The Quaternary Structure of Bovine α‐Crystallin: Size and Charge Microheterogeneity: More than 1000 Different Hybrids?

Abstract

Cortical α‐crystallin was size‐fractionated by gel filtration on Ultrogel AcA22 and charge‐fractionated by anion‐exchange chromatography on DE‐52 DEAE‐cellulose using gradient elution. Electron microscopy demonstrates that both native and reassociated α‐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 α‐crystallin and 8.5–12.5 nm (maximum at 10.0–10.5 nm) for reassociated α‐crystallin. An enormous charge heterogeneity of native α‐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 A12−151) 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 α‐crystallin very similar to that of native α‐crystallin. Therfore, 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 observed charge microheterogeneity of both reassociated and native α‐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, A12−151, B2 and B1 conceivable. Copyright © 1978, Wiley Blackwell. All rights reserved