Glycan-mediated binding to collagen by a C-type lectin-like domain of the endocytic receptor Endo180 / uPARAP

The primary role of the multidomain endocytic receptor designated Endo180 or uPARAP (urokinase-plasminogen activator receptor associated protein) is to facilitate internalization of collagen for lysosomal degradation during matrix remodeling in tissues. However, Endo180 interacts both with collagen and sugars and the relationship between these activities is not fully defined. While collagen-binding activity is associated with a fibronectin type 2 domain and an adjacent C-type lectin-like domain 1 (CTLD1), sugar-binding activity resides in CTLD2. Potential interactions of CTLD2 with various glycans have been examined by glycan array screening, solid phase binding assays, x-ray crystallography, and molecular modeling. Glycan array screening suggests potential interactions with exposed GlcNAc residues on truncated core 2 and core 4 O-linked glycans, but such structures are rare. Binding competition assays demonstrate that the disaccharide Glcα1-2Gal, found attached to hydroxylysine residues in collagens, is an alternative ligand. X-ray crystallography analysis of a complex of α-methyl fucoside with CTLD2 provides a basis for modeling how the Glcα1-2Gal disaccharide attached to hydroxylysine can be accommodated in the binding site with both sugar residues making favorable interactions with the protein. In direct binding studies, types IV and V collagen, but not type I collagen, bind to CTLD2 through the sugar-binding site. These results provide direct evidence that sugar-binding activity of CTLD2 contributes to binding of heavily glycosylated collagens. Although other endogenous mammalian glycan targets for CTLD2 are rare, CTLD2 can additionally interact with pathogen glycans such as poly-N-acetylglucosamine, contributing to the general role of Endo180 as a scavenger receptor.