Cytoplasmic sequestration of Rel proteins by I kappa B alpha requires CRM1-dependent nuclear export

Rel and I kappa B protein families form a complex cellular regulatory netwo rk A major regulatory function of I kappa B proteins is to retain Rel prote ins in the cell cytoplasm. In addition, I kappa B proteins have also been p ostulated to serve nuclear functions. These include the maintenance of indu cible NF-kappa B-dependent gene transcription, as well as termination of in ducible transcription. We show that I kappa B alpha shuttles between the nu cleus and the cytoplasm, utilizing the nuclear export receptor CRM1, A CRM1 -binding export sequence was identified in the N-terminal domain of I kappa B alpha but not in that of I kappa B beta or I kappa B epsilon. By reconst ituting major aspects of NF-kappa B-I kappa B sequestration in yeast, we de monstrate that cytoplasmic retention of p65 (also called ReIA) by I kappa B alpha requires Crm1p-dependent nuclear export. In mammalian cells, inhibit ion of CRM1 by leptomycin B resulted in nuclear localization of cotransfect ed p65 and I kappa B alpha in COS cells and enhanced nuclear relocation of endogenous p65 in T cells. These observations suggest that the main functio n of I kappa B alpha is that of a nuclear export chaperone rather than a cy toplasmic tether. We propose that the nucleus is the major site of p65-I ka ppa B alpha association, from where these complexes must be exported in ord er to create the cytoplasmic pool.