Previous studies have shown that, in unstimulated mammary epithelial cells from virgin mice, prolactin receptors are retained intracellularly because of their incomplete N-glycosylation. Activation of the nitric oxide/cGMP pathway stimulates Nacetylglucosamine (NAG) transferase I activity, completion of terminal glycosylation, and redistribution of the receptors to the cell surface. In this study, it was shown that nitric oxide could stimulate the phosphorylation of NAG transferase I in intact cells and that the cGMP-dependent protein kinase (PKG) could directly phosphorylate the purified enzyme. Furthermore, this modification was associated with enhanced enzymatic activity. Conversely, this stimulation of activity was blocked in intact cells by coincubation with a PKG inhibitor and reversed in the immunoprecipitated enzyme by alkaline phosphatase treatment. Kinetic analysis revealed that this effect on enzyme activity was due to an increase in V(max) without any change in K(m). Therefore, it appears that the nitric oxide/cGMP pathway activates NAG transferase I via direct phosphorylation by PKG.