The Aggregation of αB-Crystallin under Crowding Conditions Is Prevented by αA-Crystallin: Implications for α-Crystallin Stability and Lens Transparency

One of the most crowded biological environments is the eye lens which contains a high concentration of crystallin proteins. The molecular chaperones alpha B-crystallin (alpha Bc) with its lens partner alpha A-crystallin (alpha Ac) prevent deleterious crystallin aggregation and cataract formation. However, some forms of cataract are associated with structural alteration and dysfunction of alpha Bc. While many studies have investigated the structure and function of alpha Bc under dilute in vitro conditions, the effect of crowding on these aspects is not well understood despite its in vivo relevance. The structure and chaperone ability of alpha Bc under conditions that mimic the crowded lens environment were investigated using the polysaccharide Ficoll 400 and bovine y-crystallin as crowding agents and a variety of biophysical methods, principally contrast variation small-angle neutron scattering. Under crowding conditions, alpha Bc unfolds, increases its size/oligomeric state, decreases its thermal stability and chaperone ability, and forms kinetically distinct amorphous and fibrillar aggregates. However, the presence of alpha Ac stabilizes alpha Bc against aggregation. These observations provide a rationale, at the molecular level, for the aggregation of alpha Bc in the crowded lens, a process that exhibits structural and functional similarities to the aggregation of cataract-associated alpha Bc mutants R120G and D109A under dilute conditions. Strategies that maintain or restore alpha Bc stability, as alpha Ac does, may provide therapeutic avenues for the treatment of cataract. (C) 2020 Elsevier Ltd. All rights reserved.