The Effect of Concentration on the Structure of α-Crystallin

Abstract

Several models have been proposed for arrangement of the subunits in α-crystallin. These include the contrasting proposals that subunits are arranged in three layers and that subunits assemble into micelle-like structures. The validity of the micelle model was investigated by examining the effects of variations in protein concentration on the surface tension, conductivity, molecular weight and conformation of α-crystallin. The data were compared with those obtained for bovine serum albumin (BSA) and sodium dodecyl sulphate (SDS). Measurements of surface tension were conducted in the range, 10μgml -1to 130mgml -1, in low and high ionic strength buffers. An apparent point of inflection, independent of ionic strength, was seen in α-crystallin's surface tension at around 1.9mgml -1(95μ m). The surface tension did not plateau beyond this point, as is the case with surfactants, but continued to decrease up to 130mgml -1. BSA exhibited similar surface tension properties with an apparent inflection at 0.9mgml -1(13μ m). The conductivity of α-crystallin and BSA solutions increased smoothly with no sign of any transition up to 96mgml -1and 60mgml -1, respectively. In contrast, SDS showed a clear transition in this property at the concentration corresponding to its CMC. The aggregation state of the α-crystallin aggregates was examined by comparing molecular masses and Stokes radii. The size of the protein remained uniform over a wide concentration range and was unaffected by variations in ionic strength. Protein conformation, which was monitored by examining the microenvironment of tryptophan residues, was also found to be independent of protein concentration. It is concluded that over the concentration range that was investigated, α-crystallin does not exhibit any of the properties associated with classical micelles formed from small amphiphilic molecules.