Affiliation:
1. Molecular Basis of Carcinogenesis Laboratory 1 and
2. Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 647202
3. Special Program in Protein Chemistry, 3 ABL-Basic Research Program, National Cancer Institute, Frederick Cancer Research and Development Center, Frederick, Maryland 21702, and
Abstract
ABSTRACT
Genetic and biochemical studies have identified kinase suppressor of Ras (KSR) to be a conserved component of Ras-dependent signaling pathways. To better understand the role of KSR in signal transduction, we have initiated studies investigating the effect of phosphorylation and protein interactions on KSR function. Here, we report the identification of five in vivo phosphorylation sites of KSR. In serum-starved cells, KSR contains two constitutive sites of phosphorylation (Ser297 and Ser392), which mediate the binding of KSR to the 14-3-3 family of proteins. In the presence of activated Ras, KSR contains three additional sites of phosphorylation (Thr260, Thr274, and Ser443), all of which match the consensus motif (Px[S/T]P) for phosphorylation by mitogen-activated protein kinase (MAPK). Further, we find that treatment of cells with the MEK inhibitor PD98059 blocks phosphorylation of the Ras-inducible sites and that activated MAPK associates with KSR in a Ras-dependent manner. Together, these findings indicate that KSR is an in vivo substrate of MAPK. Mutation of the identified phosphorylation sites did not alter the ability of KSR to facilitate Ras signaling in
Xenopus
oocytes, suggesting that phosphorylation at these sites may serve other functional roles, such as regulating catalytic activity. Interestingly, during the course of this study, we found that the biological effect of KSR varied dramatically with the level of KSR protein expressed. In
Xenopus
oocytes, KSR functioned as a positive regulator of Ras signaling when expressed at low levels, whereas at high levels of expression, KSR blocked Ras-dependent signal transduction. Likewise, overexpression of
Drosophila
KSR blocked R7 photoreceptor formation in the
Drosophila
eye. Therefore, the biological function of KSR as a positive effector of Ras-dependent signaling appears to be dependent on maintaining KSR protein expression at low or near-physiological levels.
Publisher
American Society for Microbiology
Subject
Cell Biology,Molecular Biology
Cited by
155 articles.
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