Chronophin dimerization is required for proper positioning of its substrate specificity loop [Enzymology]

December 14th, 2013 by Kestler, C., Knobloch, G., Tessmer, I., Jeanclos, E., Schindelin, H., Gohla, A.

Mammalian phosphatases of the haloacid dehalogenase (HAD) superfamily have emerged as important regulators of physiology and disease. Many of these enzymes are stable homodimers, yet the role of their dimerization is largely unknown. Here, we explore the function of the obligatory homodimerization of chronophin, a mammalian HAD phosphatase known to dephosphorylate pyridoxal 5′-phosphatase (PLP) and serine/threonine-phosphorylated proteins. The exchange of two residues in the murine chronophin homodimerization interface (chronophinAla194Lys, Ala195Lys; chronophinKK) yields a constitutive monomer, both in vitro and in cells. The catalytic activity of monomeric chronophin towards PLP is strongly impaired. X-ray crystallographic studies of chronophinKK reveal that dimer formation is essential for an intermolecular arginine-arginine-tryptophan stacking interaction that positions a critical histidine residue in the substrate specificity loop of chronophin for PLP coordination. Analysis of all available crystal structures of HAD hydrolases that are grouped together with chronophin in the C2a-type structural subfamily uncovers a highly conserved mode of dimerization that results in intermolecular contacts involving the substrate specificity loop. Our results explain how the dimerization of HAD hydrolases contributes to their catalytic efficiency and substrate specificity.