Syntheses of alpha-dystroglycan derived glycosyl amino acids carrying a novel mannosyl serine/threonine linkage.

Alpha-dystroglycan (alpha-DG) is a membrane-associated, extracellular glycoprotein. It is anchored to the cell-membrane by binding to the transmembrane glycoprotein beta-dystroglycan (beta-DG) to form an alpha/beta-DG-complex. It was discovered that the bovine peripheral nerve alpha-DG possesses the Ser/Thr linked tetrasaccharide as the major constituent of the O-linked carbohydrates, which was proposed to contribute laminin binding activity of this glycoprotein. This structure has a striking feature in terms of the mode of linkage between oligosaccharide and the core protein. It has a mannose residue linked to the core protein through Ser/Thr residue. A similar structure was proposed to exist in brain derived HNK-1 immunoreactive O-glycans. Being interested in the structural novelty and potential biological significance of this type of glycan chains, the chemical synthesis of Ser/Thr linked mannose containing tetrasaccharide was investigated. Tetrasaccharide donor was constructed from monosaccharide blocks and coupled with Ser/Thr derivatives. Subsequent deprotection afforded target tetraosyl serine. Furthermore, synthetic routes to lower homologues, namely Gal-beta-(1,4)-GlcNAc-beta-(1,2)-Man-alpha-Ser and GlcNAc-beta-(1,2)-Man-alpha-Ser were also provided.