Human ras-related C3 botulinum toxin substrate 3 (GTPase Rac3) in complex with human p21(CDKN1A) activated kinase 4

Six members of the p21-activated kinase (PAK) family of protein kinases have been identified to date. PAK’s are serine/threonine kinases and can be classified into two groups named group I (PAK 1-3), and group II (PAK 4-6) based on their sequence homology and regulatory properties. In addition to the C-terminal Ste20-like catalytic domain, PAK kinases contain a Cdc42/Rac-interactive binding (CRIB) domain. PAK family members have been implicated in the regulation of multiple cellular functions, including actin reorganization, apoptotic signaling, cell motility, gene transcription, cell transformation and steroid hormone receptor signalling.

The binding of activated GTP-bound Cdc42 or Rac to group I PAKs dramatically stimulates their ability to phosphorylate exogenous substrates. In contrast, the group II PAK kinase PAK4 and PAK5 are active in the absence of GTPases and their enzymatic activity is not further stimulated by binding of activated GTPase. The reason for this difference in regulation is most likely due to the lack of an inhibitory switch domain in group II PAK’s which releases the active site upon binding of GTPases. However, GTPase binding has been shown to mediate kinase relocalization. For example upon binding of Cdc42, PAK4 relocalizes to the Golgi, and PAK5 shuttles from the microtubule network to actin-rich structures.

Here we present the crystal structure of Rac3 in complex with the CRIB domain of PAK4.