Structural Comparison of Recombinant Pulmonary Surfactant Protein SP-A Derived from Two Human Coding Sequences: Implications for the Chain Composition of Natural Human SP-A

The pulmonary surfactant-associated protein SP-A is encoded by presumably two different genes, resulting in slightly different amino acid sequences. Both gene products were expressed in Chinese hamster ovary cells. Their macromolecular structure differed significantly. SP-Aα3 exhibited a much higher amount of tetrameric to hexameric structures than SP-Aα2, for which dimeric structures predominate. These differences may be caused by the higher expression rates of SP-Aα3 presumably due to the presence of introns in the sequence. The occurrence of irregular disulfide links between individual oligomeric SP-A molecules composed of α3-chains together with the demonstrated presence of both gene products in natural human SP-A suggest that the subunits of SP-A are heterotrimers of one α2- and two α3-chains.