Kinase suppressor of Ras forms a multiprotein signaling complex and modulates MEK localization.
Genetic screens for modifiers of activated Ras phenotypes have identified a novel protein, kinase suppressor of Ras (KSR), which shares significant sequence homology with Raf family protein kinases. Studies using Drosophila melanogaster and Caenorhabditis elegans predict that KSR positively regulates Ras signaling; however, the function of mammalian KSR is not well understood. We show here that two predicted kinase-dead mutants of KSR retain the ability to complement ksr-1 loss-of-function alleles in C. elegans, suggesting that KSR may have physiological, kinase-independent functions. Furthermore, we observe that murine KSR forms a multimolecular signaling complex in human embryonic kidney 293T cells composed of HSP90, HSP70, HSP68, p50(CDC37), MEK1, MEK2, 14-3-3, and several other, unidentified proteins. Treatment of cells with geldanamycin, an inhibitor of HSP90, decreases the half-life of KSR, suggesting that HSPs may serve to stabilize KSR. Both nematode and mammalian KSRs are capable of binding to MEKs, and three-point mutants of KSR, corresponding to C. elegans loss-of-function alleles, are specifically compromised in MEK binding. KSR did not alter MEK activity or activation. However, KSR-MEK binding shifts the apparent molecular mass of MEK from 44 to >700 kDa, and this results in the appearance of MEK in membrane-associated fractions. Together, these results suggest that KSR may act as a scaffolding protein for the Ras-mitogen-activated protein kinase pathway.
Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109-0606, USA.
S
S
Stewart
M
M
Sundaram
Y
Y
Zhang
J
J
Lee
M
M
Han
K L
KL
Guan
D048948:14-3-3 Proteins; D000818:Animals; D016227:Benzoquinones; D017173:Caenorhabditis elegans / Q000502:physiology; D029742:Caenorhabditis elegans Proteins; D018797:Cell Cycle Proteins / Q000378:metabolism; D002460:Cell Line; D018833:Chaperonins; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D005816:Genetic Complementation Test; D006360:Heat-Shock Proteins / Q000378:metabolism; D015801:Helminth Proteins / Q000502:physiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D047029:Lactams, Macrocyclic; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; D048728:MAP Kinase Kinase Kinase 1; D046911:Macromolecular Substances; D051379:Mice; D020929:Mitogen-Activated Protein Kinase Kinases; D008954:Models, Biological; D018832:Molecular Chaperones; D008970:Molecular Weight; D046912:Multiprotein Complexes; D010766:Phosphorylation; D011494:Protein Kinases / Q000502:physiology*; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011506:Proteins / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D011809:Quinones / Q000494:pharmacology; D015398:Signal Transduction / Q000502:physiology*; D014157:Transcription Factors; D014446:Tyrosine 3-Monooxygenase; D050807:ets-Domain Protein Elk-1
D048948:14-3-3 Proteins; D016227:Benzoquinones; C498362:CDC37 protein, human; D029742:Caenorhabditis elegans Proteins; C513271:Cdc37 protein, mouse; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D006360:Heat-Shock Proteins; D015801:Helminth Proteins; D047029:Lactams, Macrocyclic; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D046912:Multiprotein Complexes; D011506:Proteins; D011518:Proto-Oncogene Proteins; D011809:Quinones; D014157:Transcription Factors; D050807:ets-Domain Protein Elk-1; C106207:par-5 protein, C elegans; D014446:Tyrosine 3-Monooxygenase; D011494:Protein Kinases; C097199:KSR-1 protein kinase; C482128:MAP2K2 protein, human; D011505:Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D048728:MAP Kinase Kinase Kinase 1; C482576:MAP3K1 protein, human; C482578:Map3k1 protein, mouse; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482125:MAP2K1 protein, human; C482127:Map2k1 protein, mouse; D020929:Mitogen-Activated Protein Kinase Kinases; C516678:mek-1 protein, C elegans; D018833:Chaperonins; C001277:geldanamycin