c-Jun Homodimers Can Function as a Context-Specific Coactivator

Abstract

Transcription factors can function as DNA-binding-specific activators or as coactivators. c-Jun drives gene expression via binding to AP-1 sequences or as a cofactor for PU.1 in macrophages. c-Jun heterodimers bind AP-1 sequences with higher affinity than homodimers, but how c-Jun works as a coactivator is unknown. Here, we provide in vitro and in vivo evidence that c-Jun homodimers are recruited to the interleukin-1β (IL-1β) promoter in the absence of direct DNA binding via protein-protein interactions with DNA-anchored PU.1 and CCAAT/enhancer-binding protein β (C/EBPβ). Unexpectedly, the interaction interface with PU.1 and C/EBPβ involves four of the residues within the basic domain of c-Jun that contact DNA, indicating that the capacities of c-Jun to function as a coactivator or as a DNA-bound transcription factor are mutually exclusive. Our observations indicate that the IL-1β locus is occupied by PU.1 and C/EBPβ and poised for expression and that c-Jun enhances transcription by facilitating a rate-limiting step, the assembly of the RNA polymerase II preinitiation complex, with minimal effect on the local chromatin status. We propose that the basic domain of other transcription factors may also be redirected from a DNA interaction mode to a protein-protein interaction mode and that this switch represents a novel mechanism regulating gene expression profiles.

The work was funded by a research grant from the Canadian Institutes for Health Research (T.H.), NIH grants HL56745 and CA41456 (D.T.), the Canadian Research Chair program (T.H.), studentships from the Fonds de Recherche en Santé du Québec (M.S.D.) and CIHR (M.T.), and a fellowship from Dokkyo University (K.W.).

We thank Daniel Durocher (Centre for Systems Biology, Toronto) and Marc Therrien (IRIC, Montreal) for critical reading of the manuscript and Gerhard Behre, Mona Nemer, and Luc Villeneuve for their help in establishing the gel shift and transient transfection assays. We thank Nathalie Nguyen as well as Pierre Forest for their technical assistance and Viviane Jodoin for secretarial help.