THE CARBOHYDRATE-RECOGNITION DOMAIN OF SURFACTANT PROTEIN-A MEDIATES BINDING TO THE MAJOR SURFACE GLYCOPROTEIN OF PNEUMOCYSTIS-CARINII

Pneumocystis carinii is a common cause of life-threatening pneumonia i n immunodeficient patients. Pulmonary surfactant protein A (SP-A), an alveolar glycoprotein containing collagen-like and carbohydrate recogn ition domains (CRD), binds P. carinii and enhances adherence to alveol ar macrophages. In this study, we examined the structural basis of the interaction between SP-A and the major surface glycoprotein of P. car inii (MSG). Rat SP-A bound to purified rat P. carinii-derived MSG in a saturable and calcium-dependent manner, which was partially reversibl e by coincubation with excess monosaccharides, or pretreatment of MSG with N-glycanase. Mutant recombinant SP-As with neutral amino acid sub stitutions for the predicted calcium- and carbohydrate-coordinating re sidues of the CRD were synthesized in insect cells using baculovirus v ectors and tested for binding to MSG. Substitutions of negatively char ged (Glu(195), Glu(202) and Asp(215)) and polar residues (Asn(214)) of the CRD with alanine but not substitution of the Arg(197) with glycin e reduced the binding of SP-A to mannose-Sepharose beads and to MSG. D eletion of the N-linked oligosaccharides from SP-A by mutagenesis of t he consensus sequences for glycosylation had no effect on binding. We conclude that the CRD mediates the binding of SP-A to oligosaccharides attached to MSG.