Crystallization of rat cellular retinol binding protein II. Preliminary X-ray data obtained from the apoprotein expressed in Escherichia coli

Intestinal fatty acid binding protein: characterization of mutant proteins containing inserted cysteine residues

Site-directed mutagenesis was used to introduce cysteine residues into the rat intestinal fatty acid binding protein, an almost all beta-sheet protein that in the wild-type contains neither cysteine nor proline residues. Six mutants (I23C, S53C, V60C, L72C, L89C, and A104C) with a single cysteine residue substituted for a hydrophobic residue were characterized by their stability toward denaturants at pH 7.2 and 9.6, by their fluorescent properties, and by their reactivity toward the sulfhydryl modifying reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and 4,4'-dipyridyl disulfide (4-PDS). In terms of protein stability, the substitutions were reasonably conservative with only two (V60C and L89C) being somewhat less stable than the wild-type. The mutant proteins differed considerably, however, in their reactivity toward the modifying reagents. One residue, Cys89, located in a hydrophobic core near a turn between two beta-strands, was unreactive, while two residues, Cys60 and Cys104, located in the middle of beta-strands in the cavity into which fatty acid binds, reacted only very slowly and were further protected by oleate. Cys53, located near a turn and partially buried, appeared to have an unusually low pK value. Two residues, Cys23 and Cys72, reacted more rapidly in the native protein than in the unfolded protein. Both residues are located near the portal for the fatty acid binding, and one, Cys72, was strongly protected from modification by the presence of oleate. Examination of the crystal structure indicates that Cys72 is not easily solvent-accessible. We conclude that this high reactivity for this residue may be a consequence of rapid conformational flexibility in this region of the structure.

Chromatographic method for the preparation of apo-cellular retinol-binding protein and apo-cellular retinoic acid-binding protein from their holo-types

A chromatographic method is described for the preparation of apo-cellular retinol-binding protein (CRBP) and apo-cellular retinoic acid-binding protein (CRABP) from their corresponding holoproteins. Elimination of retinoids from either purified CRBP or CRABP holoprotein complex could be performed quantitatively by DEAE-cellulose chromatography without any alteration in the inherent properties of the native proteins. In contrast, the usual methods, involving UV irradiation or acetone precipitation, resulted in some modification of these binding proteins. This chromatographic method was also applicable to the preparation of apo-fatty acid-binding protein (FABP) from FABP-palmitic acid holoprotein complex.