Post-translational modification of the major human surfactant-associated proteins.

The major protein in human pulmonary surfactant is a sialoglycoprotein of 32-36 kDa (PSP-A) that has been shown by translation of lung mRNA in vitro to be derived from precursor molecules of 29-31 kDa [Floros, Phelps & Jaeusch (1985). J. Biol. Chem. 260, 495-500]. We show here that two-dimensional gel patterns of PSP-A similar to that of the primary translation products are obtained by incorporation of [35S]methionine in the presence of tunicamycin or by N-glycanase digestion of the 32-36 kDa group. Additional gel patterns are also observed in which the isoelectric-point heterogeneity is similar to that of either tunicamycin-treated tissue or primary translation products, but with higher molecular masses. The gel patterns showing higher-molecular-mass components are obtained when terminal sialic acid addition is prevented by the incubation of lung tissue with monensin or when terminal sialic acids are digested from the fully processed protein with neuraminidase. The 32-36 kDa forms have been shown to contain [14C]mannose. Pulse-chase experiments indicate that the acidic isoforms in the protein group arise from basic isoforms that are detectable within 10 min.