Glycosylation is a widespread post-translational modification Found in glycoproteins Glycans play key roles in protein folding. quality control in the endoplasmic reticulum (ER) and protein trafficking within cells However, it remains unclear whether all positions of protein glycosylation are involved in glycan functions, or if specific positions have individual roles Here we demonstrate the integral involvement of a specific N-glycan from amongst the three glycans present oil inducible costimulator (ICOS), a T-cell costimulatory molecule. in proper protein folding and intracellular trafficking to the cell surface membrane We found that glycosylation-defective mutant proteins lacking N-glycan at amino-acid position 89 (N89). but not proteins lacking either N23 or N110. were retained within the cell and were not detected oil the cell surface membrane Additional evidence Suggested that N89 glycosylation was indirectly involved in ICOS ligand binding These data suggest that amongst the three putative ICOS glycosylation sites, N89 is requried for proper ICOS protein folding in the ER, intracellular trafficking and ligand binding activity. This study represents a substantial contribution to the current mechanistic understanding of the necessity and potential functions of a specific N-glycan among the multiple glycans of glycoproteins. (C) 2009 Elsevier Inc All rights reserved