Structural characteristics and intermolecular organization of human pulmonary-surfactant-associated proteins

The structural relationships and intermolecular organization among the proteins associated with pulmonary surfactant are largely unknown. We studied the pulmonary-surfactant-associated proteins in the bronchoalveolar lavage fluid obtained from a patient with the clinical syndrome of alveolar proteinosis. The major proteins with M(r) values of 32,000-36,000 and 62,000 formed thiol-dependent complexes (M(r) > 400,000) with intermolecular disulphide bonds present in the collgenase-sensitive domains of these proteins. In contrast, other proteins, which were collagenase-insensitive, formed thiol-dependent oligomers that were not covalently linked to the major proteins. The associations of these proteins in the surfactant of a normal individual was similar. By amino acid analysis, two-dimensional peptide mapping and bacterial-collagenase digestion the 32,000-36,000-M(r) and 62,000-M(r) proteins were nearly identical. Differences in CNBr cleavage products suggested that the larger of the proteins was formed by non-disulphide, covalent, cross-links in the collagenase-sensitive domains of the 32,000-36,000-M(r) proteins. Thus the evidence suggested that the lipid-associated proteins of M(r) 32,000-36,000 contained both disulphide and non-disulphide cross-links in the collagen-like N-terminal region of the proteins and form higher-M(r) complexes. This organization may support the three-dimensional conformation of surfactant in the alveolar space.