Jab 1 Co - activation of c - Jun Is Abrogated by the Serine 10 - phosphorylated Form

The cyclin-dependent kinase (cdk) inhibitor p27 is a central mediator in the imposition and maintenance of quiescence through the sequestration of G1-specific cyclin-cdk complexes. Previous studies have implicated the c-Jun co-activator protein Jab1 as a regulator of intracellular p27 levels. Jab1 has been reported to interact with p27 and cause its translocation to the cytoplasm as a prelude to the degradation of the cdk inhibitor. Here we describe experiments that showing phosphorylation of p27 at serine 10 leads to the suppression of Jab1 levels with the concomitant inhibition of c-Jun-dependent transcription. This repression is minimized upon quiescence exit through the rapid and preferential loss of the serine 10-phosphorylated form of p27 following serum stimulation. Our results, therefore, demonstrate an additional role for p27 in the modulation of c-Jun-dependent transcription via Jab1.