The Rho-guanine nucleotide exchange factors (Rho-GEFs) remodel the actin 
cytoskeleton via their Rho-GTPase targets and affect numerous physiological 
processes such as transformation and cell motility. They are therefore 
attractive targets to design specific inhibitors that may have therapeutic 
applications. Trio contains two Rho-GEF domains, GEFD1 and GEFD2, which activate 
the Rac and RhoA pathways, respectively. Here we have used a genetic screen in 
yeast to select in vivo peptides coupled to thioredoxin, called aptamers, that 
could inhibit GEFD2 activity. One aptamer, TRIAPalpha (TRio Inhibitory APtamer), 
specifically blocks GEFD2-exchange activity on RhoA in vitro. The corresponding 
peptide sequence, TRIPalpha, inhibits TrioGEFD2-mediated activation of RhoA in 
intact cells and specifically reverts the neurite retraction phenotype induced 
by TrioGEFD2 in PC12 cells. Thus TRIPalpha is the first Rho-GEF inhibitor 
isolated so far, and represents an important step in the design of inhibitors 
for the expanding family of Rho-GEFs.