The Propeptide Binding Site of the Bovine γ-Glutamyl Carboxylase

gamma-Glutamyl carboxylase is an integral membrane protein required for the posttranslational modification of vitamin K-dependent proteins. The main recognition between the enzyme and its substrates is through an 18-amino acid propeptide. It has been reported that this binding site resides in the amino-terminal third of the gamma-glutamyl carboxylase molecule (Yamada, M., Kuliopulos, A., Nelson, N. P., Roth, D. A., Furie, B., Furie, B. C., and Walsh, C. T. (1995) Biochemistry 34, 481-489). In contrast, we found the binding site in the carboxyl half of the gamma-glutamyl carboxylase. We show that the carboxylase may be cleaved by trypsin into an amino-terminal 30-kDa and a carboxyl-terminal 60-kDa fragment joined by a disulfide bond(s), and the propeptide binds to the 60-kDa fragment. The sequence of the amino terminus of the 60-kDa fragment reveals that the primary trypsin-sensitive sites are at residues 349 and 351. Furthermore, the tryptic fragment that cross-links to the propeptide also reacts with an antibody specific to the carboxyl portion of the gamma-glutamyl carboxylase. In addition, cyanogen bromide cleavage of bovine gamma-glutamyl carboxylase cross-linked to the peptide comprising residues TVFLDHENANKILNRPKRY of human factor IX yields a cross-linked fragment of 16 kDa from the carboxyl half of the molecule, the amino-terminal sequence of which begins at residue 438. Thus, the propeptide binding site lies carboxyl-terminal to residue 438 and is predicted to be in the lumen of the endoplasmic reticulum