Multiple crystal forms of the cell-wall invertase inhibitor from tobacco support high conformational rigidity over a broad pH range.

Plant acid invertases catalyse the breakdown of sucrose. Their activity is tightly regulated through interaction with specific protein inhibitors. The complex between the cell-wall invertase inhibitor Nt-CIF and its target enzyme is stable only at acidic pH, as found in the plant cell wall. Since the pH in this compartment can be modulated between pH 4 and 6 in planta, the rapid dissociation of the inhibitor-enzyme complex at neutral pH may represent a regulatory event. Here, it is analyzed whether the inhibitory component undergoes structural rearrangements upon changes in the pH environment. Six crystal forms grown at pH 4.6-9.5 and diffracting up to 1.63 Angstrom indicate only small structural changes in CIF. This suggests that complex dissociation at neutral pH is mediated either by rearrangements in the enzyme or by a complex pattern of surface charges in the inhibitor-enzyme binding interface.