Human SP-A: then and now.

In the short span of ten years, our understanding of human surfactant-associated protein A (SP-A) has advanced rapidly at both the level of the protein and the level of the gene. In the period 1984-1988, the protein was biochemically characterized and two SP-A precursors were identified. The molecular characterization was begun with the publication of an SP-A genomic sequence and sequences of two SP-A cDNAs, suggesting the presence of two SP-A genes. In the period 1991-1992, an SP-A pseudogene, a second SP-A genomic sequence, and an SP-A allelic variant were described. Since that time, a picture of increasing complexity has emerged from studies of the two SP-A genes. This complexity includes alternative splicing of 5' untranslated exons, allelic variants of both SP-A genes, and sequence heterogeneity within the 3' untranslated region. The challenge for the future will be to discover the physiological significance of the genetic complexity of human SP-A.