Control of NF-#kappa#B transcriptional activation by signal induced proteolysis of I#kappa#B#alpha#

摘要

In unstimulated cells the transcription factor NF-#kappa#B is held in the cytoplasm in an inactive state by I#kappa#B inhibitor proteins. Ultimately activation of NF-#kappa#B is achieved by ubiquitination and proteasome-mediated degradation of I#kappa#B#alpha# and we have therefore investigated factors which control this proteolysis. Signalinduced degradation of I#kappa#B#alpha# exposes the nuclear localization signal of NF-#kappa#B, thus allowing it to translocate into the nucleus and activate transcription from responsive genes. An autoregulatory loop is established when NF-#kappa#B induces expression of the I#kappa#B#alpha# gene and newly synthesized I#kappa#B#alpha# accumulates in the nucleus where it negatively regulates NF-#kappa#B-dependent transcription. As part of this post-induction repression, the nuclear export signal on I#kappa#B#alpha# mediates transport of NF-#kappa#B-I#kappa#B#alpha# complexes from the nucleus to the cytoplasm. As nuclear export of I#kappa#B#alpha# is blocked by leptomycin B this drug was used to examine the effect of cellular location on susceptibility of I#kappa#B#alpha# to signal-induced degradation. In the induced degradation. Thus signal-induced degradation of I#kappa#B#alpha# is mainly, if not exclusively a cytoplasmic process. An efficient nuclear export of I#kappa#B#alpha# is therefore essential for maintaining a low level of I#kappa#B#alpha# in the nucleus and allowing NF-#kappa#B to be transcriptionally active upon cell stimulation. We have detected a modified form of I#kappa#B#alpha#, conjugated to the small ubiquitin-like protein SUMO-1, which is resistant to signalinduced degradation. SUMO-1 modified I#kappa#B#alpha# remains associated with NF-#kappa#B and thus overexpression of SUMO-1 inhibits the signal-induced activation of NF-#kappa#B-dependent transcription. Reconstitution of the conjugation reaction with