Rel/NF-kappa B/I kappa B family: intimate tales of association and dissociation

Constitutive phosphorylation of IkappaBalpha by casein kinase II occurs preferentially at serine 293: requirement for degradation of free IkappaBalpha

IkappaBalpha is a phosphoprotein that sequesters the NF-kappaB/Rel transcription factors in the cytoplasm by physical association. Following induction by a wide variety of agents, IkappaBalpha is further phosphorylated and degraded, allowing NF-kappaB/Rel proteins to translocate to the nucleus and induce transcription. We have previously reported that the constitutive phosphorylation site resides in the C-terminal PEST region of IkappaBalpha and is phosphorylated by casein kinase II (CKII). Here we show that serine 293 is the preferred CKII phosphorylation site. Additionally, we show compensatory phosphorylation by CKII at neighboring serine and threonine residues. Thus, only when all five of the serine and threonine residues in the C-terminal region of IkappaBalpha are converted to alanine (MutF), is constitutive phosphorylation abolished. Finally, we show that constitutive phosphorylation is required for efficient degradation of free IkappaBalpha, in that unassociated Mutf has a half-life two times longer than wild-type IkappaBalpha. A serine residue alone at position 293, as well as aspartic acid at this position, can revert the Mutf phenotype. Therefore, the constitutive CKII phosphorylation site is an integral part of the PEST region of IkappaBalpha, and this phosphorylation is required for rapid proteolysis of the unassociated protein.

Xenopus Mad proteins transduce distinct subsets of signals for the TGF beta superfamily

Xenopus cDNAs homologous to the Drosophila Mad gene and C. elegans CEM genes have been cloned and functionally analyzed by microinjection into frog embryos. The results show that these genes (Xmad) encode intracellular proteins that act downstream of TGF beta superfamily ligands. Most interesting is the fact that different Xmad proteins produce distinct biological responses. Xmad1 produces ventral mesoderm, apparently transducing a signal for BMP2 and BMP4, whereas Xmad2 induces dorsal mesoderm like Vg1, activin, and nodal. These results suggest that an individual Xmad protein waits poised in the cytoplasm for instruction from a distinct subset of TGF beta ligands and then conveys specific information to the nucleus.

Site-specific phosphorylation of IkappaBalpha by a novel ubiquitination-dependent protein kinase activity

Signal-induced activation of the transcription factor NF-kappaB requires specific phosphorylation of the inhibitor IkappaBalpha and its subsequent proteolytic degradation. Phosphorylation of serine residues 32 and 36 targets IkappaBalpha to the ubiquitin (Ub)-proteasome pathway. Here we report the identification of a large, multisubunit kinase (molecular mass approximately 700 kDa) that phosphorylates IkappaBalpha at S32 and S36. Remarkably, the activity of this kinase requires the Ub-activating enzyme (E1), a specific Ub carrier protein (E2) of the Ubc4/Ubc5 family, and Ub. We also show that a ubiquitination event in the kinase complex is a prerequisite for specific phosphorylation of IkappaBalpha. Thus, ubiquitination serves a novel regulatory function that does not involve proteolysis.

A light-entrainment mechanism for the Drosophila circadian clock

Biochemical studies indicate that the Drosophila timeless protein (Tim) is a stoichiometric partner of the period protein (Per) in fly head extracts. A Per-Tim heterodimeric complex explains the reciprocal autoregulation of the proteins on transcription. The complex is under clock control, and many circadian features of the Tim cycle resemble those of the Per cycle. However, Tim is rapidly degraded in the early morning or in response to light, releasing Per from the complex. The Per-Tim complex is a functional unit of the Drosophila circadian clock, and Tim degradation may be the initial response of the clock to light.