Identification of amino acids in the gamma-carboxylation recognition site on the propeptide of prothrombin

A gamma-carboxylation recognition site on the propeptide of the vitamin K-dependent blood coagulation proteins directs the carboxylation of glutamic acid residues by binding to the vitamin K-dependent carboxylase. To determine residues that define this site, we evaluated the effect of mutation of certain residues in the prothrombin propeptide on the extent of carboxylation. The prothrombin cDNA modified by site-specific mutagenesis was expressed in Chinese hamster ovary cells using a system that yields functional fully carboxylated prothrombin. The cell supernatants containing recombinant prothrombin were evaluated for the extent of gamma-carboxylation by immunoassay. Conformation-specific anti-prothrombin:Ca(II)-specific antibodies measure native completely carboxylated prothrombin; anti-prothrombin:total antibodies measure all forms of prothrombin, regardless of gamma-carboxyglutamic acid content. Mutation of His-18 to Gly, Val-17 to Ser, Leu-15 to Gly or Asp, or Ala-10 to Asp was associated with a partial (30-65%) inhibition of gamma-carboxylation. Mutation of Ala-14 to Ser or Ser-8 to Val did not inhibit gamma-carboxylation. From this and earlier work, residues whose mutation leads to a significant impairment of carboxylation include His-18, Val-17, Phe-16, Leu-15, and Ala-10. Residues whose mutation does not alter the carboxylation recognition site include Ala-14, Ser-8, Arg-4, and Arg-1. To determine the size of the recognition site, the in vitro carboxylation of propeptide-containing synthetic peptides was compared. A 28-residue peptide, based upon residues -18 to +10 of prothrombin, and a 54-residue peptide, based upon residues -18 to +36 of prothrombin, were carboxylated by partially purified bovine carboxylase with similar Km values of 2-5 microM. These results indicate that the gamma-carboxyglutamic acid-rich region of prothrombin makes a minimal contribution to carboxylase binding. A molecular surface of about five amino acids located within the propeptide appears to define the carboxylation recognition site on the precursor forms of the vitamin K-dependent proteins.