Fig. 1.

Structure of protein isoaspartate O-methyltransferase. (A) Ribbon structure of PIMT [1I1N.PDB (Smith et al., 2002)]. SAH interacts with residues in the core β-sheet (β5, β4, β3, β6, β7) and is partially buried by residues in β2 (gold) and α8 (green). The isoaspartate-binding surface consists of residues in β1, β2, β9, α3 and α8, and is positioned on the opposite face of the protein from the polymorphic site. Residue 119 is positioned 13 Å from the SAM-binding site and on the opposite face of the protein from the substrate-binding site, ∼15 Å from the SD atom of SAH. SAH and residue 119 are shown in space-filling representation and colored by atom type and in cyan, respectively. Residues that contribute to the isoaspartate-binding site are shown in stick representation and colored in red. (B) SAH is buried within the PIMT structure, with its SD atom (CE in SAM) oriented towards the center of the isoaspartate-binding surface. PIMT is shown in surface representation, with isoaspartate-binding residues, SAM-binding residues, β-strands 1 and 2, α8 and residue 119 colored in red, blue, gold, light green and cyan, respectively. SAH is colored by atom type. A color version of this figure is available as supplementary data at PEDS online.