myo-Inositol monophosphatase is an activated target of calbindin D28k.

Calbindin D28k (calbindin) is a member of the calmodulin superfamily of Ca2+ -binding proteins. An intracellular target of calbindin was discovered using bacteriophage display. Human recombinant calbindin was immobilized on magnetic beads and used in affinity purification of phage-displayed peptides from a random 12-mer peptide library. One sequence, SYSSIAKYPSHS, was strongly selected both in the presence of Mg2+ and in the presence of Ca2+. Homology search against the protein sequence data base identified a closely similar sequence, ISSIKEKYPSHS, at residues 55-66 in myo-inositol-1(or 4)-monophosphatase (IMPase, EC 3.1.3.25), which constitute a strongly conserved, and exposed region in the 3D structure. IMPase is a key enzyme in the regulation of the activity of the phosphatidyl inositol signaling pathway. It catalyzes the hydrolysis of myo-inositol-1(or 4)-monophosphate to form free myo-inositol, maintaining a supply that represents the precursor for inositol phospholipid second messenger signaling systems. Fluorescence spectroscopy showed that isolated calbindin and IMPase interact with an apparent equilibrium dissociation constant, KD, of 0.9 mM. Both apo and Ca2+-bound calbindin was found to activate IMPase up to 250-fold, depending on the pH and substrate concentration. The activation is most pronounced at conditions which otherwise lead to a very low activity of IMPase, i.e. at reduced pH and at low substrate concentration.