Structure-function relationships of bovine pulmonary surfactant proteins: SP-B and SP-C.

Pulmonary surfactant contains at least three unique proteins: SP-A, SP-B and SP-C. SP-B and SP-C from bovine surfactant are markedly hydrophobic and have molecular masses between 3 and 26 kDa. We identify surfactant proteins under nonreducing conditions on polyacrylamide gels with approximate molecular mass of 5, 14, 26 kDa (SP-5, 14, 26) when organic solvent-soluble material is eluted from a Sephadex LH-20 size exclusion column followed by separation on a high-performance reverse-phase chromatography system. These bands correspond to monomeric SP-C, oligomeric SP-C and oligomeric SP-B, respectively. Computer analysis (Eisenberg-hydrophobic moment) of sequences for these proteins suggests that SP-B contains surface-seeking amphiphilic segments. In contrast, SP-C resembles a more hydrophobic transmembrane anchoring peptide. Dispersions containing dipalmitoylphosphatidylcholine, phosphatidylglycerol, palmitic acid and multimeric SP-B and SP-C duplicate the surface activity of natural surfactant when assayed in a pulsating bubble surfactometer. We speculate that oligomers of SP-B and monomers and oligomers of SP-C may act cooperatively in affecting surfactant function. An important function of SP-B and SP-C may be to affect the ordering of surfactant lipids so that rates of transport of surfactant lipids to the hypophase surface in the alveoli are enhanced.