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The Apple 1 and Apple 4 domains of factor XI act synergistically to promote
the surface-mediated activation of factor XI by factor XIIa
FA Baglia, FS Seaman and PN Walsh
Sol Sherry Thrombosis Research Center, Department of Medicine, Temple
University School of Medicine, Philadelphia, PA 19140, USA.
Binding sites for high molecular weight kininogen (HK) and for factor XIIa
are present in the Apple 1 (A1) and the A4 domains of factor XI,
respectively. To define the roles of these two sites in surface- mediated
factor-XI activation we prepared conformationally constrained synthetic
peptides and recombinant A1 domain (rA1) and determined their effects on
the activation of factor XI by factor XIIa in the presence of HK and either
kaolin or dextran sulfate. Surface-mediated factor-XI activation by factor
XIIa was inhibited by a conformationally constrained A4 peptide
(Ala317-Gly350), by an A1 peptide (Phe56-Ser86), and by rA1 (Glu1-Ser90).
When used in combination at equimolar concentrations, rA1 and A4 peptide
were 10-fold more effective than either one alone in inhibiting
surface-mediated activation of factor XI by factor XIIa. The A4 peptide was
a competitive inhibitor of factor XIIa amidolytic activity and a
noncompetitive inhibitor of factor-XI activation by factor XIIa, whereas
rA1 and the A1 peptide did not inhibit factor XIIa. The rA1 domain
inhibited factor XI binding to HK, whereas the A4 peptide did not. We
conclude that specific sequences exposed on the surfaces of the A1
(Val59-Lys83) and A4 (Ala317-Gly350) domains of factor XI act
synergistically to promote surface-mediated factor-XI activation by factor
XIIa in the presence of HK by binding factor XI to surface-bound HK (A1
domain) and by binding factor XIIa near the cleavage site (Arg369-Ile370)
of factor XI (A4 domain).
Volume 85,
Issue 8,
pp. 2078-2083,
04/15/1995
Copyright © 1995 by The American Society of Hematology
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