MLL and CREB Bind Cooperatively to the Nuclear Coactivator CREB-Binding Protein

Abstract

A fragment of the mixed-lineage leukemia (MLL) gene (Mll, HRX, ALL-1) was identified in a yeast genetic screen designed to isolate proteins that interact with the CREB–CREB-binding protein (CBP) complex. When tested for binding to CREB or CBP individually, this MLL fragment interacted directly with CBP, but not with CREB. In vitro binding experiments refined the minimal region of interaction to amino acids 2829 to 2883 of MLL, a potent transcriptional activation domain, and amino acids 581 to 687 of CBP (the CREB-binding or KIX domain). The transactivation activity of MLL was dependent on CBP, as either adenovirus E1A expression, which inhibits CBP activity, or alteration of MLL residues important for CBP interaction proved effective at inhibiting MLL-mediated transactivation. Single amino acid substitutions within the MLL activation domain revealed that five hydrophobic residues, potentially forming a hydrophobic face of an amphipathic helix, were critical for the interaction of MLL with CBP. Using purified components, we found that the MLL activation domain facilitated the binding of CBP to phosphorylated CREB. In contrast with paradigms in which factors compete for limiting quantities of CBP, these results reveal that two distinct transcription factor activation domains can cooperatively target the same motif on CBP.

ACKNOWLEDGMENTS

We thank Doug Dean and Antonio Postigo for the E1A expression plasmids and valuable discussions, Adam Shaywitz for insightful suggestions, and Scott Armstrong, Andrew Kung, and Laura Michael for critical evaluation.

P.E. is supported by the Cancer Research Fund of the Damon Runyon-Walter Winchell Foundation (DRG 1467), and J.W. is supported by an H.H.M.I. predoctoral training grant. This work was supported by grants from the NIH.