Résumé
The c-Fos proto-oncogenic transcription factor defines a multigene family controlling many processes both at the cell and the whole organism level. To bind to its target AP-1/TRE or CRE DNA sequences in gene promoters and exert its transcriptional part, c-Fos must heterodimerize with other bZip proteins, its best studied partners being the Jun proteins (c-Jun, JunB and JunD). c-Fos expression is regulated at many transcriptional and post-transcriptional levels. Yet, little is known on how its localization is dynamically regulated in the cell. We have investigated here its intranuclear mobility using FRAP-, genetic- and biochemical approaches. Whereas monomeric c-Fos is highly mobile and distributed evenly with nucleolar exclusion in the nucleus, heterodimerization with c-Jun entails intranuclear redistribution and dramatic reduction in mobility of c-Fos due to predominant association with the nuclear matrix independently of any binding to AP-1/TRE or CRE sequences. In contrast to c-Jun, dimerization with JunB does not detectably affect c-Fos mobility. However, dimerization with JunB affects intranuclear distribution with significant differences in the localization of c-Fos:c-Jun and c-Fos:JunB dimers. Moreover, c-Jun and JunB exert comparable effects on another Fos family member, Fra-1. Thus, we report a novel regulation, i.e. differentially regulated intranuclear mobility and distribution of Fos proteins by their Jun partners, and suggest the existence of intranuclear storage sites for latent c-Fos:c-Jun AP-1 complexes. This may affect the numerous physiopathological functions these transcription factors control.