PKN is a lipid-activated serine/threonine kinase. It is a member of the protein kinase C (PKC) superfamily, but lacks a C1 domain. There are at least 3 different isoforms of PKN (PRK1/PKNalpha/PAK1; PKNbeta, and PRK2/PAK2/PKNgamma). The C-terminal region contains the Ser/Thr type protein kinase domain, while the N-terminal region of PKN contains three antiparallel coiled-coil (ACC) finger domains which are relatively rich in charged residues and contain a leucine zipper-like sequence. These domains binds to the small GTPase RhoA. Following these domains is a C2-like domain. Its C-terminal part functions as an auto-inhibitory region. PKNs are not activated by classical PKC activators such as diacylglycerol, phorbol ester or Ca2+, but instead are activated by phospholipids and unsaturated fatty acids. The C2 domain was first identified in PKC. C2 domains fold into an 8-standed beta-sandwich that can adopt 2 structural arrangements: Type I and Type II, distinguished by a circular permutation involving their N- and C-terminal beta strands. Many C2 domains are Ca2+-dependent membrane-targeting modules that bind a wide variety of substances including bind phospholipids, inositol polyphosphates, and intracellular proteins. Most C2 domain proteins are either signal transduction enzymes that contain a single C2 domain, such as protein kinase C, or membrane trafficking proteins which contain at least two C2 domains, such as synaptotagmin 1. However, there are a few exceptions to this including RIM isoforms and some splice variants of piccolo/aczonin and intersectin which only have a single C2 domain. C2 domains with a calcium binding region have negatively charged residues, primarily aspartates, that serve as ligands for calcium ions.