MODULATORY FUNCTION OF CREB-CENTER-DOT-CREM-ALPHA HETERODIMERS DEPENDS UPON CREM-ALPHA PHOSPHORYLATION

The cAMP-responsive element (CRE) modulator protein CREM alpha has bee n 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 protein s 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 coul d prevent the activation of genes by PKA-phosphorylated CREB: inhibito ry CREM alpha homodimers could prevent occupancy of the CRE by CREB, o r CREM alpha could block gene activation by forming nonfunctional CREB .CREM alpha heterodimers. To determine whether CREB.CREM alpha heterod imers are indeed nonfunctional, we engineered the leucine zipper regio ns of the two proteins to direct the pattern of dimerization. We then tested the biological activities of the phosphorylated and nonphosphor ylated complexes in in vivo transcription assays. Our results indicate that CREM alpha can contribute to PKA-mediated gene activation when s electively heterodimerized with CREB. Furthermore, this transcriptiona l activity depends upon the ability of the complexes to be phosphoryla ted by PKA.