In Alzheimer's disease (AD), tau protein is abnormally hyperphosphorylated and aggregated into paired helical filaments (PHFs). It was discovered recently that tau is also O-GlcNAcylated in human brains. And O-GlcNAcylation may regulate phosphorylation of tau in a site-specific manner. In this work, we focused on the fourth microtubule-binding repeat (R4) of tau, which has an O-GlcNAcylation site-Ser356. The aggregation behavior of this repeat and its O-GlcNAcylated form was investigated by turbidity, precipitation assay and electron microscopy. In addition, conformations of these two peptides were analyzed with circular dichroism (CD). Our results revealed that O-GlcNAcylation at Ser356 could greatly slow down the aggregation speed of R4 peptide. This modulation of O-GlcNAcylation on tau aggregation implies a new perspective of tau pathology. (C) 2008 Elsevier Inc. All rights reserved.