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 Ca2+-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 NH2-terminus. The protein was purified by affinity chromatography on Ni2+-nitrilotriacetic acid agarose, digested with factor Xa, further purified by ion-exchange chromatography and characterized by two-dimensional (2-D) gel electrophoresis and NH2-terminal sequencing. The ability of rh psoriasin to bind Ca2+, Zn2+, and Mg2+ was determined by dialysis experiments. We found that rh psoriasin may bind at least seven molecules of Ca2+ in KCl and several molecules in NaCl, with an affinity for the first bound molecule of 1.3-1.6 x 10(4) M(-1). This indicates that psoriasin may cooperatively bind several molecules of Ca2+ when present in the extracellular space, or putatively, if localized in subcellular compartments where the concentration of Ca2+ is relatively high. At least eight molecules of Zn2+ were bound in KCI and four in NaCl, with an affinity just below 1 x 10(4) M(-1) for the first molecule. Thus psoriasin does not bind significant amounts of Zn2+ at physiological concentrations. Mg2+ and Ca2+ are bound anti-cooperatively and binding of each of the ions (Ca2+, Zn2+, or Mg2+), is accompanied by conformational changes that move tyrosine residues to more hydrophobic areas.