Carbohydrate recognition domain of surfactant protein D mediates interactions with Pneumocystis carinii glycoprotein A.
Pneumocystis carinii continues to cause severe pneumonia in immunocompromised patients. Surfactant protein D (SP-D), a lung collectin, markedly accumulates during P. carinii pneumonia and binds to glycoprotein A (gpA) on the surface of P. carinii, thereby enhancing interactions with alveolar macrophages. Herein, we report the structural basis of the interaction of SP-D with gpA. We demonstrate that natural SP-D binds to purified gpA in the presence of 2 mM calcium in a saturable, concentration-dependent manner, which is abolished by 10 mM ethylenediaminetetraacetic acid. Increasing concentrations 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>glucose>mannose>N-acetyl-glucosamine. Comparison of the binding of various natural and recombinant forms of SP-D to gpA reveals that the number of carbohydrate 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 substantially less binding, which is similar to that observed with a recombinant truncated molecule consisting of the CRD and neck regions, and containing trimers of this portion of the molecule. Taken together, these findings strongly indicate that the CRD of SP-D mediates interaction with P. carinii gpA through 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.
Vuk-Pavlovic
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Thoracic Diseases Research Unit, Division of Pulmonary, Critical Care, and Internal Medicine, Mayo Clinic and Foundation, Rochester, Minnesota 55905, USA.
Standing
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Crouch
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EC
Limper
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AH
D000117:Acetylglucosamine / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000502:physiology; D001667:Binding, Competitive / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000494:pharmacology; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D006023:Glycoproteins / Q000737:chemistry* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008320:Maltose / Q000378:metabolism / Q000494:pharmacology; D008358:Mannose / Q000378:metabolism / Q000494:pharmacology; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000378:metabolism*; D011010:Pneumocystis; D011020:Pneumonia, Pneumocystis / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D037663:Pulmonary Surfactant-Associated Protein D; D011663:Pulmonary Surfactants / Q000737:chemistry* / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011994:Recombinant Proteins / Q000235:genetics / Q000494:pharmacology
D002241:Carbohydrates; D005656:Fungal Proteins; D006023:Glycoproteins; D008562:Membrane Glycoproteins; D037663:Pulmonary Surfactant-Associated Protein D; D011663:Pulmonary Surfactants; D011994:Recombinant Proteins; C077556:GPA protein, Pneumocystis carinii sp.; D008320:Maltose; D005947:Glucose; D008358:Mannose; D002118:Calcium; D000117:Acetylglucosamine