I kappa B interacts with the nuclear localization sequences of the subunits of NF-kappa B: a mechanism for cytoplasmic retention.

Abstract

NF-~B is an inducible transcription factor comprised of a 50-kD (pS0) and a 65-kD (p65) subunit. Induction of NF-KB activity, which is a critical event in many signal transduction pathways, involves release from a cytoplasmic inhibitory protein, IKB, followed by translocation of the active transcription factor complex into the nucleus. Earlier studies suggested that IKB targets the p65 subunit of NF-KB. However, we demonstrate by in vitro and in vivo methods that the recently cloned IKB/MAD-3 interacts with both the p50 and p65 subunits of NF-KB, as well as c-Rel. Furthermore, an alternatively spliced, dimerization-deficient transforming variant of p65 (p65A) interacts extremely weakly with IKB/MAD-3, suggesting that dimerization is important for interaction. We demonstrate that the conserved nuclear localization sequences (NLSs) of NF-KB and c-Rel are the targets for IKB/MAD-3 interaction. Indirect immunofluorescence experiments demonstrate that IKB/MAD-3 expression retains both p65 and p50 in the cytoplasm. Furthermore, and most important, a p65 that contains an SV40 large T antigen NLS in addition to its own NLS is no longer retained in the cytoplasm in the presence of IKB/MAD-3. We propose that IKB/MAD-3 masks the NLSs of NF-KB and c-Rel and that this constitutes the mechanism for cytoplasmic retention of these proteins.

Increased expression of p50-NF-κB and constitutive activation of NF-κB transcription factors during mouse skin carcinogenesis

“…In some experimental systems p50/p50 homodimers activated transcription from promoters with speci®c kB sites (Collart et al, 1990;Fujita et al, 1992Fujita et al, , 1993Kretzschmar et al, 1992;Schouten et al, 1995;Ten et al, 1992). In other experimental systems these homodimers were unable to activate gene transcription (Beg et al, 1992;Franzoso et al, 1992;Kang et al, 1992;Plaksin et al, 1993;Schmitz and Baeuerle, 1991). It seems that p50/p50 displays high a nity binding for many distinct kB motifs but provides a strong transcriptional activation only when adopting a speci®c conformation induced by certain kB motifs but not others (Fujita et al, 1992).…”

“…However, the ®nding that IkB-a de®cient mice exhibited severe dermatitis accompanied by strong epidermal hyperplasia and hyperkeratosis (Beg et al, 1995;Klement et al, 1996) strongly suggests that NF-kB promotes keratinocyte proliferation. Moreover, the skin phenotype in mice lacking both IkB-a and p50 subunit of NF-kB developed with dramatic delay in comparison with IkB-a 7/7 mice (Beg et al, 1995). This partial rescue of the phenotype by the lack of p50 indirectly indicates that p50 is speci®cally important for NF-kB-mediated regulation of keratinocyte proliferation.…”

Regulation of NF-κB activity by IκB-related proteins in adenocarcinoma cells

“…All these proteins are characterized by the presence of repetitive sequences named`Ankyrin Repeats' because they are similar to the human protein ankyrin (Lux et al, 1990). The Ankyrin domain of the IkB proteins, with the exception of Bcl-3, covers the nuclear localization signal of NF-kB proteins and sequesters them in the cytoplasm (Haskill et al, 1991;Beg et al, 1992;Ganchi et al, 1992;Henkel et al, 1992;Lenardo and Siebenlist, 1994). A large number of signals can liberate NF-kB from its IkB inhibitor and thus allow NF-kB nuclear translocation and transactivation of target genes (Baldwin, 1996;Siebenlist, 1997).…”

Cytokine-induced nuclear translocation of signaling proteins and their analysis using the inducible translocation trap system

“…It has been shown that their actual entry is mediated by the importins/NPC complexes [72]. Unlike Stats and Smads, NFκB proteins have recognizable NLS [25,69,73,74]. Both importin α3 and α4 have been shown to mediate their nuclear translocation [72].…”