Regulation of IκBβ Degradation

doi:10.1074/jbc.272.15.9942

Regulation of IκBβ Degradation

SIMILARITIES TO AND DIFFERENCES FROM IκBα*

From the Unité de Biologie Moléculaire de l'Expression Génique, URA 1149 CNRS, and the
^§ Laboratoire d'Electrophorèse Bidimensionnelle, Institut Pasteur, 75724 Paris Cedex 15, France

^¶To whom correspondence should be addressed:
Unité de Biologie Moléculaire de l'Expression Génique, URA 1149 CNRS, Institut Pasteur, 25 Rue du Dr. Roux, 75724 Paris Cedex 15, France.
Tel.: 33-1-4568-8553; Fax: 33-1-4061-3040; E-mail: aisrael{at}pasteur.fr

Abstract

The transcription factor NF-κB (nuclear factor-κB) is neutralized in nonstimulated cells through cytoplasmic retention by IκB inhibitors. In mammalian cells, two major forms of IκB proteins, IκBα and IκBβ, have been identified. Upon treatment with a large variety of inducers, IκBα and IκBβ are proteolytically degraded, resulting in NF-κB translocation into the nucleus. Recent observations suggest that phosphorylation of serines 32 and 36 and subsequent ubiquitination of lysines 21 and 22 of IκBα control its signal-induced degradation. In this study we provide evidence that critical residues in the NH₂-terminal region of IκBβ (serines 19 and 23) as well as its COOH-terminal PEST region control IκBβ proteolysis. However Lys-9, the unique lysine residue in the NH₂-terminal region of IκBβ, is not absolutely required for its degradation. We also demonstrate that following stimulation, an underphosphorylated nondegradable form of IκBβ accumulates. Surprisingly, our data suggest that unlike IκBα, IκBβ is constitutively phosphorylated on one or two of the critical NH₂-terminal serine residues. Thus, phosphorylation of these sites is necessary for degradation but does not necessarily constitute the signal-induced event that targets the molecule for proteolysis.

Footnotes

↵^‡ Recipient of long term fellowship from ANRS.
↵* This research was sponsored in part by grants from l'Association pour la Recherche sur le Cancer, l'Agence Nationale de Recherche contre le SIDA, INSERM, and the ligue Nationale Française contre le Cancer. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

NF-κB

nuclear factor-κB

TNF

tumor necrosis factor

IL

interleukin

LPS

lipopolysaccharide

PMA

phorbol 12-myristate 13-acetate

HA

hemagglutinin

wt

wild type

Tricine

N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine

ALLN

N-acetyl-Leu-Leu-norleucinal.
↵² C. Brou, unpublished data.
↵³ S. T. Whiteside, C. Laurent-winter, and A. Israël, unpublished observations.
- Received October 31, 1996.