The Dbl-related protein, Lfc, localizes to microtubules and mediates the activation of Rac signaling pathways in cells

The possibility that the Dbl family member Lfc can activate Rad in cells is investigated in this study, Previously, we demonstrated that both Lfc and Lsc, like their closest relative Lbc, can act catalytically in stimulating the guanine nucleotide exchange activity of RhoA in vitro. Neither Lfc nor Lsc stimulated the in vitro exchange activity of Cdc42 or Rad; however, Lfc was capable of forming a tight complex with Rac1 in vitro, We show here th at Lfc stimulates c-Jun kinase (JNK) activity in COS-7 cells. This stimulat ion was blocked by a dominant negative mutant of Rad and somewhat less effe ctively by dominant negative RhoA, but not by dominant negative Cdc42, Over expression of Lfc in NIH 3T3 cells induced the formation of actin stress fi bers and membrane ruffles, consistent with the activation of both RhoA and Rad signaling pathways, whereas overexpression of Lsc led exclusively to we ll developed stress fibers. Using a recently developed assay for measuring the cellular activation of Pac, we did not find that expression of Lfc incr eased the levels of GTP-bound Rad. However, an examination of the cellular localization of Lfc showed that it was localized to microtubules, similar t o what has been reported for activated! Rad, the mixed lineage kinase (MLK) and JNK. Moreover, we have found that the Pleckstrin homology (PH) domain of Lfc specifically associates with tubulin, Taken together, these findings suggest a model where the PH domain-mediated localization of Lfc to microt ubules enables the recruitment of Rac to a site proximal to its signaling t argets, resulting in JNK activation and actin cytoskeletal changes.