SYNTHESIS AND PROCESSING OF PULMONARY SURFACTANT PROTEOLIPID SPL(Phe) IN FETAL LUNG

Abstract

Surfactant proteolipid, SPL(Phe), a small molecular weight hydrophobic protein isolated from pulmonary surfactant, has been associated with enhanced surface properties of surfactant phospholipids from various species. We have isolated cDNA's encoding SPL(Phe) and studies the induction of SPL(Phe) RNA and protein synthesis in human fetal lung tissue during explant culture usng the labelled SPL(Phe) cDNA to probe SPL(Phe) RNA. SPL(Phe) synthesis was not detected after [35S]methionine labelling and immunoprecipitation in fetal lung from 18-23 weeks gestation but increased markedly after 1-5 days in organ culture. Primary in vitro translation product of SPL(Phe) was a charge train of Mr=38-39,000, pI 5.0-5.4 processed in vitro to larger proteins of Mr=40,000. Hybrid selected and arrested translation of human lung RNA confirmed the relationship of the Mr=40,000 precursors to SPL(Phe). Synthesis of SPL(Phe) preprotein, Mr=40-41,000, pI 5.0-5.4, containing N-linked oligosaccharide, was enhanced between 0-4 days of fetal lung explant culture; Northern blot analysis of RNA of low abundance demonstrated a single 2.0 kilobase mRNA (in fetal lung), prior to explant culture and increased dramatically during 1-4 days of culture. Proteolytic processing of the SPL(Phe) to smaller peptides of Mr=6-14,000, which co-migrated with the peptides isolated from human surfactant was demonstrated in the fetal lung explants after organ culture. SPL(Phe) is a Type II cell surfactant-associated hydrophobic protein whose synthesis and RNA increased dramatically during organ culture of fetal lung in association with the morphologic saturation of the Type II epithelial cell.