Expression and divalent cation binding properties of the novel chemotactic inflammatory protein psoriasin

Psoriasin is a novel chemotactic inflammatory protein that possesses weak similarity to the S100 family members of Ca 2+ ‐binding proteins, and that is highly up‐regulated in hyperproliferative psoriatic keratinocytes. Here we have used the psoriasin cDNA to express recombinant human (rh) psoriasin in Escherichia coli as a fusion protein containing a hexa His tag and a factor Xa cleavage site in the NH 2 ‐terminus. The protein was purified by affinity chromatography on Ni 2+ ‐nitrilotriacetic acid agarose, digested with factor Xa, further purified by ion‐exchange chromatography and characterized by two‐dimensional (2‐D) gel electrophoresis and NH 2 ‐terminal sequencing. The ability of rh psoriasin to bind Ca 2+ , Zn 2+ , and Mg 2+ was determined by dialysis experiments. We found that rh psoriasin may bind at least seven molecules of Ca 2+ in KCl and several molecules in NaCl, with an affinity for the first bound molecule of 1.3–1.6 × 10 4 M −1 . This indicates that psoriasin may cooperatively bind several molecules of Ca 2+ when present in the extracellular space, or putatively, if localized in subcellular compartments where the concentration of Ca 2+ is relatively high. At least eight molecules of Zn 2+ were bound in KCl and four in NaCl, with an affinity just below 1 × 10 4 M −1 for the first molecule. Thus psoriasin does not bind significant amounts of Zn 2+ at physiological concentrations. Mg 2+ and Ca 2+ are bound anti‐cooperatively and binding of each of the ions (Ca 2+ , Zn 2+ , or Mg 2+ ), is accompanied by conformational changes that move tyrosine residues to more hydrophobic areas.