The hydrophobic pocket of 24p3 protein from mouse uterine luminal fluid: Fatty acid and retinol binding activity and predicted structural similarity to lipocalins

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Abstract
The conformation of 24p3 protein purified from mouse uterine luminal fluid was studied by circular dichroism spectroscopy in 200–300 nm. At pH 7.4, the spectrum in the UV region appears as one negative band with a minimum mean residue ellipticity of ‐3,600 deg.cm2.dmole−1 at 217 nm, suggesting a very low or no helical content, but a considerable amount of β‐form, β‐turn, and unordered form in the protein molecule. This agrees with the predicted secondary structures consisting of only one a‐helical segment of residues 150–163 and nine segments of residues 28–35, 50–60, 67–72, 78–86, 94–97, 106–114, 119–125, 136–140 and 166–172 in β‐forms, which would construct two orthonormal β‐sheets to form a less polar β‐barrel. The environments around Trp‐31 and Trp‐81 of this protein were studied by intrinsic fluorescence and solute quenching. They give an emission peak at 332 nm, and only about 21% of them are accessible to quenching by acrylamide. This together with their low accessibility to either CsCl or KI suggests that they are located in the less polar P‐barrel. Hydrophobic compounds such as fatty acids, retinoids, and cholesteryl oleate in the protein solution diminish the protein fluorescence. Analysis of the fluorescence data suggests that the protein has a binding site for hydrophobic ligand. The association constants for the complex formation are 1.03 × 106M−1, 1.92 × 105M−1, 2.38 × 105M−1 or 1.25 × 105M−1for cholesteryl oleate, oleic acid, retinol, or retinoic acid at pH 7.4. Analysis of the equilibrium binding data from binding assay using [H3]‐retinol and [H3]‐retinoic acid reveals a singular type of retinoid‐binding site in the protein with the association constant of 4.92 × 105M−1 and 1.17 × 105M−1 for retinol and retinoic acid, respectively. Trp‐31 or/ and Trp‐81 is in or very near the binding site and the gross conformation of protein changes considerably as the formation of protein‐ligand complex.