A Heterocomplex Formed by the Calcium-binding Proteins MRP8 (S100A8) and MRP14 (S100A9) Binds Unsaturated Fatty Acids with High Affinity

We show that unsaturated fatty acids (FAs) bind reversibly and with high affinity to a heterocomplex of 34 kDa (FA-p34) formed by the non-covalent association of two calcium-binding proteins of the S100 family: MRP8 (S100A8) and MRP14 (S100A9). Fatty acid-competition studies on the [3H]oleic acid.FA-p34-complex show that oleic, alpha-linoleic, gamma-linolenic, and arachidonic acids have IC50 values of about 1 microM, whereas palmitic and stearic acids are poor competitors. The binding of arachidonic acid is saturable with a single class of binding site per FA-p34, and a dissociation constant (Kd) of 0.13 microM is calculated. The individual subunits MRP8 and MRP14 show no binding properties for fatty acids, whereas a p34 complex reconstituted in vitro by the recombinant molecules exhibits binding properties, suggesting that the fatty acid-binding site of FA-p34 is created through heterocomplex formation. Furthermore, we demonstrate that lowering free Ca2+ levels to 16 nM results in a loss of the fatty acid-binding capacity of purified FA-p34. In calcium-induced differentiating keratinocytes, the amounts of FA-p34 are increased in the particulate (2.0 +/- 0.5 pmol of [3H]oleic acid/mg protein) and in the cytosolic (4.5 +/- 0.6 pmol of [3H]oleic acid/mg protein) fractions, whereas no FA-p34 can be detected in non-differentiated cultured keratinocytes. In abnormally differentiated keratinocytes (psoriasis) and in human polymorphonuclear leukocytes, FA-p34 is highly expressed (31.35 +/- 1.6 and 349.8 +/- 17.9 pmol of [3H]oleic acid/mg protein, respectively), pointing toward a role for this heteromer in mediating effects of unsaturated fatty acids in a calcium-dependent way during cell differentiation and/or inflammation.