title: Activation of the Lbc Rho exchange factor proto-oncogene by truncation of an extended C terminus that regulates transformation and targeting.
abstract: The human lbc oncogene product is a guanine nucleotide exchange factor that specifically activates the Rho small GTP binding protein, thus resulting in biologically active, GTP-bound Rho, which in turn mediates actin cytoskeletal reorganization, gene transcription, and entry into the mitotic S phase. In order to elucidate the mechanism of onco-Lbc transformation, here we report that while proto- and onco-lbc cDNAs encode identical N-terminal dbl oncogene homology (DH) and pleckstrin homology (PH) domains, proto-Lbc encodes a novel C terminus absent in the oncoprotein that includes a predicted alpha-helical region homologous to cyto-matrix proteins, followed by a proline-rich region. The lbc proto-oncogene maps to chromosome 15, and onco-lbc represents a fusion of the lbc proto-oncogene N terminus with a short, unrelated C-terminal sequence from chromosome 7. Both onco- and proto-Lbc can promote formation of GTP-bound Rho in vivo. Proto-Lbc transforming activity is much reduced compared to that of onco-Lbc, and a significant increase in transforming activity requires truncation of both the alpha-helical and proline-rich regions in the proto-Lbc C terminus. Deletion of the chromosome 7-derived C terminus of onco-Lbc does not destroy transforming activity, demonstrating that it is loss of the proto-Lbc C terminus, rather than gain of an unrelated C-terminus by onco-Lbc, that confers transforming activity. Mutations of onco-Lbc DH and PH domains demonstrate that both domains are necessary for full transforming activity. The proto-Lbc product localizes to the particulate (membrane) fraction, while the majority of the onco-Lbc product is cytosolic, and mutations of the PH domain do not affect this localization. The proto-Lbc C-terminus alone localizes predominantly to the particulate fraction, indicating that the C terminus may play a major role in the correct subcellular localization of proto-Lbc, thus providing a mechanism for regulating Lbc oncogenic potential.
authors.affiliation: Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA.
authors.forename: P
authors.initials: P
authors.lastname: Sterpetti
authors.forename: A A
authors.initials: AA
authors.lastname: Hack
authors.forename: M P
authors.initials: MP
authors.lastname: Bashar
authors.forename: B
authors.initials: B
authors.lastname: Park
authors.forename: S D
authors.initials: SD
authors.lastname: Cheng
authors.forename: J H
authors.initials: JH
authors.lastname: Knoll
authors.forename: T
authors.initials: T
authors.lastname: Urano
authors.forename: L A
authors.initials: LA
authors.lastname: Feig
authors.forename: D
authors.initials: D
authors.lastname: Toksoz
mesh_terms: D054758:A Kinase Anchor Proteins; D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D002471:Cell Transformation, Neoplastic / Q000235:genetics; D002678:Chimera / Q000235:genetics; D002884:Chromosomes, Human, Pair 15 / Q000235:genetics; D002897:Chromosomes, Human, Pair 7 / Q000235:genetics; D006224:Cricetinae; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D019204:GTP-Binding Proteins / Q000235:genetics*; D005786:Gene Expression Regulation; D015321:Gene Rearrangement; D006801:Humans; D015778:Minor Histocompatibility Antigens; D008969:Molecular Sequence Data; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D011519:Proto-Oncogenes; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D014018:Tissue Distribution; D014162:Transfection
chemical_list: D054758:A Kinase Anchor Proteins; C085467:AKAP13 protein, human; D048868:Adaptor Proteins, Signal Transducing; D017931:DNA Primers; D018076:DNA, Complementary; C000715470:MAS1 protein, human; D015778:Minor Histocompatibility Antigens; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D019204:GTP-Binding Proteins