Each protein kinase C related kinase - PKN1 (PRK1), PKN2 (PRK2) and PKN3 (PRK3) - possesses three RHO binding motifs known as HR1 (REM) domains - HR1a (REM1), HR1b (REM2) and HR1c (REM3) located at the N-termini of PKN1, PKN2 and PKN3. These domains mediate the binding of each kinase to any of the three RHO family GTPases: RHOA, RHOB and RHOC. HR1 domains of PKN1, PKN2 and PKN3 contain anti-parallel coiled-coil finger (ACC finger) folds (Maesaki et al. 1999, Hutchinson et al. 2011, Hutchinson et al. 2013). RHO GTPase RAC1 also binds and activates PKN1 (Owen et al. 2003, Modha et al. 2008), with the interaction involving HR1a and HR1b RHO-binding motifs of PKN1 and the polybasic region of RAC1, and it also binds to PKN2 (Zong et al. 1999). Binding of RAC1 to PKN3 is likely, based on sequence similarity, but has not been experimentally investigated.