Summary
Whether or not the factor VII Gla-domain is involved in the high-affinity interaction
of factor VII and tissue factor via calcium-dependent interactions with surrounding
phospholipids is unknown. To investigate this, we have purified the factor VII Gla-peptide
(FVII-GP) from digested recombinant human factor VII a and assessed its effect on
factor VII: tissue factor interactions. FVII-GP inhibited the activation of factor
X by factor Vila in the presence of either soluble or cell surface tissue factor halfmaximally
at 0.5 μM and 2.7 μM, respectively. However, FVII-GP failed to inhibit the specific
binding of factor Vila to cell-surface tissue factor, and did not inhibit the ability
of tissue factor to stimulate the amidolytic activity of factor Vila. Unrelipidated
tissue factor apoprotein stimulated the amidolytic activity of factor Vila to the
same extent as relipidated tissue factor apoprotein. These findings suggest that the
factor VII Gla-domain does not directly interact with tissue factor, but rather is
important for calcium binding and concomitant expression of other factor VII epitopes
necessary for tissue factor recognition and binding. To test this hypothesis, we have
prepared a monoclonal antibody against a putative factor VII epitope that participates
in the interaction of factor VII with cell-surface tissue factor (peptide 195-206)
and assessed its ability to bind to factor VII in the presence and absence of calcium.
Binding of this monoclonal antibody (PW-4) to intact factor VII a was calcium-dependent
and could be inhibited in a dose-dependent manner by peptide 195-206. The antibody
reacted with Gla-domainless factor Vila, but only 37% as compared to intact factor
Vila. In addition, PW4 as well as its Fab’ fragment, inhibited factor Vila binding
to cell-surface tissue factor. These studies indicate that the factor VII Gla-domain
does not provide structural elements that contribute to the formation of a stable
factor VII/VII a-tissue factor binary complex. The factor VII Gla-domain appears to
be necessary, however, in binding calcium ions and inducing a calcium-dependent conformational
change in factor VII/VII a that expresses one or more neoepitopes that participates
in the interaction of factor VII/VII a with the extracellular domain of tissue factor
apoprotein.