Pak2 (p21-activated protein kinase 2) is the serine/threonine protein kinase PAK I, which was first detected as a protease activated form in Dr. Traugh’s laboratory in early 1980s (Tahara and Traugh 1981, 1982). Pak2 was further cloned and sequenced in the same lab in 1996 (Jakobi et al. 1996). Pak2 belongs to the PAK (p21-activated protein kinase) family, which can be activated by Rac and Cdc42 (Manser et al. 1994). The PAK family consists of group I protein kinases, including the highly homologous Pak1 (α-PAK), Pak2 (γ-PAK), and Pak3 (β-PAK), and the recently identified group II, including Pak4, Pak5, and Pak6 (Jaffer and Chernoff 2002). The 60-kDa Pak1 is expressed in brain and is also detected in muscle and spleen. The 60-kDa Pak3 is expressed mainly in brain. Pak2 is 58 kDa and expressed ubiquitously in mammalian cells (Roig and Traugh 2001) (http://www.genecards.org/cgi-bin/carddisp.pl?gene=PAK2).
Pak2 Structural Information
Pak2 Inhibition and Activation
Pak2 formed a transinhibited dimer before autophosphorylation and activation. The autoinhibitory domain is located in the region 92–133 in the regulatory domain. In the inactive state, the AID interacts with the catalytic domain to inhibit its kinase activity. PAK activation is through disruption of autoinhibition, followed by autophosphorylation. The two main activation mechanisms of Pak2 are caspase-mediated activation and small G protein-mediated activation.
Pak2 can be activated in response to a lot of stresses. Moderate stresses, like hyperosmolarity, ionizing radiation, DNA-damaging agents, and serum-deprivation, induce Pak2 activation in cells and lead to cell cycle arrest at G2/M (Roig and Traugh 2001). Activated Pak2 inhibits translation by phosphorylation of various substrates. Pak2 has specific protein substrates, e.g., histone 4, myosin light chain, prolactin, c-Abl, eukaryote translation initiation factor 3 (eIF3), eIF4B, eIF4G, and Mnk1. Pak2 recognizes the consensus sequence (K/RRXS).
Pak2 is the only member of the PAK family that is directly activated by caspase 3. When Pak2 is cleaved and activated by caspase 3, Pak2 promotes the morphological and biochemical changes of apoptosis. The proapoptosis protease, caspase 3, cleaves Pak2 after Asp 212 and thus produces a p27 fragment containing primarily the regulatory domain and a p34 fragment containing a small piece of the regulatory domain and the entire catalytic domain (Fig. 2). This event loosens the autoinhibitory dimer structure and leads to a complete autophosphorylation, which then results in a constitutively active p34 kinase domain (Hsu et al. 2008). The nuclear import signal (245–251) is required for nuclear localization (Jakobi et al. 2003). Disruption of the region (197–246), containing nuclear export signal, results in the nuclear localization of the Pak2 p34 fragment.
Pak2 and Apoptosis
Pak2 is activated in response to various biological stress, such as hyperosmolarity, ionizing radiation, DNA-damaging agents, and serum deprivation (Roig and Traugh 2001). Among them, the heat shock, H2O2, and UV radiation have been shown to stimulate apoptosis.
Pak2 is proteolytically cleaved during apoptosis. This cleavage event generates two Pak2 fragments and leads to the autophosphorylation and activation of Pak2. The p34 fragment containing the kinase domain is the active fragment. It can localize to nuclear and plays an important role in regulation of the apoptotic cell. The caspase-cleaved Pak2 (p34) has also been reported posttranslationally myristoylated (Vilas et al. 2006). Myristoylation of p34 can induce the relocation from cytosol to membranes and induce cell death without mitochondria damage. Nef protein of HIV has been shown to bind to and activate Pak2 and activates Pak2 and induces apoptosis in Jurkat T cell (Krautkramer et al. 2004).
Proteolytic kinase activation and protein kinase phosphorylation have become an important method of regulation in apoptosis (Bokoch 1998). These protein kinases include Pak2, MEKK1, FAK, DNA-PK, PITSLRE, PAKCaKII, Akt and Raf-1.
Pak2 and Cancer
Pak2 can be activated by Ras-related small GTPases, which can regulate the structure, mobility, and migration of the cytoskeleton in cancer cells, indicating the role of Pak2 in tumorigenesis and metastasis of the cancer cells (Kumar et al. 2006; Dummler et al. 2009). Pak2 can react with some key substrates related to cancer development, such as PIX and MLCK in cytoskeleton remodeling, c-Myc, MNK1, prolactin, and c-Raf1 in cell growth, and BAD in cell survival.
Pak2 is a negative regulator of Myc and suggested Pak2 may be the product of a tumor suppressor gene (Huang et al. 2004). Pak2 mediates tumor invasion in breast carcinoma cells (Coniglio et al. 2008). Inhibition of RhoA in Pak2-depleted cells decreases MLC phosphorylation and restores cell invasion. Also, the NF2 tumor suppressor Merlin is a substrate of Pak2 (Kissil et al. 2002). Wilkes (2009) showed that Erbin regulates the function of Merlin through Pak2 binding to Merlin.
Pak2 is a highly regulated enzyme. It can stimulate both cell growth through small GTPase binding and cell death through caspase cleavage. This characteristic makes this enzyme important for the future researches in apoptosis and cancer.