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KSR, a novel protein kinase required for RAS signal transduction
MONITOR
A 'digest' o f , ; o m e recent p a p e r s o f interest in the primary journals.
The identificationof two novel figands of the FGFreceptorby a yeast screeningmethodand theiractivityin Xo.opns development N. KINOSHITA,J. MIN.~HULL AND.M.W.KIRSCHNER Cell83, 621-630 Many experiments have shown that embryonic development in Xenopus depends on inductions mediated by peptide growth factors and their receptors, but exactly which, and how many, ligands and receptors are involved is far from clear. Kinoshita et aL report -'t yeast-based approach leading to tile identification of two new candidate
Requirement for Xist in X chromosome inactivation G.D. PENNYk'FAL Nature379, 131-I37 Dosage compensation of X-linked genes in female mammals is achieved by the random inactivation of one X chromosome in the somatic cells of the developing embryo. A c~acting regulatory locus controlling this process, known as the X inactivation centre (Xic), has been identified, and two related functions have been ascribed to this locus: determination of the number of X chromosemes to be inactivated and the choice of the active X; and the
KSR, a novel protein kin ase required for RAS signal transduction M.THERRIENETAL. Cell 83, 879--888
The C elegans ksr.l gene encodes a novel raf.related kinase involved in ras-mediated signal transduction M. SUNDAR.*MANDM. HAN Cell 83, 889-901
The ksr-I gene encodes a novel protein kinase involved in rasmediated signaling in C. elegans K. KORNFELD.D.B.HO,~ ANDH.R. HORVrlz Cell 83, 903--913 Tyrosine kinases activate things that activate RAS, which activates RAF, which activates other things. Eventually, these other things cause cells in the
indut. ,lg factors, both of which are able tc affect normal development. They first showeO that co-expression in yeast of XenqottsFGF2 (fibroblast growth factor) together with FGFR, a receptor tyrosine kinase, leads to an increase in tyrosine phosphorylation that can be detected in colonies by .screening with an antiphosphotyrosine antilxxly. Libraries of frog cDNKs were then trar formed into yeast cells expressing FGFR alone, and a number of colonies with increased phosphotydrosine levels were obtained. None of these clones encoded FGF2, but, instead, two clones (out of six independent clones studied) were found to encode novel secreted proteins. One, FRLI, is distantly related to CRIPTO, a
mouse EGF family member, and the other, FRL2, is distantly related to the angiogenin-ribonuclea~ supeffamily. So far, only FRL2 has been shown to bind directly to the receptor domain of FGFR, but injection of mRNA for either protein into frog animal caps leads to mesodermal induction and to more extreme morphological changes than are ob~rved with FGF treatment. The FRL1 transcript is detectable only during gastrula stages z:nd appears ubiquitous, whereas the FRL2 transcript increases in abundance during embryogenesis and exhibits specific tissue localization. These results suggest that FGF receptors could interact with multiple and diverse ligands during development.
dispersal of the inactivation sigmalalong the inactive X. The recently discovered human XIST(Xist; mouse) gene, which maps to Xic. has the unique property of being expressed exclusively from the inactive X chromosome and has been proposed as a candidate for Xic. Penny et aL have now provided direct evidence that transcription of Xist is required for inactivation of the X chromo.some on which it is located. Starting with a mouse embryonic stem (ES) cell line that was heterozygous for X chromosomes carrying distinguishable alleles of Xist and several other genes, they produced a null allele of Xist by gene targeting. By examining the expression of three different X-linked
genes in these cells, they determined that it was always the X chromosome not bearing the knockout which was inactivated. The authors then examined the effect of the Xist knockout in vivo, using the targeted ES cells to construct chimeric embryos with normal 8-cell host embryos. As was the case in vitro, the counting mechanism operated and only the unmodified X was inactivated. This work, thus, establishes the identity of Xic and the requirement of gist transcription for the spreading of the inactivation signal. The extremely interesting questions of how the counting ntechanism works and how Xistpromulgates the inactivation signal remain to be answered.
Drosophila embryo to take on terminal fates, cells in file C. eleganshypodermis to make vulvae and cells in hun~tns to divide. A burst of biochemical and genetic experiments over the last few years has identified all the things upstream of RAS and downstream of RAF, or so one might have thought until now. These three papers annc, unce tile discovery of a new thing in the RAS pathway. All three begin the same way: IW isolating suppressors cf a RAS gainof-function mutation. They all get the same result: identification of a new protein kinase, KSR (for kinase suppressor of RAS). Reduction of KSR function can suppress the effects of mutationally activated RAS in womls and flies, but what does it do for a living in nornml aninmls? In flies, the answer appears to be simple. Embryos lacking KSR have a torso phenotype, as expected if the RAS pathway were not operating. Activated RAF inlected into these embryos restores
terminal development. Thus, it appears that KSR is necessary for RAS to talk to RAF. In the eye disk, KSR also appears to sit between RAS and RAF. In worms, tile :;tory is more complicated. RAS signaling is weakened but not abolished in animals lacking KSR. The extent to which signaling is weakened is different in different cells. Most unexpected is Sundamm and Han's result that KSR is necessary for the effects of mutationally activated RAS, hut barely affects signaling by wild-type RAS activated through tile normal upstream channels. And there is another puzzle: two of the papers report that ti'.ey have been unable to see interactions between KSR and either RAS, RAF, or MEK (the kinase downstreant of RAF) in the two-hybrid system. In sutmnary, there is .something new in the RAg pathway. It is doing something important, but it will take more work to figure out what. ,~
TIG MARCH 1996 VOL. 12 No. 3
89
A 'digest' o f , ; o m e recent p a p e r s o f interest in the primary journals.
The identificationof two novel figands of the FGFreceptorby a yeast screeningmethodand theiractivityin Xo.opns development N. KINOSHITA,J. MIN.~HULL AND.M.W.KIRSCHNER Cell83, 621-630 Many experiments have shown that embryonic development in Xenopus depends on inductions mediated by peptide growth factors and their receptors, but exactly which, and how many, ligands and receptors are involved is far from clear. Kinoshita et aL report -'t yeast-based approach leading to tile identification of two new candidate
Requirement for Xist in X chromosome inactivation G.D. PENNYk'FAL Nature379, 131-I37 Dosage compensation of X-linked genes in female mammals is achieved by the random inactivation of one X chromosome in the somatic cells of the developing embryo. A c~acting regulatory locus controlling this process, known as the X inactivation centre (Xic), has been identified, and two related functions have been ascribed to this locus: determination of the number of X chromosemes to be inactivated and the choice of the active X; and the
KSR, a novel protein kin ase required for RAS signal transduction M.THERRIENETAL. Cell 83, 879--888
The C elegans ksr.l gene encodes a novel raf.related kinase involved in ras-mediated signal transduction M. SUNDAR.*MANDM. HAN Cell 83, 889-901
The ksr-I gene encodes a novel protein kinase involved in rasmediated signaling in C. elegans K. KORNFELD.D.B.HO,~ ANDH.R. HORVrlz Cell 83, 903--913 Tyrosine kinases activate things that activate RAS, which activates RAF, which activates other things. Eventually, these other things cause cells in the
indut. ,lg factors, both of which are able tc affect normal development. They first showeO that co-expression in yeast of XenqottsFGF2 (fibroblast growth factor) together with FGFR, a receptor tyrosine kinase, leads to an increase in tyrosine phosphorylation that can be detected in colonies by .screening with an antiphosphotyrosine antilxxly. Libraries of frog cDNKs were then trar formed into yeast cells expressing FGFR alone, and a number of colonies with increased phosphotydrosine levels were obtained. None of these clones encoded FGF2, but, instead, two clones (out of six independent clones studied) were found to encode novel secreted proteins. One, FRLI, is distantly related to CRIPTO, a
mouse EGF family member, and the other, FRL2, is distantly related to the angiogenin-ribonuclea~ supeffamily. So far, only FRL2 has been shown to bind directly to the receptor domain of FGFR, but injection of mRNA for either protein into frog animal caps leads to mesodermal induction and to more extreme morphological changes than are ob~rved with FGF treatment. The FRL1 transcript is detectable only during gastrula stages z:nd appears ubiquitous, whereas the FRL2 transcript increases in abundance during embryogenesis and exhibits specific tissue localization. These results suggest that FGF receptors could interact with multiple and diverse ligands during development.
dispersal of the inactivation sigmalalong the inactive X. The recently discovered human XIST(Xist; mouse) gene, which maps to Xic. has the unique property of being expressed exclusively from the inactive X chromosome and has been proposed as a candidate for Xic. Penny et aL have now provided direct evidence that transcription of Xist is required for inactivation of the X chromo.some on which it is located. Starting with a mouse embryonic stem (ES) cell line that was heterozygous for X chromosomes carrying distinguishable alleles of Xist and several other genes, they produced a null allele of Xist by gene targeting. By examining the expression of three different X-linked
genes in these cells, they determined that it was always the X chromosome not bearing the knockout which was inactivated. The authors then examined the effect of the Xist knockout in vivo, using the targeted ES cells to construct chimeric embryos with normal 8-cell host embryos. As was the case in vitro, the counting mechanism operated and only the unmodified X was inactivated. This work, thus, establishes the identity of Xic and the requirement of gist transcription for the spreading of the inactivation signal. The extremely interesting questions of how the counting ntechanism works and how Xistpromulgates the inactivation signal remain to be answered.
Drosophila embryo to take on terminal fates, cells in file C. eleganshypodermis to make vulvae and cells in hun~tns to divide. A burst of biochemical and genetic experiments over the last few years has identified all the things upstream of RAS and downstream of RAF, or so one might have thought until now. These three papers annc, unce tile discovery of a new thing in the RAS pathway. All three begin the same way: IW isolating suppressors cf a RAS gainof-function mutation. They all get the same result: identification of a new protein kinase, KSR (for kinase suppressor of RAS). Reduction of KSR function can suppress the effects of mutationally activated RAS in womls and flies, but what does it do for a living in nornml aninmls? In flies, the answer appears to be simple. Embryos lacking KSR have a torso phenotype, as expected if the RAS pathway were not operating. Activated RAF inlected into these embryos restores
terminal development. Thus, it appears that KSR is necessary for RAS to talk to RAF. In the eye disk, KSR also appears to sit between RAS and RAF. In worms, tile :;tory is more complicated. RAS signaling is weakened but not abolished in animals lacking KSR. The extent to which signaling is weakened is different in different cells. Most unexpected is Sundamm and Han's result that KSR is necessary for the effects of mutationally activated RAS, hut barely affects signaling by wild-type RAS activated through tile normal upstream channels. And there is another puzzle: two of the papers report that ti'.ey have been unable to see interactions between KSR and either RAS, RAF, or MEK (the kinase downstreant of RAF) in the two-hybrid system. In sutmnary, there is .something new in the RAg pathway. It is doing something important, but it will take more work to figure out what. ,~
TIG MARCH 1996 VOL. 12 No. 3
89