Binding of dexamethasone by alpha-crystallin

Long-term steroid therapy is a known risk factor for the development of posterior subcapsular cataract. Previous work in this laboratory has found soluble lens proteins to bind dexamethasone, but this binding is not due to a glucocorticoid receptor. This study was undertaken to identify the soluble protein or proteins involved in lens glucocorticoid binding.

Bovine lens extract was incubated with 5.2 x 10(-)(8) M [(3)H]-dexamethasone for 3 hours, and the distribution of label assessed in the soluble and insoluble fractions after centrifugation. Soluble lens extract was fractionated using gel permeation chromatography to isolate and identify proteins involved in the binding. Total lens proteins, high-molecular-weight proteins, or alpha-crystallin were exposed to dexamethasone and the protein bound steroid measured after separation of free and bound ligand on a gel chromatography column. Scatchard analysis was used to determine dexamethasone-binding parameters. Sequence comparisons between bovine alphaA- and alphaB-crystallins and glucocorticoid-binding proteins were performed using a sequence-alignment program.

Of the total dexamethasone bound in lens extract, soluble proteins were found to account for 52%. The majority of the soluble protein-bound dexamethasone coeluted with the high-molecular-weight proteins that consisted mainly of alpha-crystallin. Binding studies with isolated proteins showed that alpha-crystallin accounted for more than 98% of total soluble dexamethasone binding in the lens. Scatchard analysis of steroid binding showed it to be a nonspecific partitioning event. Sequence comparisons between alphaA- and alphaB-crystallins and various glucocorticoid-binding proteins showed the lens proteins to have three regions of sequence homology with yeast corticosteroid-binding protein.

alpha-Crystallin is the principal soluble glucocorticoid binding protein in the lens. The steroid association is described by nonspecific partitioning and may be related to the unique structural characteristics of the protein. The nonspecific association with alpha-crystallin is not thought to be functional; however, it may aid in the increased covalent steroid modification observed for this protein.