Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 275, Issue 28, Pages 21539-21548Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M002366200
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- NIGMS NIH HHS [GM50565] Funding Source: Medline
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The mannose receptor of macrophages and liver endothelium mediates clearance of pathogenic organisms and potentially harmful glycoconjugates. The extracellular portion of the receptor includes eight C-type carbohydrate recognition domains (CRDs), of which one, CRD-4, shows detectable binding to monosaccharide ligands, We have determined the crystal structure of CRD-4, Although the basic C-type lectin fold is preserved, a loop extends away from the core of the domain to form a domain-swapped dimer in the crystal. Of the two Ca2+ sites, only the principal site known to mediate carbohydrate binding in other C-type lectins is occupied. This site is altered in a way that makes sugar binding impossible in the mode observed in other C-type lectins, The structure is likely to represent an endosomal form of the domain formed when Ca2+ is lost from the auxiliary calcium site, The structure suggests a mechanism for endosomal ligand release in which the auxiliary calcium site serves as a pH sensor. Acid pH-induced removal of this Ca2+ results in conformational rearrangements of the receptor, rendering it unable to bind carbohydrate ligands.
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