Vitamin K-dependent carboxylation of pulmonary surfactant-associated proteins

Rat type II pneumocytes expressed vitamin K-dependent carboxylase activity that incorporated 14CO2 into microsomal protein precursors of molecular weights similar to those of surfactant-associated proteins (SAP). Compared to carboxylated precursor proteins present in the liver, these molecules appeared to be unique to the lung. Antibodies raised against purified rat surfactant reacted with SAP resolved by NaDodSO4/PAGE and with surfactant-containing lamellar bodies in type II pneumocyte cytoplasm. NaDodSO4/PAGE of microsomal proteins, after carboxylase-catalyzed incorporation of 14CO2, demonstrated radiolabeled, immunoreactive products identical to SAP. The presence of gamma-carboxyglutamic acid in these proteins was confirmed by HPLC analysis of SAP hydrolysates. Furthermore, lung carboxylase activity and SAP matured over similar time courses during fetal lung development. These results show that SAP are carboxylated by type II cells via a vitamin K-dependent pathway analogous to that for hepatic carboxylation of clotting factors. Further analogy to the clotting system suggests that gamma-carboxyglutamic acid residues in SAP polypeptides play a role in Ca2+ binding and thus in the known requirements for both the cation and SAP in the physiological function of pulmonary surfactant.