The Transcription Factor B-Myb Is Maintained in an Inhibited State in Target Cells through Its Interaction with the Nuclear Corepressors N-CoR and SMRT

Abstract

The B-Myb transcription factor has been implicated in coordinating the expression of genes involved in cell cycle regulation. Although it is expressed in a ubiquitous manner, its transcriptional activity is repressed until the G₁-S phase of the cell cycle by an unknown mechanism. In this study we used biochemical and cell-based assays to demonstrate that the nuclear receptor corepressors N-CoR and SMRT interact with B-Myb. The significance of these B-Myb-corepressor interactions was confirmed by the finding that B-Myb mutants, which were unable to bind N-CoR, exhibited constitutive transcriptional activity. It has been shown previously that phosphorylation of B-Myb by cdk2/cyclin A enhances its transcriptional activity. We have now determined that phosphorylation by cdk2/cyclin A blocks the interaction between B-Myb and N-CoR and that mutation of the corepressor binding site within B-Myb bypasses the requirement for this phosphorylation event. Cumulatively, these findings suggest that the nuclear corepressors N-CoR and SMRT serve a previously unappreciated role as regulators of B-Myb transcriptional activity.

We thank Roger J. Watson for the B-Myb expression plasmids pcDNA3-B-Myb and pcDNA3-B-Myb_1-561, Ed Harlow for pCMV-cdk2 and pCMV-cdk2DN, and Timothy P. Bender for pR3SV and pRMb3SV-c-Myb. We thank Anthony R. Means and Tso-Pang Yao for helpful insight and suggestions.

This work was supported by a U.S. Army breast cancer research grant to X.L. (DAMD17-00-1-0234) and an NIH grant to D.P.M. (DK50494).