INTRACELLULAR MATURATION OF THE GAMMA-CARBOXYGLUTAMIC ACID (GLA) REGION IN PROTHROMBIN COINCIDES WITH RELEASE OF THE PROPEPTIDE

Vitamin K-dependent coagulation factors undergo several post-translati onal modifications before the proteins are secreted into the blood as functional zymogens of the coagulation system. The modifications inclu de Asn-linked glycosylation, Asn/Asp hydroxylation, removal of a signa l peptide for translocation of the polypeptide into the endoplasmic re ticulum and removal of a propeptide which, when attached to the intrac ellular coagulation factor precursor, directs the protein for vitamin K-dependent gamma-carboxylation. Gamma-carboxylation of targeted Glu r esidues results in formation of Ca2+-binding gamma-carboxyglutamic aci d (Gla) residues. Ca2+ binding by these residues induces a conformatio nal change in the protein which is a necessary event for optimal activ ation or activity of the clotting factor in blood. In the present stud y we have monitored the intracellular prothrombin precursor in the sec retory pathway of liver cells to determine the effect that the propept ide has on Ca2+-dependent folding of the protein. The data provide evi dence that the Ca2+-induced conformational change required for activat ion of prothrombin coincides with release of the propeptide in the tra ns-Golgi apparatus of the liver cell and elucidates an important funct ion for the endoproteinase furin in biosynthesis of vitamin K-dependen t clotting factors.