Recombinant cyclic AMP response element binding protein (CREB) phosphorylated on Ser-133 is transcriptionally active upon its introduction into fibroblast nuclei.

To date, it has not been possible to determine whether the single phosphorylation of the cyclic AMP response element binding factor (CREB) at Ser-133 is sufficient for the transcriptional activation by cAMP-mediated pathways. Previous in vivo studies investigating this point have relied upon transfection of cyclic AMP-dependent kinase (cAPK) or its activation by treatment of cells with cell-permeable cAMP analogs. However, as numerous cellular proteins, including CREB, are substrates for activated cAPK, the possibility remains that cAPK substrates other than CREB are required for the transcriptional activity of CRE-containing promoters. To further address this, we compared the activity of recombinant CREB phosphorylated on Ser-133 in both cell-free transcription assays and in vivo after introduction of the same preparations into fibroblasts by microinjection. The activity of phosphorylated CREB, nonphosphorylated CREB, and a mutant form of CREB, containing Ala substituted for Ser at position 133, was found to be nearly identical in cell-free in vitro transcription assays. In contrast, we found that only the phosphorylated CREB microinjected into fibroblasts resulted in the stimulation of expression of CRE-regulated genes. These results suggest that phosphorylation of CREB on Ser-133 directly stimulates its ability to transactivate gene expression in intact cells.