Activation of the Lbc Rho exchange factor proto-oncogene by truncation of an extended C terminus that regulates transformation and targeting

The human lbc oncogene product is a guanine nucleotide exchange factor that specifically activates the Rho small GTP binding protein, thus resulting i n biologically active, GTP-bound Rho, which in turn mediates actin cytoskel etal reorganization, gene transcription, and entry into the mitotic S phase , In order to elucidate the mechanism of onco-Lbc transformation, here we r eport that while proto- and onco-lbc cDNAs encode identical N-terminal dbl oncogene homology (DH) and pleckstrin homolog (PH) domains, proto-Lbc encod es a novel C terminus absent in the oncoprotein that includes a predicted a lpha-helical region homologous to cyto-matrix proteins, followed by a proli ne-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, unre lated C-terminal sequence from chromosome 7. Both onco- and proto-Lbc can p romote formation of GTP-bound Rho in vivo. Proto-Lbc transforming activity is much reduced compared to that of onco-lhc, and a significant increase in transforming activity requires truncation of both the or-helical and proli ne-rich regions in the proto-Lbc C terminus. Deletion of the chromosome 7-d erived C terminus of onco-lhc does not destroy transforming activity, demon strating that it is loss of the proto-Lbc C terminus, rather than gain of a n unrelated C-terminus by onco-lbc, that confers transforming activity.,Mut ations of onco-lbc DH and PH domains demonstrate that both domains are nece ssary for full transforming activity. The proto-Lbc product localizes to th e particulate (membrane) fraction, while the majority of the onco-lhc produ ct is cytosolic? and mutations of the PH domain do not affect this localiza tion, The proto-Lbc C-terminus alone localizes predominantly to the particu late 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.