The role of the amino-terminal domain and the collagenous region in the structure and the function of rat surfactant protein D.

Surfactant protein D (SP-D) is a member of the C-type lectin superfamily with four distinct structural domains: an amino terminus involved in forming intermolecular disulfides, a collagen-like domain, a neck region, and a carbohydrate recognition domain. A collagen domain deletion mutant (CDM) of SP-D was created by site-directed mutagenesis. A second variant lacking both the amino-terminal region and the collagen-like domain was generated by collagenase treatment and purification of the collagenase-resistant fragment (CRF). The CDM expressed in CHO-K1 cells formed the covalent trimers, but not the noncovalent dodecamers, typical of native SP-D. The CRF derived from recombinant SP-D formed only monomers. The CDM bound mannose-Sepharose and phosphatidylinositol (PI) as well as SP-D, but the binding to mannosyl bovine serum albumin and glucosylceramide was diminished by approximately 60%. The CRF displayed weak binding to mannose-Sepharose and PI and essentially no binding to mannosyl bovine serum albumin and glucosylceramide. Both SP-D and CDM altered the self-aggregation of PI-containing liposomes. SP-D reduced the density and the light scattering properties of PI aggregates. These results demonstrate that the collagen-like domain is required for dodecamer but not covalent trimer formation of SP-D and plays an important, but not essential, role in the interaction of SP-D with PI and GlcCer. Removal of the amino-terminal domain of SP-D along with the collagen-like domain diminishes PI binding and effectively eliminates GlcCer binding.