Semi-synthesis of a tag-free O-GlcNAcylated tau protein by sequential chemoselective ligation.

In this paper, the first semi-synthesis of the Alzheimer-relevant tau protein carrying an O-GlcNAcylation is demonstrated by using sequential chemoselective ligation. The 52-amino acid C-terminus of tau was obtained by native chemical ligation between two synthetic peptide fragments, one carrying the O-GlcNAc moiety on Ser400, which has recently been demonstrated to inhibit tau phosphorylation and to hinder tau oligomerization, and the other equipped with a photocleavable biotin handle. After desulfurization to deliver a native alanine at the ligation junction, the N-terminal cysteine was unmasked, and the peptide was further used for expressed protein ligation to generate the full-length tau protein, which was purified by a photocleavable biotin tag. We thus provide a synthetic route to obtain a homogenous tag-free O-GlcNAcylated tau protein that can further help to elucidate the significance of posttranslational modification on the tau protein and pave the way for evaluating possible drug targets in Alzheimer's disease.