PAKs, p21 activated kinases, are a family of serine/threonine protein kinases comprised of six isoforms, PAK1-6, and they play important roles in cytoskeleton dynamics, cell survival and proliferation (Ye et al, 2012). Each of these isoforms contains a C-terminal catalytic domain and an N-terminal regulatory domain with a small G protein binding motif (Chen et al, 2004). OSR1, oxidative stress response 1, is activated only by osmotic stresses, like sorbital or NaCl (Chen et al, 2004). It has been predicted that OSR1 phosphorylates PAK1 in the regulatory domain at thr84 and inhibits activation of JNK and MAPK pathway. (Chen et al, 2004). It has also been suggested that OSR1 may have a regulating function with actin cytoskeleton because it can phosphorylate PAK1 at Thr-84 and bind to gelsolin (Chen et al, 2004).
