Molecular cloning and characterization of endosialin, a C-type lectin-likecell surface receptor of tumor endothelium

Endosialin, the antigen identified with monoclonal antibody FB5, is a highl y restricted 165-kDa cell surface glycoprotein expressed by tumor blood ves sel endothelium in a broad range of human cancers but not detected in blood vessels or other cell types in many normal tissues.:Functional analysis of endosialin has been hampered by a lack of information about its molecular structure. In this study, we describe the purification and partial amino ac id sequencing of endosialin, leading to the Cloning of a full-length cDNA w ith an open reading frame of 2274 base pairs. The endosialin cDNA encodes a type I membrane protein of 757 amino acids with a predicted molecular mass of 80.9 kDa. The sequence matches with an expressed sequence tag of unknow n function in public data bases, named TEM1, which was independently linked to tumor endothelium by serial analysis of gene expression profiling. Bioi nformatic evaluation classifies endosialin as a C-type lectin-like protein, : composed of a signal leader peptide, five globular extracellular domains (including a C-type lectin domain,: one domain with similarity to the Sushi /ccp/scr pattern, and three EGF repeats), followed by a mucin-like region, a transmembrane segment, and a short cytoplasmic tail. Carbohydrate analysi s shows that the endosialin core protein carries abundantly sialylated, O-l inked oligosaccharides and is sensitive to O-sialoglycoprotein endopeptidas e, placing it in the group of sialomucin-like molecules. The N-terminal 360 amino acids of endosialin show homology to thrombomodulin, a receptor invo lved in regulating blood coagulation, and to complement receptor C1qRp. Thi s structural kinship may indicate a function for endosialin as a tumor endo thelial receptor for as yet unknown ligands, a notion now amenable to molec ular investigation.