Posttranslational processing of surfactant protein C in rat type II cells

Pulmonary surfactant consists of phospholipids and proteins that form a stable monolayer at the surface of the alveoli to prevent lung collapse. Surfactant protein C (SP-C) is a hydrophobic 4-kDa palmitoylated protein derived from a 21-kDa precursor. We determined the membrane insertion, proteolytic processing, and subcellular location of 21-kDa proSP-C. In vitro, proSP-C associated with canine microsomes, and the NH2-terminal of proSP-C was protected from digestion with proteinase K, suggesting that proSP-C was inserted in a type III transmembrane configuration. Treatment of freshly isolated rat type II cells with cerulenin blocked acylation of the 21-kDa precursor. Pulse-chase labeling of type II cells demonstrated proSP-C processing intermediates of 19, 16, and 13 kDa that contained the NH2-terminal of proSP-C. Proteolytic processing of proSP-C was inhibited by incubation at 20 degrees C, suggesting that processing of proSP-C begins in a late Golgi or post-Golgi compartment. Immunogold labeling of rat lung with an antiserum to the NH2-terminal of proSP-C identified proSP-C in the trans-Golgi and multivesicular bodies but not in lamellar bodies. These findings suggest that proSP-C processing takes place in the trans-Golgi and multivesicular bodies before SP-C is incorporated into lamellar bodies.