Volume 272, Number 14, Issue of April 4, 1997 pp. 9371-9377
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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

(Received for publication, September 20, 1996, and in revised form, January 21, 1997)

From Clinique de Dermatologie, Hôpital Cantonal Universitaire, CH-1211 Genève 14, Switzerland and the ^§ Ludwig Institute for Cancer Research, S-75124 Uppsala, Sweden

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, -linoleic, -linolenic, and arachidonic acids have IC₅₀ values of about 1 µM, 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 (K_d) of 0.13 µM 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.

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