counterproductive domain-domain interaction between EGF2 and EGF4 if EGF3 is in a bent conformation. We speculate that the ability of EGF3 to adopt different conformations may be of functional significance in protein-protein interactions involving protein S.
authors.affiliation: Department of Biophysical Chemistry, University of Lund, P.O. Box 124, SE-221 00 Lund, Sweden. torbjorn.drakenberg@bpc.lu.se
authors.forename: Torbjörn
authors.initials: T
authors.lastname: Drakenberg
authors.forename: Houman
authors.initials: H
authors.lastname: Ghasriani
authors.forename: Eva
authors.initials: E
authors.lastname: Thulin
authors.forename: Ann-Marie
authors.initials: AM
authors.lastname: Thämlitz
authors.forename: Andreas
authors.initials: A
authors.lastname: Muranyi
authors.forename: Arto
authors.initials: A
authors.lastname: Annila
authors.forename: Johan
authors.initials: J
authors.lastname: Stenflo
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015675:Osteocalcin / Q000737:chemistry* / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017293:Protein S / Q000737:chemistry* / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D010446:Peptide Fragments; D017293:Protein S; C504026:protein S (159-245), human; D015675:Osteocalcin; D002118:Calcium