Regulation of IkappaBbeta degradation. Similarities to and differences from IkappaBalpha

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