Functional relationship of serine 90 phosphorylation and the surrounding putative salt bridge in bovine prolactin

The biological activity of bovine prolactin (PRL) is reduced by in vivo phosphorylation of serine 90 (S90) that is located within a putative N+4 salt bridge (R89 and D93). We substituted hydrophobic, polar, or acidic residues for S90 and/or replaced members of the putative R89/D93 salt bridge to determine if a functional relationship between the putative salt bridge and the phosphorylation could be observed. At position 90 the bulk of the residue was the most important factor in modulating biological activity in either the rat Nb2 cell bioassay or PRL receptor binding. Charge played a smaller role. Replacement of either partner of the salt bridge reduced both biological and binding activities indicating the presence of a salt bridge at this position. The combination of replacing a salt bridge member and substituting glutamic acid at S90 produced greater than additive changes in our experimental endpoints, indicating a functional coupling between the salt bridge and phosphorylation site. We interpret the data to indicate that either in vivo phosphorylation or specific mutations that destabilize the salt bridge impairs biological activity.