CHP, A HOMOLOG OF THE GTPASE CDC42HS, ACTIVATES THE JNK PATHWAY AND IS IMPLICATED IN REORGANIZING THE ACTIN CYTOSKELETON

The p21-activated protein kinases (PAKs) are activated through direct interaction with the GTPases Rac and Cdc42Hs, which are implicated in the control of the mitogen-activated protein kinase (MAP kinase) c-Jun N-terminal kinase (JNK) and the reorganization of the actin cytoskele ton [1-3]. The exact role of the PAK proteins in these signaling pathw ays is not entirely clear, To elucidate the biological function of Pak 2 and to identify its molecular targets, we used a novel two-hybrid sy stem, the Ras recruitment system (RRS), that aims to detect protein-pr otein interactions at the inner surface of the plasma membrane (descri bed in the accompanying paper by Broder et al. [4]), The Pak2 regulato ry domain (PakR) was fused at the carboxyl terminus of a RasL61 mutant protein and screened against a myristoylated rat pituitary cDNA libra ry. Four clones were identified that interact specifically with PakR a nd three were subsequently shown to encode a previously unknown homolo gue of the GTPase Cdc42Hs. This similar to 36 kDa protein, designated Chp, exhibits an overall sequence identity to Cdc42Hs of similar to 52 %. Chp contains two additional sequences at the amino and carboxyl ter mini that are not found in any known GTPase. The amino terminus contai ns a polyproline sequence, typically found in Src homology 3 (SH3)-bin ding domains, and the carboxyl terminus appears to be important for Pa k2 binding. Results from the microinjection of Chp into cells implicat ed Chp in the induction of lamellipodia and showed that Chp activates the JNK MAP kinase cascade. (C) Current Biology Ltd ISSN 0960-9822.