The Quaternary Structure of Bovine alpha-Crystallin. Size and Charge Microheterogeneity: More than 1000 Different Hybrids?

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Abstract
Cortical .alpha.-crystallin [from calf lenses] was size-fractionated by gel filtration on Ultrogel AcA22 and charge-fractionated by anion-exchange chromatography on DE-52 DEAE-cellulose using gradient elution. EM demonstrates that both native and reassociated .alpha.-crystallin are heterogeneous populations of spherical or slightly ellipsoidal molecules with diameters of 13.5 - 16.0 nm (maximum at 14.0 - 15.0 nm) for native .alpha.-crystallin and 8.5 - 12.5 nm (maximum at 10.0 - 10.5 nm) for reassociated .alpha.-crystallin. An enormous charge heterogeneity of native .alpha.-crystallin was detected, which is shown to arise from variations in the stoichiometry of the 5 main types of subunits. The molar ratio of acidic chains (A2, A1 and .**GRAPHIC**. to basic chains (B2 and B1) varies from 70/30 - 80/20 (averaging about 3/1) and the amount of deamidated chains (A1 and B1) varies from 7 - 37%. Recombination of the subunits after dissociation in 6 M urea, leads to a charge heterogeneity of reassociated .alpha.-crystallin very similar to that of native .alpha.-crystallin. Therefore, specific formation of pure A or B chain aggregates is not preferred. Instead, random combination of subunits is theoretically shown to be sufficient to describe the charge microheterogeneity of both reassociated and native .alpha.-crystallin. No obvious relationship exists between size and charge heterogeneity. Within these ranges of molecular weight and subunit composition there are more than 1000 different combinations of A2, A1, .**GRAPHIC**. B2 and B1 conceivable.