TI - Pak1 and Pak2 mediate tumor cell invasion through distinct signaling mechanisms . AB - Pak kinases are thought to play critical roles in cell migration and invasion . Here , we analyze the roles of Pak1 and Pak2 in breast carcinoma cell invasion using the transient transfection of small interfering RNA . We find that although both Pak1 and Pak2 contribute to breast carcinoma invasion stimulated by heregulin , these roles are mediated by distinct signaling mechanisms . Thus , whereas the depletion of Pak1 interferes with the heregulin-mediated dephosphorylation of cofilin , the depletion of Pak2 does not . The depletion of Pak1 also has a stronger inhibitory effect on lamellipodial protrusion than does the depletion of Pak2 . Interestingly , Pak1 and Pak2 play opposite roles in regulating the phosphorylation of the myosin light chain ( MLC ) . Whereas the depletion of Pak1 decreases phospho-MLC levels in heregulin-stimulated cells , the depletion of Pak2 enhances MLC phosphorylation . Consistent with their opposite effects on MLC phosphorylation , Pak1 and Pak2 differentially modulate focal adhesions . Pak2-depleted cells display an increase in focal adhesion size , whereas in Pak1-depleted cells , focal adhesions fail to mature . We also found that the depletion of Pak2 , but not Pak1 , enhances RhoA activity and that the inhibition of RhoA signaling in Pak2-depleted cells decreases MLC phosphorylation and restores cell invasion . In summary , this work presents the first comprehensive analysis of functional differences between the Pak1 and Pak2 isoforms .