Role of the H helix in heparin binding to protein C inhibitor.
Protein C inhibitor (PCI) is a plasma serine proteinase inhibitor (serpin) that is a major physiological regulator of activated protein C. Inhibition of its target proteinase is accelerated by heparin in a reaction that involves the binding of both inhibitor and proteinase to heparin to form a ternary complex. This study was undertaken to understand the role of the H helix region (residues 264-278) of PCI in heparin binding and used (i) a recombinant truncated PCI fusion protein of the first 294 residues, (ii) H helix synthetic peptides containing single Arg/Lys-->Glu substitutions, and (iii) site-directed Ala mutagenesis of 4 basic residues (Arg-269, Lys-270, Lys-276, and Lys-277) in the H helix region of full-length recombinant PCI (rPCI) expressed in Baculovirus. The PCI fusion protein interfered in heparin-accelerated PCI-proteinase inhibition reactions, and it bound to heparin-Sepharose. Compared to the wild-type PCI fusion protein, deletion of the H helix from the fusion protein resulted in a reduction of both heparin-Sepharose binding and the ability to compete for heparin during PCI-proteinase inhibition reactions. Competition assays with H helix synthetic peptides revealed that the R269E altered peptide was the least effective at blocking heparin-catalyzed PCI-proteinase inhibition reactions. Compared with full-length active wild-type rPCI, R269A: K270A and K276A:K277A rPCI both had reduced heparin-Sepharose binding, but only R269A:K270A rPCI showed a loss of heparin-accelerated proteinase inhibition for both activated protein C and thrombin. We conclude that a major heparin-binding site of PCI is the H helix, unlike its heparin-binding serpin homologues antithrombin and heparin cofactor II, which bind heparin primarily through the D helix.
Shirk
R A
RA
Department of Pathology, University of North Carolina School of Medicine, Chapel Hill 27599.
Elisen
M G
MG
Meijers
J C
JC
Church
F C
FC
D000595:Amino Acid Sequence; D016367:Baculoviridae / Q000235:genetics; D001483:Base Sequence; D001665:Binding Sites; D006493:Heparin / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009838:Oligodeoxyribonucleotides; D011485:Protein Binding; D018046:Protein C Inhibitor / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D009838:Oligodeoxyribonucleotides; D018046:Protein C Inhibitor; D011993:Recombinant Fusion Proteins; D006493:Heparin