The Aggregation of aB-Crystallin under Crowding Conditions Is Prevented by aA-Crystallin: Implications for a-Crystallin Stability and Lens Transparency

Abstract

One of the most crowded biological environments is the eye lens which contains a high concentration of crystallin proteins. The molecular chaperones aB-crystallin (aBc) with its lens partner aA-crystallin (aAc) prevent deleterious crystallin aggregation and cataract formation. However, some forms of cataract are associated with structural alteration and dysfunction of aBc. While many studies have investigated the structure and function of aBc 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 aBc under conditions that mimic the crowded lens environment were investigated using the polysaccharide Ficoll 400 and bovine a-crystallin as crowding agents and a variety of biophysical methods, principally contrast variation small-angle neutron scattering. Under crowding conditions, aBc 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 aAc stabilizes aBc against aggregation. These observations provide a rationale, at the molecular level, for the aggregation of aBc in the crowded lens, a process that exhibits structural and functional similarities to the aggregation of cataract-associated aBc mutants R120G and D109A under dilute conditions. Strategies that maintain or restore aBc stability, as aAc does, may provide therapeutic avenues for the treatment of cataract. Show more