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Profactor IX: the propeptide inhibits binding to membrane surfaces and activation by factor XIa
J A Bristol1, S J Freedman, B C Furie
1Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts.
Biochemistry
|November 29, 1994
Summary
The propeptide of profactor IX, though fully carboxylated, prevents its activation and membrane binding. Removal of this propeptide by furin/PACE restores factor IX activity and function.
Area of Science:
- Biochemistry
- Molecular Biology
- Hematology
Background:
- Vitamin K-dependent proteins, like factor IX, require gamma-carboxylation for function.
- The propeptide of profactor IX contains a recognition site crucial for gamma-carboxylation.
- Understanding the processing of coagulation factors is vital for studying hemostasis.
Purpose of the Study:
- To investigate the role of the propeptide in factor IX processing and activity.
- To characterize the properties of incompletely processed factor IX (profactor IX).
- To elucidate the mechanism of profactor IX inactivation.
Main Methods:
- Isolation and purification of profactor IX from mammalian cells expressing human factor IX cDNA.
- Immunoaffinity chromatography using specific antibodies.
- Enzymatic cleavage assays with factor XIa and furin/PACE.
- Assessment of Ca(II)-dependent phospholipid binding.
Main Results:
- Purified profactor IX is fully gamma-carboxylated but lacks procoagulant activity.
- Profactor IX does not bind to acidic phospholipid vesicles, unlike mature factor IX.
- The propeptide of profactor IX inhibits its cleavage by factor XIa.
- In vitro removal of the propeptide by furin/PACE yields active factor IX.
Conclusions:
- The N-terminal propeptide of profactor IX is essential for its biological inactivity.
- Propeptide presence prevents factor IX activation and proper membrane interaction.
- Furin/PACE-mediated propeptide removal is a key step in generating active factor IX.