Requirement of the collagenous domain for carbohydrate processing and secretion of a surfactant protein, SP-A

Two distinct intracellular forms of surfactant protein M_r = 35 000 (SP-A) were demonstrated in both purified type II epithelial cells and rat lung in vivo. High-mannose precursors of M_r = 30 000 and 34 000 comprised a significant fraction of intracellular SP-A in vivo and in vitro. A second intracellular pool was demonstrated in lamellar body enriched fractions which contained endoglycosidase-H resistant, sialylated forms of SP-A. Intracellular transport and secretion of SP-A was not altered by inhibitors of carbohydrate processing. However, incubation of type II cells with α,α′-dipyridyl or cis-4-hydroxy-l-proline, agents which disrupt triple-helix formation within collagenous peptide domains, inhibited sialylation, intracellular transport to the lamellar body fraction and secretion. In the presence of either α,α′-dipyridyl or cis-4-hydroxy-l-proline, high mannose precursors accumulated intracellularly and were not secreted after 16-18 h. Thus, high-mannose precursors in proximal intracellular pool(s) and sialylated forms in lamellar body-enriched fractions represent two major intracellular storage forms of SP-A in vitro and in vivo. SP-A is routed by processes dependent upon the hydroxylation of the collagenous domain of the polypeptide. Transport and secretion of SP-A are not dependent upon the addition or processing of asparagine-linked carbohydrate.