Phosphorylation of CREB by CaM-kinase IV activated by CaM-kinase IV kinase.

Previous reports have shown that CaM-kinase IV can phosphorylate the transcription factor CREB in vitro on Ser133. Furthermore, transfected CaM-kinase IV can activate CREB-dependent transcription, but at a lower efficiency than the cAMP-kinase. In this paper we examine the kinetics and site-specificity of CREB phosphorylation in vitro by CaM-kinase IV after its phosphorylation and activation by a newly discovered brain CaM-kinase IV kinase. Our results show that activated CaM-kinase IV has the same Km (1-5 microM) for CREB phosphorylation, but the Vmax is about 30-fold higher than with non-activated CaM-kinase IV. Activated CaM-kinase IV still shows specificity for phosphorylation of Ser133, the site necessary for transactivation by CREB. It is likely that the lower efficiency of transcriptional activation by transfected CaM-kinase IV in previous studies was due to the fact that the CaM-kinase IV was not activated by CaM-kinase IV kinase.