Cytoplasmic sequestration of Rel proteins by IκBα requires CRM1-dependent nuclear export

Rel and I kappa B protein families form a complex cellular regulatory network A major regulatory function of I kappa B proteins is to retain Rel proteins in the cell cytoplasm. In addition, I kappa B proteins have also been postulated to serve nuclear functions. These include the maintenance of inducible NF-kappa B-dependent gene transcription, as well as termination of inducible transcription. We show that I kappa B alpha shuttles between the nucleus 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 reconstituting major aspects of NF-kappa B-I kappa B sequestration in yeast, we demonstrate that cytoplasmic retention of p65 (also called ReIA) by I kappa B alpha requires Crm1p-dependent nuclear export. In mammalian cells, inhibition of CRM1 by leptomycin B resulted in nuclear localization of cotransfected 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 function of I kappa B alpha is that of a nuclear export chaperone rather than a cytoplasmic tether. We propose that the nucleus is the major site of p65-I kappa B alpha association, from where these complexes must be exported in order to create the cytoplasmic pool.