INTERMOLECULAR EXCHANGE AND STABILIZATION OF RECOMBINANT HUMAN ALPHA-A-CRYSTALLIN AND ALPHA-B-CRYSTALLIN

Lens alpha-crystallin subunits alpha A and alpha B are differentially expressed and have a 3-to-1 ratio in most mammalian lenses by intermol ecular exchange. The biological significance of this composition and t he mechanism of exchange are not clear. Preparations of human recombin ant alpha A- and alpha B-crystallins provide a good system in which to study this phenomenon. Both recombinant alpha A- and alpha B-crystall ins are folded and aggregated to the size of the native cu-crystallin. During incubation together, they undergo an intermolecular exchange a s shown by native isoelectric focusing. Circular dichroism measurement s indicate that the protein with a 3-to-1 ratio of alpha A- and alpha B-crystallins has the same secondary structure but somewhat different tertiary structures after exchange: the near-UV CD increases after exc hange. The resulting hybrid aggregate is more stable than the individu al homogeneous aggregates: at 62 degrees C, alpha B-crystallin is more susceptible to aggregation and displays a greater light scattering th an alpha A-crystallin. This heat-induced aggregation of alpha B-crysta llin, however, was suppressed by intermolecular exchange with alpha A- crystallin, These phenomena are also observed by fast performance liqu id chromatography gel filtration patterns. The protein structure of al pha B-crystallin is stabilized by intermolecular exchange with alpha A -crystallin.