The effect of dextran on subunit exchange of αA-crystallin

α-Crystallin, a member of small heat shock protein (sHsp) family, is comprised of αA and αB subunits and acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation. The αA-crystallin homopolymer consists of 30-40 subunits that are undergoing dynamic exchange. α-Crystallin and αA-crystallin are poorer chaperones in the presence of the crowding agent, dextran. Using fluorescence resonance energy transfer, it is shown that the αAcrystallin subunit exchange rate strongly increased with temperature. Binding of reduced ovotransferrin to αA-crystallin markedly decreases the rate of subunit exchange, as does the presence of dextran. In addition, in the presence of dextran the effect of reduced ovotransferrin on decreasing the rate of subunit exchange of αA-crystallin is stronger than in the absence of dextran. Under the conditions of molecular crowding, the αA-crystallin subunit exchange rate is not temperature-dependent. The exchange rate of αA-crystallin subunits correlates with its chaperone efficiency i.e. the variation in chaperone ability of αA-crystallin increases with temperature. However, in the presence of dextran the temperature dependence of the chaperone action of αA-crystallin is eliminated.