Background: Inositol-monophosphatase (IMPase) is a key enzyme regulating the phosphatidylinositol (PI) signaling system. Lithium inhibits IMPase activity at therapeutically-relevant concentrations. Reduced IMPase activity may lead to myo-inositol depletion. Since myo-inositol is used in the re-synthesis of the signal precursor PI, its depletion may attenuate hyperactive signaling. Calbindin-D28k (calbindin) interacts with residues 55-66 of IMPase, enhancing its activity.
Methods:Calbindin-IMPase interaction was modeled by docking these proteins using MolFit. Brain homogenate IMPase activity was assessed measuring liberated phosphate from inositol-1-phosphate in the presence/absence of 30 mM LiCl. To study the effect of short peptides on the enhancing calbindin-IMPase interaction the reaction was carried out with recombinant calbindin and in the presence/absence of each of the peptides.
Results:The calculated modeled interaction found that IMPase's residues 55-66 form an exposed loop interacting with a groove between the N- and C-terminals of calbindin and suggested that the Lys-Glu-Lys motif interacts with Asp24 and Asp26 of calbindin. Calbindin's effect on mouse and human IMPase was strongly reduced by specific six but not five amino-acid peptides which included the Lys-Glu-Lys motif or part of it. A three amino acid peptide comprised of the Lys-Glu-Lys motif had negligible effect.
Conclusions:The elucidation of the molecular characteristics of the activating interaction between calbindin and IMPase may lead to the development of molecules capable of inhibiting endogenous IMPase activity as alternative mood stabilizing drugs.