Modulatory function of CREB.CREM alpha heterodimers depends upon CREM alpha phosphorylation

The cAMP-responsive element (CRE) modulator protein CREM alpha has been proposed to be a negative regulator of the CRE-binding protein (CREB). Precisely how CREM alpha inhibits CREB function is unclear, however. CREM alpha and CREB have highly related structures, and both proteins bind to consensus CRE sequences with similar affinities. Furthermore, both proteins can be phosphorylated by cAMP-dependent protein kinase A (PKA). Two models have been proposed to explain how CREM alpha could prevent the activation of genes by PKA-phosphorylated CREB: inhibitory CREM alpha homodimers could prevent occupancy of the CRE by CREB, or CREM alpha could block gene activation by forming non-functional CREB.CREM alpha heterodimers. To determine whether CREB-CREM alpha heterodimers are indeed non-functional, we engineered the leucine zipper regions of the two proteins to direct the pattern of dimerization. We then tested the biological activities of the phosphorylated and nonphosphorylated complexes in in vivo transcription assays. Our results indicate that CREM alpha can contribute to PKA-mediated gene activation when selectively heterodimerized with CREB. Furthermore, this transcriptional activity depends upon the ability of the complexes to be phosphorylated by PKA.