Trafficking of surfactant protein A in fetal rabbit lung in organ culture

Surfactant protein A (SP-A), a major protein component of pulmonary surfactant, is a developmentally and hormonally regulated sialoglycoprotein expressed in type II pneumonocytes. Surfactant proteins and glycerophospholipids are transported to multilamellar structures termed lamellar bodies, which serve to store surfactant lipoprotein until secretion by exocytosis into the alveolar lumen. The cellular mechanism(s) for targeting of SP-A and other surfactant components to lamellar bodies is unknown. In the present study, we have investigated the transport of SP-A to lamellar bodies in fetal rabbit lung tissue in organ culture using pulse- chase analysis of [³⁵S]-methionine-labeled SP-A protein. SP-A accumulated in lamellar bodies within 1-3 h of synthesis; lamellar body SP-A was found to be endoglycosidase H resistant and represented 30-40% of the radiolabeled SP- A recovered from the tissue for periods of up to 12 h postlabeling. Based on our estimates of lamellar body recovery from tissue homogenates, lamellar body-associated SP-A may account for 60-80% of the SP-A present in the fetal lung explants. Treatment of fetal rabbit lung explants with inhibitors of oligosaccharide addition (tunicamycin) and processing (castanospermine), which act within the endoplasmic reticulum, significantly reduced the rate of transport of newly synthesized SP-A to lamellar bodies. An inhibitor of oligosaccharide processing that acts on a processing step that takes place within the Golgi apparatus (swainsonine) reduced the rate of transport of radiolabeled SP-A to lamellar bodies by ~50%. These results suggest that a large proportion of newly synthesized SP-A is transported to lamellar bodies and that the transport of SP-A to lamellar bodies is enhanced by the presence of an N-linked oligosaccharide side chain and may depend on N-linked oligosaccharide processing.