Rac, a small GTPase in the ras superfamily, regulates at least two bio
logical processes in animal cells: (i) the polymerization of actin and
the assembly of integrin complexes to produce lamellipodia and ruffle
s; and (ii) the activity of an NADPH oxidase in phagocytic cells. NADP
H oxidase activation is mediated through a rac effector protein, p67(p
hox), and using chimeras made between rac and the closely related GTPa
se, rho, we have identified two distinct effector sites in rac, one N-
terminal and one C-terminal, both of which are required for activation
of p67(phox). The same two effector sites are essential far rac-induc
ed actin polymerization in fibroblasts. p65(PAK), a ubiquitous serine/
threonine kinase, interacts with rac at both the N- and C-terminal eff
ector sites, but the GTPase-activating protein, bcr interacts with rac
at a different region, This makes p65(PAK), but not bcr, a candidate
effector of rac-induced lamellipodium formation.