Chronophin dimerization is required for proper positioning of its substrate specificity loop.

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Mammalian phosphatases from the haloacid dehalogenase (HAD) superfamily regulate physiological processes and disease.
• Many HAD enzymes function as homodimers, but the functional significance of this dimerization remains largely unexplored.

Purpose of the Study:

• To investigate the functional role of obligatory homodimerization in chronophin, a mammalian HAD phosphatase.
• To elucidate how chronophin's dimerization impacts its catalytic activity and substrate specificity, particularly for pyridoxal 5 omino-phosphate (PLP).

Main Methods:

• Site-directed mutagenesis of murine chronophin to create a monomeric mutant (chronophin(A194K,A195K)).
• In vitro and cellular assays to assess the catalytic activity of wild-type and mutant chronophin.
• X-ray crystallography to determine the structural basis of chronophin dimerization and its impact on PLP binding.

Main Results:

• The chronophin(A194K,A195K) mutant forms a constitutive monomer, both in vitro and in cells.
• Monomeric chronophin exhibits significantly impaired catalytic activity towards PLP.
• X-ray crystallography revealed that dimer formation facilitates an intermolecular stacking interaction essential for positioning a key histidine residue for PLP coordination.

Conclusions:

• Homodimerization is essential for the catalytic efficiency and substrate specificity of chronophin.
• A conserved dimerization mode involving the substrate specificity loop is observed across C2a-type HAD hydrolases.
• Dimerization-mediated structural rearrangements are critical for the function of HAD superfamily phosphatases.