In vitro dephosphorylation of alpha-crystallin is dependent on the state of oligomerization

alphaA- and alphaB-crystallin, members of the small heat shock protein family, are present in lens cell extracts as large aggregates. Both alpha-crystallins are found partially phosphorylated. This study tests the ability of five phosphatases (protein phosphatase PP1, PP2A, PP2B, alkaline and acid phosphatases) to dephosphorylate alphaA- and alphaB-crystallin in vitro. Activity of a phosphatase was dependent on the size of the aggregate. Each of the phosphatases tested showed different specificity and efficiency towards alphaA- and alphaB-crystallins, which depended on the oligomeric state of the alpha-crystallin aggregate. Alkaline phosphatase dephosphorylated both alphaA- and alphaB-crystallin. The reaction was faster when alpha-crystallin was in a tetrameric form. PP2A dephosphorylated primarily alphaA-crystallin but only after the conversion of alpha-crystallin to tetramers. PP1 and PP2B did not dephosphorylate either alphaA- or alphaB-crystallins present as large aggregates but could not be tested on the lower molecular weight form of alphaA-crystallin. Acid phosphatase dephosphorylated both alphaA- and alphaB-crystallin. The results suggest that an important relationship exists between the structure of alpha-crystallin and its level of phosphorylation in the cell.