Requirement of p21-activated kinase (PAK) for Salmonella typhimurium-induced nuclear responses

Salmonella typhimurium has sustained a long-standing association with its h ost and therefore has evolved sophisticated strategies to multiply and surv ive within this environment. Central to Salmonella pathogenesis is the func tion of a dedicated type III secretion system that delivers bacterial effec tor proteins into the host cell cytoplasm. These effecters stimulate nuclea r responses and actin cytoskeleton reorganization leading to the production of proinflammatory cytokines and bacterial internalization. The stimulatio n of these responses requires the function of Cdc42, a member of the Rho fa mily of small molecular weight GTPases, and SopE, a bacterial effector prot ein that stimulates guanine nucleotide exchange on Rho GTPases. However, no thing is known about the role of Cdc42 effector proteins in S. typhimurium- induced responses. We showed here that S. typhimurium infection of cultured epithelial cells results in the activation of p21-activated kinase (PAK), a serine/threonine kinase that is an effector of Cdc42-dependent responses. Transient expression of a kinase-defective PAK blocked both S. typhimurium - and SopE-induced c-Jun NH2-terminal kinase (JNK) activation but did not i nterfere with bacteria-induced actin cytoskeleton rearrangements. Similarly , expression of SH3-binding mutants of PAK did not block actin-mediated S. typhimurium entry into cultured cells. However, expression of an effector l oop mutant of Cdc42Hs (CdC42Hs(C40)) unable to bind PAK and other CRIB (for Cdc42/Rac interacting binding)-containing target proteins resulted in abro gation of both S. typhimurium-induced nuclear and cytoskeletal responses. T hese results show that PAK kinase activity is required for bacteria-induced nuclear responses but it is not required for cytoskeletal rearrangements, indicating that S. typhimurium stimulates cellular responses through differ ent Cdc42 downstream effector activities. In addition, these results demons trate that the effector loop of Cdc42 implicated in the binding of PAK and other CRIB-containing target proteins is required for both responses.