Characterization of the small hydrophobic proteins associated with pulmonary surfactant

Lipid extracts of bovine pulmonary surfactant, which retain many of the biophysical characteristics of natural surfactant, contain approx. 98% lipid and 2% protein, as determined by amino acid analysis. Polyacrylamide/urea gel electrophoresis reveals that lipid extract surfactant contained a major apoprotein band with apparent Mr 3500 and minor apoprotein bands with apparent Mr 15,000 and 7000. After reduction, the 15 kDa band disappears and is replaced by a prominent band with apparent Mr = 5000. Reduction also results in a relative diminution of the 7 kDa band and a relative increase in the intensity of the 3.5-kDa band. Edman degradation reveals two major peptide sequences which have been designated surfactant-associated peptide (N-terminal Phe) and surfactant-associated peptide (N-terminal Leu) and a minor sequence designated surfactant-associated peptide (N-terminal Ile). The latter surfactant-associated peptide appears to be related to the N-terminal Leu peptide but lacks the terminal Leu. N-Terminal analysis by dansylation demonstrates that the 15 and 5 kDa (reduced) apoprotein species contain N-terminal Phe, Leu and Ile. The 3.5 and 7 kDa bands contain only N-terminal Leu and Ile. Chromatography of lipid extracts on silicic acid columns gives rise to fraction I, which contains protein and phosphatidylglycerol, and fraction II, which contains protein, phosphatidylglycerol and phosphatidylethanolamine. Fraction I was primarily composed of the 15-kDa apoproteins, while fraction II contained mainly the 3.5 and 7 kDa apoproteins. Both fractions exhibited biophysical activity after reconstitution with dipalmitoylphosphatidylcholine. These results indicate that lipid extracts contain an oligomer of 15 kDa containing surfactant-associated peptide (N-terminal Phe) and surfactant-associated peptides (N-terminal Leu or Ile) which interact through sulfhydryl and perhaps other bonds. Lipid extracts also contain 3.5 kDa monomers of surfactant-associated peptides with N-terminal Leu and N-terminal Ile which can dimerize through sulfhydryl and perhaps hydrophobic interactions.