Figure 5.
Determinants of Target Specificity.
(A) Dose-dependent inhibition of Arabidopsis PME by protein chimera between PMEI and the invertase inhibitor CIF.
(B) Schematic representation of protein chimera. In XPMEI-CIF, the α-hairpin of PMEI has been connected to the bundle core of CIF. XCIF-PMEI combines the corresponding hairpin segment of CIF with the four-helix bundle of PMEI. The color coding follows the structural representations in Figure 1.
(C) Proposed model for the inactivation of plant PME by PMEI. Schematic view, in which the four-helix bundle covers the enzyme's pectin binding cleft, which measures ∼40 Å in length. The helical hairpin anchors the inhibitor to its target enzyme and mediates specificity.
(D) Manual docking of PMEI onto the pectin binding cleft of plant PME. Carrot PME (PDB ID 1gq8) is shown in both ribbon and molecular surface representation in a view along the cleft region. The inhibitor (in green) covers the entire cleft. Active-site residues and residues W227/252 discussed in the text are highlighted in red and blue, respectively. The longer loop regions of the bacterial enzyme (PDB ID 1qjv) are indicated in magenta.