Phosphorylation of the transcription factor CREB leads to the recruitment of the coactivator, CREB binding protein (CBP). Recent studies have suggested that CBP recruitment is not sufficient for CREB function, however. We have identified a conserved protein-protein interaction motif within the CBP-binding domains of CREB and another transcription factor, SREBP (sterol-responsive element binding protein). In contrast to CREB, SREBP interacts with CBP in the absence of phosphorylation. We have exploited the conservation of this interaction motif to test whether CBP recruitment to CREB is sufficient for transcriptional activation. Substitution of six nonconserved amino acids from SREBP into the activation domain of CREB confers high-affinity, phosphorylation-independent CBP binding. The mutated CREB molecule, CREB DIEDML , activates transcription in F9 teratocarcinoma and PC12 cells even in the absence of protein kinase A (PKA). Addition of exogenous CBP augments the level of transcription mediated by CREB DIEDML , and adenovirus 12S E1A blocks transcription, implicating CBP in the activation process. Thus, recruitment of CBP to CREB is sufficient for transcriptional activation. Addition of PKA stimulates transcription induced by CREB DIEDML further, suggesting that a phosphorylation event downstream from CBP recruitment augments CREB signaling.The signaling mechanism that activates genes through the cyclic AMP (cAMP)-regulated enhancer (CRE) (23) represents one of the most intensively studied transcriptional pathways. This pathway consists of protein kinase A (PKA), the transcription factor CREB, and the coactivator CREB binding protein (CBP) (3,7,18). CBP has been shown to participate in many additional transcriptional pathways as well (11), but the mechanism by which it activates gene expression remains uncertain. CBP and its homologue p300 interact with the basal transcription factors TFIID and TFIIB, as well as with the RNA polymerase II holoenzyme component, RNA helicase A (9, 24, 34), suggesting that one function of this coactivator is to stabilize the preinitiation complex. Other studies have argued that transcriptional activation through CBP/p300 occurs only in the context of chromatin, however (17). The involvement of chromatin in CBP function is consistent with the finding that CBP, and several associated proteins including P/CAF, steroid receptor coactivator 1, and p/CIP, have the ability to acetylate the amino-terminal tails of histone proteins (6,31,36,38). These and other posttranslational modifications of chromatin components induced by the cAMP signaling cascade may stimulate transcription through nucleosome remodeling.Of all transcription factor-CBP associations, only the interaction with phosphorylated CREB has been characterized in detail. Our lab has studied this association by using a fluorescence polarization binding assay and a genetic interaction assay in yeast (18,30). These studies indicated that CREB phosphorylated at Ser 133 binds to CBP with an affinity of approximately 350 nM and that...

In the current study, we show that bone morphogenetic proteins (BMPs) play a role in hematopoiesis that is independent of their function in specifying ventral mesodermal fate. When BMP activity is upregulated or inhibited in Xenopus embryos hematopoietic precursors are specified properly but few mature erythrocytes are generated. Distinct cellular defects underlie this loss of erythrocytes: inhibition of BMP activity induces erythroid precursors to undergo apoptotic cell death, whereas constitutive activation of BMPs causes an increase in commitment of hematopoietic progenitors to myeloid differentiation and a concomitant decrease in erythrocytes that is not due to enhanced apoptosis. These blood defects are observed even when BMP activity is misregulated solely in non-hematopoietic (ectodermal) cells, demonstrating that BMPs generate extrinsic signals that regulate hematopoiesis independent of mesodermal patterning. Further analysis revealed that endogenous calmodulin-dependent protein kinase IV (CaM KIV) is required to negatively modulate hematopoietic functions of BMPs downstream of receptor activation. Our data are consistent with a model in which CaM KIV inhibits BMP signals by activating a substrate, possibly cAMP-response element-binding protein (CREB), that recruits limiting amounts of CREB binding protein (CBP) away from transcriptional complexes functioning downstream of BMPs.