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 Å 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.