Equilibrium Folding Studies of Cellular Retinoic Acid Binding Protein, a Predominantly β-Sheet Protein

We have examined the conformational behavior under various unfolding conditions of a predominantly β-sheet protein, cellular retinoic acid binding protein (CRABP). Urea unfolding-refolding of CRABP is a highly cooperative process that can be approximated by a two-state model. Acid denaturation is also cooperative and reversible and leads to a state containing nonnative residual structure: Below pH 2.6, CRABP contains a substantially larger amount of α-helix than under native conditions. CRABP adopts up to 75% α-helix in solutions containing a high percentage of 2,2,2-trifluoroethanol. The acid-denatured state of CRABP undergoes a conformational change to a state containing predominantly β-sheet structure upon the addition of small amounts of Na₂SO₄. This conformational malleability may be important for the folding mechanism of CRABP. The possible implication of nonnative α-helical structure in the folding of CRABP is discussed.