Protein Kinase C
Calcium-independent protein pkc
ATP + a protein ↔ ADP + a phosphoprotein
Protein Kinase C aka PKC EC 18.104.22.168, a family of protein kinases, where first discribed by Nishizuka et. al. in 1977 as nucleotide-independent, Ca2+-dependent serine kinases.  In the unstimulated, state they are found within the cytosol and are involved in numerous cell functions and signal transduction pathways. Molecular cloning has identified at least 11 izozymes that are subclassified into 3 groups based on their structure similarities and modes of stimulation. . Its ubiquitous expression and number of isozymes has made drug development difficult. Further studies have also shown that PCK is the intercellular receptor for tumor promoting phorbal esters and plays an important role in cancer research.
PKCs when synthesized are single polypeptides with a C-terminal catalytic domain, which show a high degree of homology across the isozymes and a N-terminal regulatory domain. Both separated by a flexible hinge region. The regulatory domains are more diverse and are the bases for the subdivision into three groups. The N-terminals inhibit the catalytic activity via a pseudosubstrate region that presents two zinc like fingers and allows for the binding of phorbol esters.  The dissociation of the pseudosubstrate region is provoked in an allosteric manner by the binding of cofactors such as diacylglycerol and Ca2+. In general, the isozymes contain four similar regions identified as C1-C4 . The interdomain influences play an important role in the adequate isozume-specific functioning and PKC should not be treated as simple string of independent domains. 
- Regulatory Domain
C1 and C2 where first discovered in PKC. However, they are found in a wide range of proteins and function in a similar way. 
- C2 domains are found in conventional and unconventional isozymes. They contain the recognition site for anionic phospholipids in a calcium-dependent manner.
- Catalytic Domain
The catalytic domains present 50-70 amino acid sequences and contain the highly conserved turn and hydrophobic phosphorylation motifs. 
- C3 presents the ATP binding site. 
- C4 is the kinase substrate binding lobe. 
Protein Kinase C is a family of at least 11 known isozymes. They are classified based on their structural similarities and activation requirements into conventional, unconventional/novel and atypical. 
- Conventional: activated by phosphatidylserine, diacylglycerol and Ca2+.
- Unconventional: activated by phosphatidylserine and diacylglycerol but do not require Ca2+.
- Atypical: activated by phosphatidylserine.
In its inactive form, the pseudosubstrate region of the regulatory domain prohibits interaction of the substrate with the catalytic domain. Most of the unstimulated PKC is found in the cell cytosol  and upon stimulation translocates to the plasma membrane.  The basic signal transduction pathway involves the allosteric activation of PKC by the intracellular messengers diacylglycerol and Calcium ions. An external stimulus, such as, growth factors, hormones, and neurotransmitters active a G protein-coupled receptor (GPRC) found on the plasma membrane. This in turn activates a stimulating G protein which in turn activates phospholipase C (PLC). The PLC cleaves phosphoinositol-4,5-bisphosphate (PIP2) into 1,2-diacylglycerol (DAG) and inositol-1.,4,5-triphosphate (IP3). The IP3 binds to calcium ion channels in the endoplasmic reticulum allowing for an increase of Ca2+ in the cytoplasm. The increased levels of Ca2+ avtivate PKC and it then translocates to the plasma membrane where it binds to DAG and form the active enzyme. It is important to note that, PKC activity is also affected by other inputs. These include, several kinases, phosphatases and intercellular binding proteins. 
The PKC family plays a major role in cellular signal transduction. Most PKC isozymes are ubiquitous and multiple members of the family can be coexpressed within the same cell leading to complex pathways. Their main roles consist in the regulation of cell proliferation, diferentiation, survival and apoptosis. 
General Functions of PCK Isozymes
|PKC-α||Insulin receptor feedback|
|PKC-β||B-cell receptor signaling
Mast cell activation
Transcription factor in hypoxia
Learning, addiction, anxiety
|PKC-δ||Smooth muscle cell homeostasis
B-cell receptor signaling
Various malignancies have been associated with altered PKC activity. High levels of PKC isozymes have been reported in breast, thyroid and lung cancers. Lower than expected levels have been seen in colon adenocarcinomas . Therefore, PKC inhibitors have become more important in the fight against cancer .
- A new Protein Kinase C Monocyte Assay has been developed to measure the PKC activity within vascular tissue. This method is based on the correlation between PKC activity in vascular tissue and that in mononuclear cells. It may be used to study disorders such as, diabetes, diabetic retinopathy, diabetic nephropathy, renal failure, hypertension, atherosclerosis and a number of cardiovascular disorders. 
- A 2007 study suggests that PKC inhibitors can be useful in the treatment of cognitive deficiencies in aging patients. 
- Another study shows that PKC isozymes could inhibit atherosclerotic disease progression.
- Meejung, 'et.at.' showed that the levels of retinal PKC-α increases with maturity and sugests that it plays a rold in the signal transduction pathways for postnatal development in porcine retina.. The human eye/retina shares similarities with that of the porcine. This suggests it can be used as a model for human disease. 
- Clinical studies of Ruboxistaurin, a PKC inhibitor, have shown to be successful in delaying loss of vision and macula degeneration. 
- A 2008 study determined that the stimulation of PKC has a role in the disruption of eipthelial apical junctions. This is likely to be important for cancer cell dissociataion and tumor metastasis. 
- Takai, Y., Kishimoto, A., Inoue, M., & Nishizuka, Y. (1977). Studies on a cyclic nucleotide-independent protein kinase and its proenzyme in mammalian tissues. I. Purification and characterization of an active enzyme from bovine cerebellum. J. Biol. Chem., 252(21), 7603-7609. 
- Churchill, E., Budas, G., Vallentin, A., Koyanagi, T., & Mochly-Rosen, D. (2008). PKC Isozymes in Chronic Cardiac Disease: Possible Therapeutic Targets?. Annual review of pharmacology and toxicology, 48, 569-599. Retrieved May 15, 2009, from Pubmed.
- Acs, P., Wang, Q., Bogi, K., Marquez, A., Lorenzo, P., Biro, T., Szallai, Z., Mushinski, F., & Blue, P. (1997). Both the Catalytic and Regulatory Domains of Protein Kinase C Chimeras Modulate the Proliferative Properties of NIH 3T3 Cells. J Biol Chem., 272(45), 28793-28799. Retrieved May 15, 2009, from Pubmed
- D Breitkreutz, L Braiman-Wiksman, N Daum, M F Denning, T Tennenbaum. (2007). Protein kinase C family: On the crossroads of cell signaling in skin and tumor epithelium. Journal of Cancer Research & Clinical Oncology, 133(11), 793-808. Retrieved May 16, 2009, from Pubmed
- Dekker, L (2004). Protein Kinase C (Molecular Biology Intelligence Unit). New York: Springer.
- (2003). Protein Kinase C Protocols (Methods in Molecular Biology). Totowa: Humana Press. Cite error: Invalid
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- Steinberg, S. (2008). Structural Basis of Protein Kinase C Isoform Function. Physiol. Rev., 88, 1341-1378. Retrieved October 5, 2009, from Pubmed
- Signal Transduction Resource. (n.d.). Retrieved May 15, 2009, from Promega
- http://www.joslinresearch.org/inventions/ Joslin Research Center
- Brennana, A., Yuanb, P., Dicksteinc, D., Rocherc, A., Hofc, P., Manjib, H., & Arnsten, A. (2007). Protein kinase C activity is associated with prefrontal cortical decline in aging . Neurobiology of Aging, 30(5), 785-792. Retrieved May 18, 2009, from Pubmed
- Meejung, A., Changjong, M., Chanwoo, J., Heechul, K., Jae-Kwang, J., & Taekyun, S. (2009). Immunohistochemical localization of protein kinase C-alpha in the retina of pigs during postnatal development . Neuroscience Letters, 455(2), 93-96 . Retrieved May 17, 2009, from Pubmed
- Garca, M., Ruiz-Ederra, J. & Vecina, E. (2005). Topography of pig retinal ganglion cells. The Journal of Comparative Neurology 486(4), 361 - 372 Pubmed
- Duh, E. (2008). The Role of Protein Kinase C in Diabetic Retinopathy . Boston: Humana Press.
- Ivanov, A., Samarin, S., Bachar, M., Parkos, & Nusrat, A. (2009). Protein kinase C activation disrupts epithelial apical junctions via ROCK-II dependent stimulation of actomyosin contractility. BMC Cell Biology. 10(36).