MUTATION OF SERINE-90 TO GLUTAMIC-ACID MIMICS PHOSPHORYLATION OF BOVINE PROLACTIN

Phosphorylated prolactin has been identified and isolated from bovine pituitaries. The biological activity of this phosphoprotein is severel y reduced in comparison with nonphosphorylated prolactin, The sites of phosphorylation are serines 26, 34, and 90, and the stoichiometry is 1:1:10, respectively, In this report, the phosphoserine residues have been individually replaced with glutamic acid in recombinant methionyl bovine prolactins in order to mimic phosphorylation at each site, Sub stitution of glutamic acid for serine at positions 26, 34, and 90 redu ced protein helical contents by 10, 6, and 14%, respectively, UV absor bances for S26E and S34E bovine prolactins were blue-shifted, similar to the biological isolates of phosphorylated bovine prolactin, but the biological activities of the S26E and S34E mutants (ED(50) values of 16.3 and 18.8 pM, respectively) were similar to that of wild-type prol actin (ED(50) value of 18.6 pm) in the Nb2 rat lymphoma assay, S90E bo vine prolactin had the greatest reduction in helical content but showe d similar UV and fluorescent spectra to the wildtype bovine prolactin, The biological activity of S90E bovine prolactin (ED(50) value of 672 pm) was reduced to an activity similar to that of phosphorylated bovi ne prolactin, The data indicate that the phosphorylation of serine 90 is responsible for the reduction in biological activity.