GEF at work: Vav in protruding filopodia

The Dbl family proto-oncogene vav is a nucleotide exchange factor for Rho f amily GTPases and is involved in triggering cytoskeletal changes contributi ng to the alterations of cell shape and motility, as well as in the inducti on of gene expression. In vitro and in vivo Vav is regulated by multiple ty rosine phosphorylation and binding to phosphatidylinositol phosphates. Alth ough recruitment of Vav to the plasma membrane appears important for the ac tivation of Vav function, there is little information on the precise subcel lular localization of Vav in living cells. Employing live video fluorescenc e and immunoelectron microscopy, we show that GFP-tagged full-length Vav, a nd several mutants in which the N-terminal regulatory calponin homology (CH ) domain has been deleted, specifically localize to the tips of filopodia. This localization was congruent with a high content of tyrosine phosphoryla tion in these regions. Consistent with earlier observations, mutants lackin g the C-terminal SH domain region were unable to translocate to the filopod ia tips. The enrichment in filopodial tips persisted despite their lateral movement but was dependent on forward growth. Upon retraction, the signal w as rapidly lost, indicating that Vav undergoes a specific and transient tra nslocation in response to actin-based, protrusive (C) 2001 Wiley-Liss, Inc.