Membrane recruitment of Raf-1 is not the only function of Ras in Raf-1 activation.
Ras interacts with Raf-1 and stimulates its kinase activity, which results in activation of the mitogen-activated protein (MAP) kinase cascade. It has been proposed that the main function of Ras in Raf-1 activation is to recruit Raf-1 to the plasma membrane, where a separate activation event such as phosphorylation takes place. Here, we examined the activities of various mutants of human Ha-Ras to induce membrane translocation of Raf-1 and to activate Raf-1 in vivo. Overexpression of an activator region mutant Ha-Ras(V45E) in COS7 cells induced membrane translocation of Raf-1 as effectively as wild-type Ha-Ras. However, the activity of this mutant to activate Raf-1 and extracellular signal-regulated kinase-2 (ERK2) was attenuated by approximately 70% compared to that of wild-type Ha-Ras. The decrease in the specific activity was further demonstrated by measuring the activity of the Ha-Ras(V45E)-associated Raf-1 purified from the membrane fraction. These results imply that the association of Ras with Raf-1 has another important consequence, presumably dependent on the interaction between its activator region and Raf-1, than the simple recruitment of Raf-1 to the plasma membrane.
Tamada
M
M
Department of Physiology II, Kobe University School of Medicine, Japan.
Hu
C D
CD
Kariya
K
K
Okada
T
T
Kataoka
T
T
D000818:Animals; D001692:Biological Transport / Q000235:genetics; D019556:COS Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics; D019950:Mitogen-Activated Protein Kinase 1; D009154:Mutation; D015689:Oncogene Protein p21(ras) / Q000235:genetics; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D018631:ras Proteins / Q000235:genetics / Q000502:physiology*
D008565:Membrane Proteins; D019908:Proto-Oncogene Proteins c-raf; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; D015689:Oncogene Protein p21(ras); D018631:ras Proteins