Characterization of the protein Z-dependent protease inhibitor interactive-sites of protein Z.

BACKGROUND:Protein Z (PZ) has been reported to promote the inactivation of factor Xa (FXa) by PZ-dependent protease inhibitor (ZPI) by about three orders of magnitude. Previously, we prepared a chimeric PZ in which its C-terminal pseudo-catalytic domain was grafted on FX light-chain (Gla and EGF-like domains) (PZ/FX-LC). Characterization of PZ/FX-LC revealed that the ZPI interactive-site is primarily located within PZ pseudo-catalytic domain. Nevertheless, the cofactor function and apparent Kd of PZ/FX-LC for interaction with ZPI remained impaired ~6-7-fold, suggesting that PZ contains a ZPI interactive-site outside pseudo-catalytic domain. X-ray structural data indicates that Tyr-240 of ZPI interacts with EGF2-domain of PZ. Structural data further suggests that 3 other ZPI surface loops make salt-bridge interactions with PZ pseudo-catalytic domain. To identify ZPI interactive-sites on PZ, we grafted the N-terminal EGF2 subdomain of PZ onto PZ/FX-LC chimera (PZ-EGF2/FX-LC) and also generated two compensatory charge reversal mutants of PZ pseudo-catalytic domain (Glu-244 and Arg-212) and ZPI surface loops (Lys-239 and Asp-293). METHODS:PZ chimeras were expressed in mammalian cells and ZPI derivatives were expressed in Escherichia coli. RESULTS:The PZ EGF2 subdomain fusion restored the defective cofactor function of PZ/FX-LC. The activities of PZ and ZPI mutants were all impaired if assayed individually, but partially restored if the compensatory charge reversal mutants were used in the assay. CONCLUSIONS:PZ EGF2 subdomain constitutes an interactive-site for ZPI. Data with compensatory charge reversal mutants validates structural data that the identified residues are part of interactive-sites. GENERAL SIGNIFICANCE:Insight is provided into mechanisms through which specificity of ZPI-PZ-FXa complex formation is determined.

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