The phosphorylation of the primary gene products of alpha-crystallin

The alpha-crystallin primary gene product A2 and its post-translational modified counterpart A1 were isolated from calf lens cortex. The amino acid compositions determined from both chains were almost identical and in excellent agreement with that calculated from the reported sequence of A2. Chemical analysis of phosphate revealed 1 mol/mol of A1 and was negative in A2. Phosphoamino acid analysis demonstrated the presence of phosphoserine only in A1. Chymotryptic peptide maps of A2 and A1 resolved approximately 50 peptides and were strikingly similar. An apparent change in the relative mobility of one peptide was the only difference observed between A1 and A2. Phosphate analysis of this peptide obtained from A1 and A2 was positive only in the peptide from A1. Identical amino acid composition and the sequence Arg-Leu-Pro-Ser-Asn-Val-Asp-Gln-Ser-Ala-Leu was found for the peptide isolated from both chains, corresponding to residues 119 to 129 in the reported sequence of A2. These results indicate that the post-translational modification of A2 to A1 is the result of a phosphorylation reaction rather than a spontaneous nonenzymatic deamidation as previously suggested.