Carbohydrate recognition domain of surfactant protein D mediates interactions with Pneumocystis carinii glycoprotein A

Pneumocystis carinii continues to cause severe pneumonia in immunocompromis ed patients. Surfactant protein D (SP-D), a lung collectin, markedly accumu lates during P. carinii pneumonia and binds to glycoprotein A (gpA) on the surface of P. carinii, thereby enhancing interactions with alveolar macroph ages, Herein, we report the structural basis of the interaction of SP-D wit h gpA. We demonstrate that natural SP-D binds to purified gpA in the presen ce of 2 mM calcium in a saturable, concentration-dependent manner, which is abolished by 10 mM ethylenediaminetetraacetic acid. Increasing concentrati ons of calcium under otherwise cation-free conditions significantly enhance SP-D binding to gpA, whereas manganese and magnesium cations have minimal effect. Maximal SP-D binding occurs at pH 7.4, with significant inhibition at pH 4. SP-D binding to gpA is also competitively inhibited by maltose > g lucose > mannose >N-acetyl-glucosamine. Comparison of the binding of variou s natural and recombinant forms of SP-D to gpA reveals that the number of c arbohydrate recognition domains (CRDs) in a given SP-D form determines the relative extent of binding to gpA. Maximal binding is observed with natural SP-D (dodecamers and higher order SP-D complexes) followed by recombinant dodecamers. In contrast, recombinant full-length trimers exhibit substantia lly less binding, which is similar to that observed with a recombinant trun cated molecule consisting of the CRD and neck regions, and containing trime rs of this portion of the molecule. Taken together, these findings strongly indicate that the CRD of SP-D mediates interaction with P. carinii gpA thr ough its attached oligosaccharides and that the extent of SP-D binding to P . carinii is greatest with dodecamers and higher order forms of SP-D.