Protein kinase C

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Protein kinase C ('PKC', EC 2.7.11.13) is a family of protein kinases consisting of ~10 isozymes.[1] They are divided into three subfamilies, based on their second messenger requirements: conventional (or classical), novel, and atypical.[2] Conventional (c)PKCs contain the isoforms α, βI, βII, and γ. These require Ca2+, diacylglycerol (DAG), and a phospholipid such as phosphatidylcholine for activation. Novel (n)PKCs include the δ, ε, η, and θ isoforms, and require DAG, but do not require Ca2+ for activation. Thus, conventional and novel PKCs are activated through the same signal transduction pathway as phospholipase C. On the other hand, atypical (a)PKCs (including protein kinase Mζ and ι / λ isoforms) require neither Ca2+ nor diacylglycerol for activation. The term "protein kinase C" usually refers to the entire family of isoforms.

Isozymes

Structure

The structure of all PKCs consists of a regulatory domain and a catalytic domain tethered together by a hinge region. The catalytic region is highly conserved among the different isoforms, as well as, to a lesser degree, among the catalytic region of other serine/threonine kinases. The second messenger requirement differences in the isoforms are a result of the regulatory region, which are similar within the classes, but differ among them. Most of the crystal structure of the catalytic region of PKC has not been determined, except for PKC theta and iota. Due to its similarity to other kinases whose crystal structure have been determined, the structure can be strongly predicted.

Regulatory

The regulatory domain or the amino-teminus of the PKCs contains several shared subregions. The C1 domain, present in all of the isoforms of PKC has a binding site for DAG as well as non-hydrolysable, non-physiological analogues called phorbol esters. This domain is functional and capable of binding DAG in both conventional and novel isoforms, however, the C1 domain in atypical PKCs is incapable of binding to DAG or phorbol esters. The C2 domain acts as a Ca2+ sensor and is present in both conventional and novel isoforms, but functional as a Ca2+ sensor only in the conventional. The pseudosubstrate region, which is present in all three classes of PKC, is a small sequence of amino acids that mimic a substrate and bind the substrate-binding cavity in the catalytic domain keeping the enzyme inactive. When Ca2+ and DAG are present in sufficient concentrations, they bind to the C2 and C1 domain, respectively, and recruit PKC to the membrane. This interaction with the membrane results in release of the pseudosubstrate from the catalytic site and activation of the enzyme. In order for these allosteric interactions to occur, however, PKC must first be properly folded and in the correct conformation permissive for catalytic action. This is contingent upon phosphorylation of the catalytic region, discussed below.

Catalytic

The catalytic region or kinase core of the PKA allows for different functions to be processed, PKB (also known as Akt) and PKC kinases contains approximately 40% amino acid sequence similarity. This similarity increases to ~ 70% across PKCs and even higher when comparing within classes. For example, the two atypical PKC isoforms, ζ and ι/λ, are 84% identical (Selbie et al., 1993). Of the over-30 protein kinase structures whose crystal structure has been revealed, all have the same basic organization. They are a bilobal structure with a β sheet comprising the N-terminal lobe and an α helix constituting the C-terminal lobe. Both the ATP- and substrate-binding sites are located in the cleft formed by these two lobes. This is also where the pseudosubstrate domain of the regulatory region binds.

Another feature of the PKC catalytic region that is essential to the viability of the kinase is its phosphorylation. The conventional and novel PKCs have three phosphorylation sites, termed: the activation loop, the turn motif, and the hydrophobic motif. The atypical PKCs are phosphorylated only on the activation loop and the turn motif. Phosphorylation of the hydrophobic motif is rendered unnecessary by the presence of a glutamic acid in place of a serine, which, as a negative charge, acts similar in manner to a phosphorylated residue. These phosphorylation events are essential for the activity of the enzyme, and 3-phosphoinositide-dependent protein kinase-1 (PDK1) is the upstream kinase responsible for initiating the process by transphosphorylation of the activation loop.(Balendran et al., 2000)

The consensus sequence of protein kinase C enzymes is similar to that of protein kinase A, since it contains basic amino acids close to the Ser/Thr to be phosphorylated. Their substrates are, e.g., MARCKS proteins, MAP kinase, transcription factor inhibitor IκB, the vitamin D3 receptor VDR, Raf kinase, calpain, and the epidermal growth factor receptor.

Activation

Upon activation, protein kinase C enzymes are translocated to the plasma membrane by RACK proteins (membrane-bound receptor for activated protein kinase C proteins). The protein kinase C enzymes are known for their long-term activation: They remain activated after the original activation signal or the Ca2+-wave is gone. This is presumably achieved by the production of diacylglycerol from phosphatidylcholine by a phospholipase; fatty acids may also play a role in long-term activation.

Function

PKC phosphorylates other proteins, altering their function. However, what proteins are available for phosphorylation depends on in what kind of cell the PKC activity is present, since protein composition varies from cell type to cell type. Thus, effects of PKC are cell-type specific:

Overview table

Cell type Organ/system Activators
ligands --> Gq-GPCRs
Effects
smooth muscle cell (gastrointestinal tract sphincters) Digestive system contraction
smooth muscle cell (iris dilator muscle) Sensory system contraction
smooth muscle cell (iris constrictor muscle) Sensory system contraction
smooth muscle cell (ciliary muscle) Sensory system contraction
smooth muscle cell (urethral sphincter) Urinary system contraction
smooth muscle cell (uterus) Reproductive system contraction
smooth muscle cell (arrector pili muscles) Integumentary system contraction
smooth muscle cell (ureter) Urinary system contraction
smooth muscle cell (urinary bladder) Urinary system
smooth muscle cell (vascular) Circulatory system
smooth muscle cell (seminal tract[8]) Reproductive system ejaculation
smooth muscle cell (GI tract) Digestive system
  • contraction [9]
smooth muscle cell (bronchi) Respiratory system bronchoconstriction [6]
proximal convoluted tubule cell kidney
neurons in autonomic ganglia nervous system acetylcholine --> M1 receptor EPSP
neurons in CNS nervous system
  • neuronal excitation (5-HT) [6]
  • memory? (acetylcholine) [13]
platelets Circulatory system 5-HT --> 5-HT2A receptor[6] Aggregation[6]
ependymal cells (choroid plexus) Ventricular system 5-HT --> 5-HT2C receptor[6] cerebrospinal fluid secretion[6]
heart muscle circulatory system Positive ionotropic effect[4]
Serous cells (salivary gland) Digestive system
Serous cells (lacrimal gland) Digestive system
  • ↑Secretion[7]
adipocyte Digestive system/Endocrine system
hepatocyte Digestive system
sweat gland cells Integumentary system
  • ↑Secretion[4]
parietal cells Digestive system acetylcholine --> M1 receptors[11] gastric acid secretion

See also

References

  1. Mellor H, Parker PJ (1998). "The extended protein kinase C superfamily". Biochem. J. 332 ( Pt 2): 281–92. PMID 9601053. 
  2. Nishizuka Y (1995). "Protein kinase C and lipid signaling for sustained cellular responses". FASEB J. 9 (7): 484–96. PMID 7737456. 
  3. 3.0 3.1 Signal transduction in lower esophageal sphincter circular muscle Piero Biancani, Ph.D. and Karen M. Harnett, Ph.D.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2". Neuroscience (Third Edition ed.). Sunderland, Mass: Sinauer. ISBN 0-87893-725-0. 
  5. Chou EC, Capello SA, Levin RM, Longhurst PA (2003). "Excitatory α1-adrenergic receptors predominate over inhibitory β-receptors in rabbit dorsal detrusor". J. Urol. 170 (6 Pt 1): 2503–7. PMID 14634460. doi:10.1097/01.ju.0000094184.97133.69. 
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4.  Page 187
  7. 7.0 7.1 Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4.  Page 127
  8. Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4.  Page 163
  9. G Protein-Coupled Receptors in Gastrointestinal Physiology IV. Neural regulation of gastrointestinal smooth muscle Kenton M. Sanders
  10. Keith Parker; Laurence Brunton; Goodman, Louis Sanford; Lazo, John S.; Gilman, Alfred (2006). Goodman & Gilman's the pharmacological basis of therapeutics (11th Ed. ed.). New York: McGraw-Hill. pp. page 185. ISBN 0-07-142280-3. 
  11. 11.0 11.1 "Entrez Gene: CHRM1 cholinergic receptor, muscarinic 1". 
  12. 12.0 12.1 Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3.  Page 787
  13. Rang HP, Dale MM, Ritter JM, Moore PK (2003). "Ch. 10". Pharmacology (5th edition ed.). Elsevier Churchill Livingstone. pp. page 139. ISBN 0-443-07145-4. 

External links

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