| cAMP Response Element Binding (CREB) protein is a ubiquitously expressed and highly conserved bZIP transcription factor involved in a diverse array of cellular processes involving cell growth, neuronal development and function, hematopoiesis, and metabolism. As such, it is a heavily modified protein with phosphorylations observed on fourteen different residues via twenty different kinases acting to modulate CREB activity. CREB regulates these processes by active transcription of genes induced in response to cellular signals such as cAMP, calcium, and mitogens predominantly through phosphorylation on serine (Ser)-133 via Protein Kinase A. CREB activity is also negatively impacted in response to DNA damage signals via phosphorylation by other protein kinases. Our lab first identified phosphorylations on a conserved cluster of serine residues, the ATM/CK cluster, by the concerted efforts of Ataxia-telangiectasia Mutated (ATM), Casein Kinase 1 (CK1), and Casein Kinase 2 (CK2).;To determine the role for these sequential modifications of CREB, I have generated a gene-targeted mouse strain, CREBS111A/S111A, which encodes a CREB Ser-111 to Ala mutated version of CREB (CREB S111A) that cannot be phosphorylated on the ATM/CK cluster. Cells from CREBS111A/S111A mice exhibited elevated cAMP-induced transcriptional activity that correlated with increased binding of CREB to its co-activator, CREB Binding Protein (CBP). The enhanced affinity of CREB S111A for CBP correlated with hyper-activation of several CREB target genes upon stimulation with cAMP. These findings suggest that phosphorylation of the ATM/CK cluster attenuates CREB activity in response to cAMP. Additionally, we unexpectedly found that DNA damage-induced activation of p53 was reduced in CREB S111A mice. One plausible explanation for this finding is that CREB S111A functionally sequesters limiting pools of CBP/p300 coactivators, thereby diminishing the p53 response to genotoxic stress.;Other published studies led me to identify two novel phosphorylation sites (Ser-270/271) residing outside of the kinase-inducible domain. These novel phosphorylations on CREB accumulate during the G2/M phase of the cell cycle and require the mitotic cyclin-dependent kinase, CDK1 (Cyclin-Dependent Kinase 1). CDK1-dependent phosphorylation of CREB decreased its interaction with DNA. The combined studies further illuminate the complicated mechanisms of CREB transcription factor regulation by phosphorylation. |
