CHUK Redux: More Answers in Mystery of NF-KAPPAB Activation

CHUK Redux:
More Answers in Mystery of NF-KB Activation

by Jeremy Cherfas

WHAT'S HOT IN BIOLOGY...

Rank	Paper	Citations This Period May-Jun 99	Rank Last Period Mar-Apr 99
1	S. F. Altschul, et al., "Gapped BLAST and PSI-BLAST: a new generation of protein database search programs," Nucleic Acids Res., 25(17):3389-3402, 1 September 1997. [NIH, Bethesda, MD; Pennsylvania St. U., University Park] *XU793	167	1
2	P. Li, et al., "Cytochrome c and dATP-dependent formation of Apaf-1/Caspase-9 complex initiates an apoptotic protease cascade," Cell, 91(4):479-89, 14 November 1997. [Howard Hughes Med. Inst., U. Texas Southwest. Med. Ctr. Dallas; Thomas Jefferson U., Philadelphia, PA] YG492	72	3
3	H. Zou, et al., "Apaf-1, a human protein homologous to C. elegans CED-4, participates in cytochrome c-dependent activation of caspase-3," Cell,90(3):405-13, 8 August 1997. [U. Texas Southwestern Med. Ctr. Dallas; Genentech, South San Francisco, CA] *XQ063	59	7
4	F.R. Blattner, et al., "The complete genome sequence of Escherichia coli K-12," Science, 277(5331):1453-74, 5 September 1997. [U. Wisconsin, Madison; U. Michigan Sch. Med., Ann Arbor; FMC Bioproducts, Rockland, ME; U. Natl. Autonoma Mexico, Moreles] *XV429	52	2
5	M. Enari, et al., "A caspase-activated DNase that degrades DNA during apoptosis, and its inhibitor ICAD," Nature, 391(6662):43-50, 1 January 1998. [Osaka U. Med. Sch., Japan; Kirin Brewery Co., Kanagawa, Japan; Osaka Biosci. Inst., Japan] *YP888	50	8
6	D.A. Doyle, et al., "Structure of the potassium channel: Molecular basis of K<^>+ conduction and selectivity," Science, 280(5360):69-77, 3 April 1998. [Rockefeller U., New York, NY; Howard Hughes Med. Inst., Rockefeller U., NY] *ZF314	50	5
7	F. Kunst, et al., "The complete genome sequence of the Gram-positive bacterium Bacillus subtilis," Nature, 390(6657):249-56, 20 November 1997. [46 institutions worldwide] *YG667	48	4
8	J.-F.Tomb, et al., "The complete genome sequence of the gastric pathogen Helicobacter pylori," Nature, 388(6642):539-47, 7 August 1997. [6 U.S. and Swedish institutions] *XP722	44	†
9	F. Mercurio, et al., "IKK-1 and IKK2: Cytokine-activated IKAPPAB kinases essential for NF-KAPPAB activation," Science, 279(5339):860-6, 31 October 1997. [Signal Pharmaceuticals, Inc., San Diego, CA; European Molec. Biol. Lab., Heidelberg, Germany; Harvard Med. Sch., Boston, MA] *YD479	41	†
10	C.H. Regnier, et al., "Identification and characterization of an IKAPPAB kinase," Cell, 90(2):373-83, 25 July 1997. [Tularik, Inc., South San Francisco, CA] *XN249	40	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

year ago four groups were racing to unravel the chain of command that—among other things—controls the life and death of cells (see Science Watch, 9[4]: 8, July/August 1998). The mystery is that a great many external stimuli, from X-rays to tumor necrosis factor, all converge on a single path to gene activation. What, then, allows different stimuli to have different effects? And how, precisely, does the pathway transmit the signals? An answer to the second question makes the running today, as some of those same researchers fill the new slots in the What's Hot list with their identification of a key link in the chain.

The story was that the transcriptional activator protein, called NF-kB, was held captive in the cytoplasm by inhibitor proteins called I-kB. An activation signal triggered some change that released NF-kB, involving the specific phosphorylation of two serine residues of the I-kB. Was this change the activation of an IkB kinase? Or the inactivation of a phosphatase? (Although, given the predominance of kinases, kinase kinases, and even kinase kinase kinases, this seems highly unlikely.) Many papers reported results implicating a whole range of kinases in this crucial event, but none of them did everything needed of the putative activation signal. At last, entering the lists at #9, #10, and #11 are papers that identify and characterize the I-k kinases that release NF-kB to enter the nucleus.

Two of the groups adopted an essentially similar approach. First to report was that of Michael Karin at the University of San Diego (see J.A. DiDonato et al., Nature, 388[6642]:548-54, 7 August 1997; #11, with 39 citations this period), followed closely by a team led by Frank Mercurio at Signal Pharmaceuticals Inc, also of San Diego (#9). Both used predominantly physico-chemical methods to purify two IK kinases - IKK-1 and IKK-2 - from cells activated by exposure to TNF. The third paper (#10), from Mike Rothe, David Goeddel and their colleagues at Tularik Inc. in San Francisco, used a biological screen to nab one of the kinases and then searched DNA databases to identify the other.

The first kinase, IKK-1, turns out to be identical to CHUK, a serine-threonine kinase that, when it was isolated in 1995, had no known function. IKK-2 is very closely related to IKK-1, and both have exactly the properties that the missing IKK needs. Both contain the motif that activates MAP kinases, and so, as Mercurio demonstrated, they are targets for kinases such as NIK that are known to activate NF-kB. The two IKKs form homo- and hetero-dimers with each other, though as Karin's group has shown, the heterodimer may be the biologically more relevant version. The crucial evidence, however, comes from transfecting cells with modified version of the IKK genes.

For both versions of IKK, overexpression activates NF-kB, while mutations that destroy the kinase domain suppress the ability of TNF and interleukin-1 to induce activation of NF-kB, as does the expression of antisense to IKK-1 mRNA. All three groups also demonstrated that IKK-1 and IKK2 both directly phosphorylate IkB-1.

Pinpointing the identify and mechanism of IKK allows a more detailed model of activation of NF-kB. NIK activates the IkB complex by phosphorylating IKK-1 and IKK-2. The activated heterodimer attracts IkB-1, bound to NF-kB, and phosphorylates it. Ubiquitin now enters the story, marking IkB-1 for destruction by the 26S proteasome. That releases NF-kB, which passes through the nuclear membrane and finds its gene targets.

Sounds good, and it is indeed a considerable achievement, but intriguing mysteries remain. CHUK, as originally isolated, needed ubiquitination to function as a kinase. But Karin's group, at least, could find no evidence that IKK-1 required ubiquitin. Then there is the question of whether IKK-1 is, as it were, a unique bottleneck through which all the disparate stimuli that activate NF-kB must pass. If so, it would be a perfect target for the therapeutics that are almost always promised in selling cell signals. But then, how do the different external stimuli exert their disparate effects? Perhaps a year from now answers to those questions will be hot topics.