The chaperone activity of bovine alpha crystallin. Interaction with other lens crystallins in native and denatured states.

Abstract

It has previously been reported that alpha crystallin (alpha) a major lens protein composed of alpha A and alpha B subunits, can act as a chaperone interacting with other proteins to prevent heat-induced insolubilization. It is now shown that with gamma (gamma), beta L (beta L), and beta H (beta H), other major crystallin groups, this interaction occurs exclusively with soluble denatured protein. Based on studies primarily conducted with the gamma and the beta L crystallins, there is at least one binding site per alpha monomer (alpha m). This conclusion is derived from the following evidence. The binding of soluble denatured protein to alpha increases in a linear stoichiometric manner until a 1:1 ratio of alpha m to gamma or to the presumed (beta L)m is achieved. This is based upon determination of the apparent molecular weight of the alpha-denatured protein aggregate with a calibrated TSK-G4000 SW column and on the determination of the relative masses from the areas of the aggregate peak and those of the reactants. SDS-polyacrylamide gel electrophoresis confirms that the aggregates contain the presumed components following reaction with either gamma, beta L, or beta H. Studies in which gamma or beta L crystallins have been independently heat denatured indicate that species representing 50% of the gamma population and 30% of beta L population have not heat denatured under the conditions employed. When the abundance of the denatured soluble protein exceeds that of the alpha subunits present in the macromolecular complex, further interaction occurs leading to a loss of solubility of the complex. A small increase in the size of the complex remaining in solution is also observed. The results contribute to understanding earlier observations that soluble native low molecular weight alpha species cannot be found in the inner regions of old human lenses but have shifted to large aggregates containing other crystallin components. The work tentatively suggests that the interaction rates of alpha with denatured soluble crystallins are gamma > beta L > beta H.