Investigation of γE-crystallin target protein binding to bovine lens alpha-crystallin by small-angle neutron scattering

α-Crystallin, one of the main constituent proteins in the crystalline lens, is an important molecular chaperone both within and outside the lens. Presently, the structural relationship between α-crystallin and its target proteins during chaperone action is poorly understood. It has been hypothesised that target proteins bind within a central cavity. Small-angle neutron-scattering (SANS) experiments in conjunction with isotopic substitution were undertaken to investigate the interaction of a target lens protein (γE-crystallin) with α-crystallin (αH) and to measure the radius of gyration (Rg) of the proteins and their binary complexes in solution under thermal stress. The size of the αH in D2O incubated at 65 °C increased from 69 ± 3 to 81 ± 5 Å over 40 min, in good agreement with previously published small-angle X-ray scattering (SAXS) and SANS measurements. Deuterated γE-crystallin in H2O buffer (γED/H2O) and hydrogenous γE-crystallin in D2O buffer (γEH/D2O) free in solution were of insufficient size and/or too dilute to provide any measurable scattering over the angular range used, which was selected primarily to investigate γE:αH complexes. The evolution of the aggregation size/shape as an indicator of αH chaperone action was monitored by recording the neutron scattering in different H:D solvent contrasts under thermally stressed conditions (65 °C) for binary mixtures of αH, γEH, and γED. It was found that Rg(αH:γED/D2O) > Rg(αH:γEH/D2O) > Rg(αH/D2O) and that Rg(αH:γEH/D2O) ≈ Rg(αH/D2O). The relative sizes observed for the complexes weighted by the respective scattering powers of the various components imply that γE-crystallin binds in a central cavity of the α-crystallin oligomer, during chaperone action.