Protein kinases N (PKN), also known as protein kinase C-related kinases (PKR) feature a C-terminal serine/threonine kinase domain and three RHO-binding motifs at the N-terminus. RHO GTPases RHOA, RHOB, RHOC and RAC1 bind PKN1, PKN2 and PKN3 (Maesaki et al. 1999, Zhong et al. 1999, Owen et al. 2003, Modha et al. 2008, Hutchinson et al. 2011, Hutchinson et al. 2013), bringing them in proximity to the PIP3-activated co-activator PDPK1 (PDK1
) (Flynn et al. 2000, Torbett et al. 2003). PDPK1 phosphorylates PKNs on a highly conserved threonine residue in the kinase activation loop, which is a prerequisite for PKN activation. Phosphorylation of other residues might also be involved in activation (Flynn et al. 2000, Torbett et al. 2003, Dettori et al. 2009). PKNs are activated by fatty acids like arachidonic acid and phospholipids in vitro, but the in vivo significance of this activation remains unclear (Palmer et al. 1995, Yoshinaga et al. 1999).
PKNs play important roles in diverse functions, including regulation of cell cycle, receptor trafficking, vesicle transport and apoptosis. PKN is also involved in the ligand-dependent transcriptional activation by the androgen receptor. More than 20 proteins and several peptides have been shown to be phosphorylated by PKN1 and PKN2, including CPI-17 (Hamaguchi et al. 2000), alpha-actinin (Mukai et al. 1997), adducin (Collazos et al. 2011), CDC25C (Misaki et al. 2001), vimentin (Matsuzawa et al. 1997), TRAF1 (Kato et al. 2008), CLIP170 (Collazos et al. 2011) and EGFR (Collazos et al. 2011). There are no known substrates for PKN3 (Collazos et al. 2011).