The effect of concentration on the structure of alpha-crystallin

Several models have been proposed for arrangement of the subunits in alpha-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 alpha-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, 10 micrograms ml-1 to 130 mg ml-1, in low and high ionic strength buffers. An apparent point of inflection, independent of ionic strength, was seen in alpha-crystallin's surface tension at around 1.9 mg ml-1 (95 microM). The surface tension did not plateau beyond this point, as is the case with surfactants, but continued to decrease up to 130 mg ml-1. BSA exhibited similar surface tension properties with an apparent inflection at 0.9 mg ml-1 (13 microM). The conductivity of alpha-crystallin and BSA solutions increased smoothly with no sign of any transition up to 96 mg ml-1 and 60 mg ml-1, respectively. In contrast, SDS showed a clear transition in this property at the concentration corresponding to its CMC. The aggregation state of the alpha-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, alpha-crystallin does not exhibit any of the properties associated with classical micelles formed from small amphiphilic molecules.