Definition and comparison of the phosphorylation sites of the A and B chains of bovine alpha-crystallin.

The major phosphorylation sites of bovine alpha-crystallin Ser122 in the A chain, Ser59 and Ser43 and/or Ser45 in the B chain have been previously characterized. Further analysis of total alpha-crystallin, isolated from the cortex of calf lenses incubated in the presence of [32P]orthophosphate, demonstrated the presence of additional phosphorylation sites in both chains. At least three additional phosphorylation sites were found in the A chain and at least one in the B chain. These additional sites accounted for approximately 25% of the radioactivity incorporated in the protein. Two general sequences were found in most phosphorylation sites of both chains of alpha-crystallin: (Arg/Lys)-(X)-Pro-Ser and Ser-(X)-Ser-Leu-Ser. In spite of the 57% homology in the sequences of the A and B chains, the phosphorylation sites are located, in the A polypeptide, at the C-terminal third and in the B polypeptide, at the N-terminal third. The alignment of the regions containing the phosphorylation sites of both chains (C-terminal third of the A and N-terminal third of the B chain) revealed an unexpected similarity in the relative positions of the sites in each chain.