Effect of Ca2+ and Mg2+ ions on oligomeric state and chaperone-like activity of αB-crystallin in crowded media.

The main function of small heat shock proteins acting as suppressors of aggregation of non-native proteins is greatly influenced by crowded environment in the cell and the presence of divalent metal ions. The goal of the present work was to study the effects of Ca(2+) and Mg(2+) ions on the quaternary structure and anti-aggregation activity of αB-crystallin under crowding conditions. We showed that Ca(2+) and Mg(2+) ions induced formation of suboligomeric forms of αB-crystallin. This effect was retained in the presence of crowder (polyethylene glycol), although to a lesser degree. The chaperone-like activity of αB-crystallin was analyzed using heat-induced aggregation of myosin subfragment 1 (S1) at 40°C. In the absence of crowding agents chaperone-like activity of αB-crystallin exhibited low sensitivity to the presence of Ca(2+) and Mg(2+) ions. The addition of the crowding agents (polyethylene glycol 20000, Ficoll 70) dramatically increased S1 aggregation rates and significantly depressed anti-aggregation activity of αB-crystallin. Low concentrations of Ca(2+) (0.1mM) and Mg(2+) (10mM) partially restored the chaperone-like activity of αB-crystallin in the presence of crowders. This effect was observed at relatively low values of [αB-crystallin]/[S1] molar ratio, however, at [αB-crystallin]/[S1]>0.2 the stimulating effect of Ca(2+) became less pronounced. These findings might indicate that under crowded cell conditions different factors, including divalent cations, can effectively modulate chaperone-like activity of protein chaperones, which otherwise cannot properly cope with crowding-provoked accelerated rates of substrates aggregation.