Conversion from oligomers to tetramers enhances autophosphorylation by lens alpha A-crystallin. Specificity between alpha A- and alpha B-crystallin subunits.

Previously we showed that alpha-crystallins are autophosphorylated (Kantorow, M., and Piatigorsky, J. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 3112-3116). Here we report that addition of 1% deoxycholate converted alpha A-crystallin aggregates into 80-kDa tetramers which were 10-fold more active for autophosphorylation. Circular dichroism (CD) spectra of alpha-crystallin revealed little or no change in secondary and tertiary structures in 1% deoxycholate, alpha A2D, a truncated form of bovine alpha A that exists as a tetramer, was as active for autophosphorylation in the absence of deoxycholate as intact alpha A was in the presence of deoxycholate. At least one serine between amino acids 131 and 145 of bovine alpha A was autophosphorylated in peptide mapping experiments. Chicken alpha A-crystallin, which lacks the Ser-122 cAMP-dependent kinase site of bovine alpha A, was also autophosphorylated in the presence of deoxycholate. In contrast to alpha A-crystallin, autophosphorylation by alpha B-crystallin was not activated by deoxycholate despite its conversion to a tetrameric form, and alpha B was also more efficiently phosphorylated by cAMP-dependent kinase than alpha A. These data suggest metabolic differences between the alpha-crystallin subunits that may be related to specific expression of alpha A in the lens and ubiquitous expression of alpha B in numerous normal and diseased tissues.