DQ2 confers susceptibility to celiac disease (CD) and intestinal CD4(+) T cells of DQ2(+) CD patients preferentially recognize deamidated gliadin peptides. This modification can be mediated by tissue transglutaminase (tTG). We have investigated what role the tTG-modified residues play in DQ2 binding and T cell presentation using a model gamma-gliadin peptide (residues 134 - 153). Treatment of this peptide with tTG resulted in deamidation of Gln residues at positions 140, 148 and 150. Two of these residues act as DQ2 anchors at position P7 (148) and P9 (150) and increased the affinity of the modified peptide for DQ2 50-fold. Testing of a mutant DQ2 molecule demonstrated that the Lys residue at beta71 of DQ2 is important for binding of the deamidated peptide. A variant DQ2 molecule (with the same beta-chain but different alpha-chain) that does not confer susceptibility to CD was capable of presenting the gliadin peptide, but not pepsin/trypsin-digested gliadin, equally well to a T cell. This suggests that processing events might be involved in the preferential presentation of the gliadin peptide by the DQ2 molecule. Substitution of Gln with Glu in some positions not targeted by tTG, but in positions likely to be deamidated via non-enzymatic mechanisms, disrupted T cell recognition. This provides additional evidence that tTG is responsible for modification of gliadin in vivo.