Calf lens α-crystallin quaternary structure: A three-layer tetrahedral model☆

Abstract Calf lens α-crystallins are polydisperse globular particles made of a large number of two types of subunits, A and B, both of molecular weight ∼- 20,000. α-Crystallin populations consisting on average of 40 subunits or more were subjected to various changes in pH, ionic strength, temperature and urea concentration. Modifications in quaternary structure induced by variation of these physicochemical parameters were followed by means of X-ray and quasi-elastic light-scattering and quantified in terms of weight average molecular weight ( M ), radius of gyration ( R g ) and hydrodynamic radius ( R h ). High-Pressure liquid chromatography was used as a control of polydispersity. Increasing the pH, decreasing the ionic strength and incubating at temperatures from 20 °C to 45 °C all resulted in the formation of particles of decreasing M , R g and R h values. These effects are cumulative. All monomodal α-crystallin populations encountered in this study, which covers a wide range of sizes and molecular weights, may be accounted for by a three-layer model with partial filling up of the layers. Applying basic principles of symmetry and postulating specific contacts between protein subunits to construct this three-layer model leads to tetrahedral symmetry, with 12, 24 and 24 sites in the first, second and third layers, respectively. Variations in probabilities of site occupancy account for both the observed quaternary structure modifications and the intrinsic polydispersity of α-crystallins.