Lfc and Lsc Oncoproteins Represent Two New Guanine Nucleotide Exchange Factors for the Rho GTP-binding Protein

doi:10.1074/jbc.271.44.27374

Lfc and Lsc Oncoproteins Represent Two New Guanine Nucleotide Exchange Factors for the Rho GTP-binding Protein*

From the ^‡ Department of Pharmacology, Cornell University, Ithaca, New York 14853, the
^§Department of Pharmacology and the Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, North Carolina 27599, and the
^¶Terry Fox Laboratory, British Columbia Cancer Agency, Vancouver, British Columbia V5Z 4E6, Canada

^″To whom correspondence should be addressed.

Abstract

Lfc and Lsc are two recently identified oncoproteins that contain a Dbl homology domain in tandem with a pleckstrin homology domain and thus share sequence similarity with a number of other growth regulatory proteins including Dbl, Tiam-1, and Lbc. We show here that Lfc and Lsc, like their closest relative Lbc, are highly specific guanine nucleotide exchange factors (GEFs) for Rho, causing a >10-fold stimulation of [³H]GDP dissociation from Rho and a marked stimulation of GDP-[³⁵S]GTPγs (guanosine 5′-O-(3-thiotriphosphate) exchange. All three proteins (Lbc, Lfc, and Lsc) are able to act catalytically in stimulating the guanine nucleotide exchange activity, such that a single molecule of each of these oncoproteins can activate a number of molecules of Rho. Neither Lfc nor Lsc shows any ability to stimulate GDP dissociation from other related GTP-binding proteins such as Rac, Cdc42, or Ras. Thus Lbc, Lfc, and Lsc appear to represent a subgroup of Dbl-related proteins that function as highly specific GEFs toward Rho and can be distinguished from Dbl, Ost, and Dbs which are less specific and show GEF activity toward both Rho and Cdc42. Consistent with these results, Lbc, Lfc, and Lsc each form tight complexes with the guanine nucleotide-depleted form of Rho and bind weakly to the GDP- and GTPγS-bound states. None of these oncoproteins are able to form complexes with Cdc42 or Ras. However, Lfc (but not Lbc nor Lsc) can bind to Rac, and this binding occurs equally well when Rac is nucleotide-depleted or is in the GDP- or GTPγS-bound state. These findings raise the possibility that in addition to acting directly as a GEF for Rho, Lfc may play other roles that influence the signaling activities of Rac and/or coordinate the activities of the Rac and Rho proteins.

Footnotes

↵^§ Supported by a National Institutes of Health Predoctoral Training Grant T32 GM08210.
↵* This work was supported by National Institutes of Health Grant GM47458, Grant 4104 from the Council for Tobacco Research, and Department of Defense Grant DAMD17-94-J-4123. 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.
↵¹ The abbreviations used are:

GEF

guanine nucleotide exchange factor

DH

Dbl homology

GST

glutathione S-transferase

JNK

c-Jun kinase

GTPγS

guanosine 5′-O-(3-thiotriphosphate)

Mant

N-methylanthraniloyl

PH

pleckstrin homology.
↵²J. A. Glaven, I. P. Whitehead, R. Kay, and R. A. Cerione, unpublished data.
↵³J. Westwick, I. P. Whitehead, and C. J. Der, unpublished data.
- Received May 25, 1996.
- Revision received August 16, 1996.