Abstract
The scavenger receptor C-type lectin (SRCL) is an endothelial receptor that is similar in organization to type A scavenger receptors for modified low density lipoproteins but contains a C-type carbohydrate-recognition domain (CRD). Fragments of the receptor consisting of the entire extracellular domain and the CRD have been expressed and characterized. The extracellular domain is a trimer held together by collagen-like and coiled-coil domains adjacent to the CRD. The amino acid sequence of the CRD is very similar to the CRD of the asialoglycoprotein receptor and other galactose-specific receptors, but SRCL binds selectively to asialo-orosomucoid rather than generally to asialoglycoproteins. Screening of a glycan array and further quantitative binding studies indicate that this selectivity results from high affinity binding to glycans bearing the Lewis(x) trisaccharide. Thus, SRCL shares with the dendritic cell receptor DC-SIGN the ability to bind the Lewis(x) epitope. However, it does so in a fundamentally different way, making a primary binding interaction with the galactose moiety of the glycan rather than the fucose residue. SRCL shares with the asialoglycoprotein receptor the ability to mediate endocytosis and degradation of glycoprotein ligands. These studies suggest that SRCL might be involved in selective clearance of specific desialylated glycoproteins from circulation and/or interaction of cells bearing Lewis(x)-type structures with the vascular endothelium.
Highlights
The binding properties of scavenger receptor C-type lectin (SRCL) have only been partially characterized
Expression and Characterization of the SRCL Extracellular Domains—To initiate a study of the ligand-binding properties of SRCL, the C-terminal carbohydrate-recognition domain (CRD) was expressed in bacteria, whereas the entire extracellular domain was expressed in Chinese hamster ovary cells (Fig. 1)
Similar to SRCL, serum mannose-binding protein and other collectins contain C-type CRDs, collagen-like domains and regions consisting of coiled coils of ␣ helices [31]
Summary
SRCL and the collectins represents independent gene shuffling events that have converged on a similar set of protein structural elements. To assess the biological functions of SRCL, both the sugarbinding characteristics and intracellular trafficking properties of the receptor need to be better defined. To this end, the results of this study demonstrated that the CRD of SRCL binds with unusually high selectivity to glycans containing the Lewisx epitope, that the CRDs are clustered in a trimer, and that the receptor is able to direct uptake and degradation of glycoprotein ligands
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