α-B- and α-A-Crystallin Prevent Irreversible Acidification-Induced Protein Denaturation

Abstract

α-Crystallin (α), a major structural protein of the mammalian lens, is a large, physically heterogeneous macromolecule with an average molecular weight of approximately 800 kDa and is composed of two 20-kDa polypeptides designated as αA and αB. A line of evidence strongly suggests that αB may have an essential nonlenticular function. Here it is demonstrated that αB can bind partially denatured enzymes effectively at acidic pH and prevent their irreversible aggregation, but cannot prevent loss of enzyme activity. However, when the inactive luciferase bound to αB was treated with reticulocyte lysate (a rich source of molecular chaperones) and an ATP-generating system, more than 50% of the original luciferase activity could be recovered. Somewhat less activation was observed when αA-bound enzyme or the α-bound enzyme was renatured similarly. The overall results suggest that α acts as a chaperone to stabilize denaturing proteins at acidic pH so that at a later time they can be reactivated by other chaperones.