Processing of surfactant protein B proprotein by a cathepsin D-like protease

Surfactant protein B (SP-B) is a hydrophobic peptide of relative molecular weight (M(r)) = 8,000 that is associated with pulmonary surfactant phospholipids. SP-B is synthesized by the alveolar type II epithelial cell as a proprotein of M(r) = 42,000 which requires at least two proteolytic cleavages to generate the 79 residue mature SP-B peptide. We have previously reported that cleavage of the NH2-terminal propeptide, to generate a processing intermediate of M(r) = 25,000, occurs in close temporal approximation to secretion. In the present study we demonstrate that SP-B proprotein, isolated from stably transfected Chinese hamster ovary cells, is processed to M(r) = 25,000 by a crude type II cell membrane fraction but not by intact type II cells or type II cell conditioned media. In vitro processing of the proprotein by the type II cell membrane preparation resulted in release of a single peptide of M(r) = 16,000-17,000, which was detected by antiserum directed against antigenic epitopes in propeptide of the precursor. SP-B processing activity was extracted by Na2CO3 lysis of the type II cell membrane preparation, had a pH optimum of 5.0-6.0, and was inhibited by 10(-7) M pepstatin A, suggesting that the NH2-terminal peptide of the precursor is cleaved by an aspartyl protease. Consistent with this hypothesis, processing of SP-B by a crude type II cell membrane preparation was blocked by antiserum directed against the aspartyl protease cathepsin D; further, purified cathepsin D efficiently processed the SP-B precursor to M(r) = 25,000. Collectively these results suggest that cleavage of the NH2-terminal propeptide of the SP-B precursor is mediated by cathepsin D or a cathepsin D-like protease localized within the secretory pathway of the type II epithelial cell.