Last Updated: November 16, 2022
The Protein Kinase C (PKC) Family of Serine/Threonine Kinases
The protein kinase C (PKC) family of serine/threonine kinases are integrated into numerous signal transduction pathways elicited by a wide range of GPCR and other growth factor-dependent cellular responses. The original PKC enzyme was demonstrated to be lipid- and calcium-sensitive. This lipid-sensitive family of kinases is known to be activated by growth factor receptors that stimulate phospholipase C (PLC) family member enzymes. The PLC enzymes hydrolyze phosphatidylinositol-4,5-bisphosphate (PIP2; also designated PtdIns-4,5-P2) to generate membrane-bound diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP3; also designated Ins-1,4,5-P3) which mobilizes intracellular calcium (Ca2+). Both DAG and Ca2+ influence the activity of PKC enzymes.
Expression of most of the PKC family members is observed in most tissues and many different PKC subfamilies are expressed in the same cell types. However, some of the PKC isoforms are expressed in a tissue-specific manner. Expression of PKCθ is primarily found in skeletal muscle and lymphoid/hematopoietic cells. Expression of PKCγ is limited to neuronal tissues.
PKC isoforms are members of the AGC (PKA, PKG, PKC) family of protein serine/threonine kinases that contain a highly conserved catalytic domain and a regulatory domain that maintains the enzyme in an inactive conformation. The PKC isoforms have been subdivided into three distinct subfamilies based upon differences in their N-terminal regulatory domain structure. These three subfamilies are referred to as the conventional PKC isoforms (cPKC), the novel PKC isoforms (nPKC), and the atypical PKC isoforms (aPKC).
The regulatory domain of PKC isoforms resides in the N-terminus and contains an autoinhibitory pseudosubstrate domain. This pseudosubstrate domain contains a serine residue in place of the serine/threonine phosphoacceptor site, but otherwise resembles a natural PKC substrate. In addition to the catalytic and regulatory domains these enzymes contain two discrete membrane targeting modules, termed C1 and C2.
Conventional PKC Isoforms
The conventional PKC (cPKC) family includes cPKCα (cPKC-alpha), cPKCβI (cPKC-beta I), cPKCβII (cPKC-beta II), and cPKCγ (cPKC-gamma). The regulatory domains of cPKC isoforms contain a C1 domain consisting of tandem ~50 amino acid long sequences termed C1A and C1B. The C1A and C1B subdomains each have six cysteines and two histidines that coordinate two Zn2+ ions. The cPKCβII enzyme is an alternatively spliced version of cPKCβI. The C1A/C1B motifs function as a DAG-/PMA-binding motif (PMA: phorbol myristic acid). The regulatory domains of the cPKC isoforms also contain a C2 domain that binds anionic phospholipids in a calcium-dependent manner. All the cPKC isoforms require DAG, Ca2+, and phospholipids for activation.
Table of Conventional PKC Isoforms
| Protein Name | Gene Symbol | Comments |
| PKCα | PRKCA | located on chromosome 17q24.2; composed of 25 exons; encodes a 672 amino acid protein; involved in events related to epithelial cell function, bile acid mediated effects in the liver, hypertrophic responses in the heart, and within the CNS related to memory |
| PKCβ1 | PRKCB | located on chromosome 16p12.2–p12.1; composed of 18 exons; two alternatively spliced mRNAs; the PKCβ1 isoform is the shorter of the two isoforms; results from use of an alternative splice site at the 5′-end of the terminal exon; the PKCβ1 isoform has a different C-terminus than the PKCβ2 isoform; is a 671 amino acid protein; involved in B cell activation, intestinal carbohydrate absorption, endothelial cell proliferation, induction of apoptosis, and within the CNS the protein plays a role in stress responses |
| PKCβ2 | PRKCB | located on chromosome 16p12.2–p12.1; composed of 18 exons; two alternatively spliced mRNAs; the PKCβ2 isoform is the longer of the two isoforms and is a 673 amino acid protein |
| PKCγ | PRKCG | located on chromosome 19q13.42; composed of 20 exons; two alternatively spliced mRNAs; isoform 1 encodes a 710 amino acid protein, isoform 2 encodes a 697 amino acid protein; expression limited to the brain and spinal cord; only expressed in neurons; required for long term potentiation (LTP) and long term depression (LTD) responses in the brain |
Novel PKC Isoforms
The novel PKC (nPKC) family includes four isoforms identified as nPKCδ (nPKC-delta), nPKCθ (nPKC-theta), nPKCε (nPKC-epsilon), and nPKCη (nPKC-eta). The nPKCδ protein is encoded by the PRKCD gene, the nPKCθ protein is encoded by the PRKCQ gene, the nPKCε protein is encoded by the PRKCE gene, and the nPKCη protein is encoded by the PRCKH gene.
Similar to the conventional PKC family members, the four nPKC family members also have twin C1 domains (C1A and C1B) and a C2 domain. However, the nPKC C2 domains lack the critical calcium-coordinating acidic residues. It is this structural difference between cPKC and nPKC isoforms that accounts for the fact that nPKC isoforms are activated by DAG but not by Ca2+ and accounts for the distinct pharmacology of these two subclasses.
Atypical PKC Isoforms
The atypical PKC (aPKC) family includes two family members identified as aPKCζ (aPKC-zeta) and aPKCι (aPKC-iota; also referred to as aPKC-iota/lambda). The aPKCζ protein is encoded by the PRKCZ gene and the aPKCι protein is encoded by the PRKCI gene.
The atypical PKC proteins are so-called because they contain an atypical C1 domain harboring only a single cysteine-rich membrane-targeting structure. In addition, the aPKC isoforms lack a calcium-sensitive C2 domain. The C1 domains of aPKC enzymes bind PIP3 or ceramide not DAG or PMA. This accounts for the fact that the aPKC proteins are not activated by DAG nor by Ca2+.
The aPKC isoforms also contain a protein-protein interaction domain (PB1: Phox and Bem 1) that mediates interactions with other PB1-containing scaffolding proteins. The PBI-scaffolding proteins includes p62 (encoded by the SQSTM1 gene: sequestosome 1), partitioning defective-6 (PAR-6; encoded by the PARD6A gene), and mitogen-activated protein kinase kinase 5 (MAPK/ERK kinase 5: MEK5; encoded by the MAP2K5 gene). The activity of the aPKC isoforms is regulated via the PBI domain-mediated protein-protein interactions as well as via phosphoinositide-dependent kinase 1 (PDK1)-mediated phosphorylation.
Regulation of PKC Activities
Although the classic idea that the PKC act as generic kinases and achieve substrate specificity as a result of translocation events to the membrane, recent data indicates that this family of enzymes can also be regulated via phosphorylation on both serine/threonine and tyrosine residues. These phosphorylation events influence the stability, protease/phosphatase resistance, protein-protein interactions, subcellular localization, and substrate specificity and activity of the enzyme.
Some PKC isoforms have also been shown to be substrates for caspase-mediated cleavage that generates a catalytically active kinase domain and a released regulatory domain fragment. In some cases the released catalytic domain exhibits altered substrate specificity relative to that of the intact enzymes. The released regulatory fragment can act both as an inhibitor of the full-length enzyme and as an activator of certain signaling responses.
As indicated above, some PKC isoforms (e.g. the aPKC) are activated by interaction with less traditional lipid cofactors such as ceramides. In addition, some PKC can be activated via lipid-independent mechanisms that include oxidative modifications or tyrosine nitration.
Regulation of PKC activities has also been shown to be exerted by a family of proteins called receptors for activated C kinase (RACK) which are a family of membrane-associated PKC anchoring proteins. RACK serve as molecular scaffolds that localize individual PKC to distinct membrane microdomains in close proximity to their allosteric activators and substrates. It has been proposed that cells express a unique RACK for each PKC isoform and that resultant PKC-RACK interactions are essential for isoform-specific cellular responses. Definitive RACK proteins have been identified for PKCβ (RACK1) and PKCε (RACK2).