The phosphorylation sites of the B2 chain of bovine alpha-crystallin

The B2 chain of bovine lens alpha-crystallin is phosphorylated in a cAMP-dependent reaction. By analysis of 32P-labelled chymotryptic peptides isolated from alpha-crystallin obtained from lenses labelled in organ culture, two phosphorylated B2 chain fragments were found. Sequence analysis of the fragments gave the following results: Arg-Ala-Pro-Ser-Trp-Ile-Asp-Thr-Gly-Leu and Ser-Leu-Ser-Pro-Phe corresponding to residues 56 to 65 and 43 to 47, respectively. It is established by this work that B1 is a phosphorylated post-translational product of B2. Both the A2 and B2 chains of alpha-crystallin are phosphorylated at a similar site with the sequence Arg-(X)-Pro-Ser. This is an unusual site for cAMP-phosphorylation since the phosphorylated serine is preceded by a proline residue. It may also be of significance that the other B2 chain phosphorylation site even more radically differs from previously reported cAMP-dependent phosphorylation sites.