Regulation of the MAP kinase pathway by mammalian Ksr through direct interaction with MEK and ERK.
BACKGROUND
Genetic screens in Drosophila melanogaster and Caenorhabditis elegans identified the kinase suppressor of Ras, Ksr, as a new component in the Ras intracellular signaling pathway. In these organisms, mutations in Ksr resulted in attenuation of Ras-mediated signaling. Homologs of Ksr have also been isolated from mice and humans; their precise role in Ras signaling is not well defined. Here, we present data showing interactions between the murine form of Ksr (mKsr-1) and other components of the Ras pathway.


RESULTS
To gain insight into the biological function of Ksr, we used a yeast two-hybrid screen and found an interaction between the carboxy-terminal region of mKsr-1 and mitogen-activated protein (MAP) kinase kinase 1 (MAPKK-1 or MEK-1). An interaction was also detected between MAP kinase (also called extracellular signal-regulated kinase; ERK), and the amino-terminal region of mKsr-1. These interactions were recapitulated in COS-7 cells. Further, when COS-7 cells were transfected with either full-length mKsr-1 or only its carboxy-terminal region, an inhibition of serum-stimulated MAP kinase activation was observed. Microinjection of full-length mKsr-1 or its carboxy-terminal, but not its amino-terminal region, blocked serum-induced DNA synthesis in rat embryo fibroblasts. Co-injection of mKsr-1 with MEK-1 reversed the blockade.


CONCLUSIONS
Together with the data from genetic analyses, our findings lead us to propose that mKsr-1 may control MAP kinase signaling by serving as a scaffold protein that links MEK and its substrate ERK.
Yu
W
W
Chiron Corporation, 4560 Horton Street, Emeryville, California 94608, USA.
Fantl
W J
WJ
Harrowe
G
G
Williams
L T
LT
D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D004261:DNA Replication / Q000187:drug effects; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D048369:MAP Kinase Kinase 1; D051379:Mice; D008845:Microinjections; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism*; D020929:Mitogen-Activated Protein Kinase Kinases; D008969:Molecular Sequence Data; D011485:Protein Binding; D011494:Protein Kinases / Q000378:metabolism / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D051381:Rats; D015398:Signal Transduction; D018631:ras Proteins / Q000378:metabolism
D011494:Protein Kinases; C097199:KSR-1 protein kinase; D011505:Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048369:MAP Kinase Kinase 1; C482127:Map2k1 protein, mouse; D020929:Mitogen-Activated Protein Kinase Kinases; D018631:ras Proteins