Four Human Ras Homologs Differ in Their Abilities to Activate Raf-1, Induce Transformation, and Stimulate Cell Motility*
Abstract
Human cells contain four homologous Ras proteins, but it is unknown whether each of these Ras proteins participates in distinct signal transduction cascades or has different biological functions. To directly address these issues, we assessed the relative ability of constitutively active (G12V) versions of each of the four Ras homologs to activate the effector protein Raf-1 in vivo. In addition, we compared their relative abilities to induce transformed foci, enable anchorage-independent growth, and stimulate cell migration. We found a distinct hierarchy between the four Ras homologs in each of the parameters studied. The hierarchies were as follows: for Raf-1 activation, Ki-Ras 4B > Ki-Ras 4A >>> N-Ras > Ha-Ras; for focus formation, Ha-Ras ≥ Ki-Ras 4A >>> N-Ras = Ki-Ras 4B; for anchorage-independent growth, Ki-Ras 4A ≥ N-Ras >>> Ki-Ras 4B = Ha-Ras = no growth; and for cell migration, Ki-Ras 4B >>> Ha-Ras > N-Ras = Ki-Ras 4A = no migration. Our results indicate that the four Ras homologs significantly differ in their abilities to activate Raf-1 and induce distinctly different biological responses. These studies, in conjunction with our previous report that demonstrated that the Ras homologs can be differentially activated by upstream guanine nucleotide exchange factors (Jones, M. K., and Jackson, J. H. (1998)J. Biol. Chem. 273, 1782–1787), indicate that each of the four Ras proteins may qualitatively or quantitatively participate in distinct signaling cascades and have significantly different biological roles in vivo. Importantly, these studies also suggest for the first time that the distinct and likely cooperative biological functions of the Ki-ras-encoded Ki-Ras 4A and Ki-Ras 4B proteins may help explain why constitutively activating mutations of Ki-ras, but not N-ras or Ha-ras, are frequently detected in human carcinomas.
Footnotes
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↵* This work was supported by National Institutes of Health Grant CA 54298 (to J. H. J.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed: Dept. of Immunology, IMM-12, The Scripps Research Institute, 10550 N. Torrey Pines Rd., La Jolla, CA 92037. Tel.: 619-784-8748; Fax: 619-784-8150; E-mail:jjackson{at}scripps.edu.
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↵2 J. Jackson and S. Wong, unpublished results.
- Abbreviations:
- GEF
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guanine nucleotide exchange factor
- GFP
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green fluorescent protein
- MBP
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myelin basic protein
- CRD
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cysteine-rich domain
- ERK
-
extracellular signal-regulated kinase
- MEK
-
mitogen-activated protein kinase /ERK kinase
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- Received February 1, 1999.
- Revision received March 31, 1999.
- The American Society for Biochemistry and Molecular Biology, Inc.
