Interactions of surfactant protein D with fatty acids.
Surfactant Protein D (SP-D) plays important roles in antimicrobial host defense, inflammatory and immune regulation, and pulmonary surfactant homeostasis. The best-characterized endogenous ligand is phosphatidylinositol; however, this lipid interaction at least in part involves the carbohydrate moiety. In this study we observed that SP-D binds specifically to saturated, unsaturated, and hydroxylated fatty acids (FA). Binding of biotinylated-SP-D to FAs or biotinylated FA to SP-D was dose-dependent, saturable, and specifically competed by the corresponding unlabeled probe. Specific binding to FA chains was also demonstrated by solution phase competition for FA binding to acrylodan-labeled FA binding protein (ADIFAB), and by overlay of thin layer chromatograms with SP-D. Maximal binding to FA was dependent on calcium, and binding was localized to the neck and carbohydrate recognition domains (CRD) using recombinant trimeric neck+CRDs. Saccharide ligands showed complex, dose-dependent effects on FA binding, and FAs showed dose- and physical state-dependent effects on the binding of SP-D to mannan. In addition, CD spectroscopy suggested alterations in SP-D structure associated with binding to monomeric FA. Together, the findings indicate specific binding of FA to one or more sites in the CRD. We speculate that the binding of SP-D to the fatty acyl chains of surfactant lipids, microbial ligands, or other complex lipids contributes to the diverse biological functions of SP-D in vivo.
DeSilva
Nihal S
NS
Dept. of Pathology and Immunology, Barnes-Jewish Hospital, North campus, Box 8118 216 S. Kingshighway, St. Louis, MO 63110, USA.
Ofek
Itzhak
I
Crouch
Erika C
EC
D000818:Animals; D019921:Biotinylation; D002118:Calcium / Q000378:metabolism; D050260:Carbohydrate Metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002417:Cattle; D002942:Circular Dichroism; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids / Q000737:chemistry / Q000378:metabolism*; D008024:Ligands; D008351:Mannans / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D037663:Pulmonary Surfactant-Associated Protein D / Q000378:metabolism*; D051381:Rats; D011994:Recombinant Proteins; D012710:Serum Albumin, Bovine / Q000378:metabolism
D002352:Carrier Proteins; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids; D008024:Ligands; D008351:Mannans; D010716:Phosphatidylinositols; D037663:Pulmonary Surfactant-Associated Protein D; D011994:Recombinant Proteins; C079339:acrylodated intestinal fatty acid binding protein, recombinant; D012710:Serum Albumin, Bovine; D002118:Calcium