Definition
Protein kinase A (PKA) refers to a family of enzymes whose activity is dependent on the level of cyclic AMP (cAMP) in the cell. PKA is also known as cAMP-dependent protein kinase. PKA is a member of the Ser/Thr protein kinase family, which is involved in the regulation of a large number of processes, including cell proliferation and metabolism, as well as higher level functions of learning and memory 1.
Structural Characteristics
Unlike many protein kinases, the catalytic and regulatory functions of PKA are on separate polypeptide chains. The catalytic (C) subunits are responsible for catalyzing phosphoryl transfer, whereas the regulatory (R) subunits confer cAMP dependence and localize the holoenzyme to discrete subcellular locations via interactions with protein kinase A anchoring proteins2. At low intracellular cAMP concentrations, PKA is maintained as an inactive tetrameric holoenzyme complex (R2C2) consisting of a homodimeric R2 subunit and two C subunits. When intracellular concentrations of cAMP are increased in response to specific stimuli, two cAMP molecules bind allosterically to each R subunit, which releases inhibition of the C subunits and allows them to phosphorylate their protein targets.
There are four major isoforms of PKA that differ with respect to their R subunits (RIa, RIß, RIIa, and RIIß). These isoforms have different biological functions, as determined by genetic studies using mice 3.
Mode of Action
PKA holoenzyme is one of the major receptors for cAMP, where an extracellular stimulus is translated into a signaling response. It is found that the RI regulatory subunit upon binding to the catalytic subunit, RI undergoes a dramatic conformational change in which the two cAMP-binding domains uncouple and wrap around the large lobe of the catalytic subunit. This large conformational reorganization reveals the concerted mechanism required to bind and inhibit the catalytic subunit. Furthermore, the structure also reveals a holoenzyme-specific salt bridge between two conserved residues, Glu261 and Arg366, which tethers the two adenine capping residues far from their cAMP-binding sites. Mutagenesis of these residues demonstrates their importance for PKA activation. These structural insights, combined with the mutagenesis results, provide a molecular mechanism for the ordered and cooperative activation of PKA by cAMP 4.
Functions
Molecular Mechanisms for PKA Mediated Modulation of Immune Function: In lymphocytes, phosphorylation by PKA has been demonstrated to regulate antigen receptor-induced signalling both by altering protein-protein interactions and by changing the enzymatic activity of target proteins. PKA substrates involved in immune activation include transcription factors, members of the MAP kinase pathway and phospholipases. The ability of PKA type I to regulate activation of signalling components important for formation of the immunological synapse, demonstrates that the cAMP signalling pathway can directly modulate proximal events in lymphocyte activation. Furthermore, the recent discovery that PKA regulates Src kinases through modulation of Csk, supports the notion that PKA is involved in the fine-tuning of immune receptor signalling in lipid rafts 5.
PKA Deficiency Causes Axially Localized Neural Tube Defects in Mice: A study has investigated the function of PKA during embryonic development using a PKA-deficient mouse that retains only one functional catalytic subunit allele, either Ca or Cß, of PKA. It has been shown that the reduced PKA activity results in neural tube defects that are specifically localized posterior to the forelimb buds and lead to spina bifida. The affected neural tube has closed appropriately but exhibits an enlarged lumen and abnormal neuroepithelium. Furthermore, decreased PKA activity causes dorsal expansion of Sonic hedgehog signal response in the thoracic to sacral regions correlating with the regions of morphological abnormalities. Other regions of the neural tube appear normal. The regional sensitivity to changes in PKA activity indicates that downstream signaling pathways differ along the anterior-posterior axis and suggesting a functional role for PKA activation in neural tube development 6.
Genetically Lean Mice Result from Targeted Disruption of the RIIß Subunit of PKA: In mice, RIIß isoform is abundant in brown and white adipose tissue and brain, with limited expression elsewhere. To elucidate, a study investigated the function by generating RIIß knockout mice. It as been reported that mutants appear healthy but have markedly diminished white adipose tissue despite normal food intake. They are protected against developing diet-induced obesity and fatty livers. Further it has been sown that mutant brown adipose tissue exhibits a compensatory increase in RIa, which almost entirely replaces lost RIIß, generating an isoform switch. These results demonstrate a role for the RIIß holoenzyme in regulating energy balance and adiposity 3.
References
1. Kirschner LS, Carney JA, Pack SD, Taymans SE, Giatzakis C, Cho YS, Cho-Chung YS & Stratakis CA (2000). Mutations of the gene encoding the protein kinase A type I-a regulatory subunit in patients with the Carney complex. Nat. Genet., 26:89–92.
2. Wong W, Scott JD (2004). AKAP signalling complexes: focal points in space and time. Nat. Rev. Mol. Cell. Biol., 5:959–970.
3. Cummings DE, Brandon EP, Planas JV, Motamed K, Idzerda RL and McKnight GS. (1996). Genetically lean mice result from targeted disruption of the RIIß subunit of protein kinase A. Nature, 382: 622–626.
4. Kim C, Cheng CY, Saldanha SA, Taylor SS (2007). PKA-I holoenzyme structure reveals a mechanism for cAMP-dependent activation. Cell, 130(6):1032-1043.
5. Torgersen KM, Vang T, Abrahamsen H, Yaqub S, Taskén K (2002). Molecular mechanisms for protein kinase A-mediated modulation of immune function. Cell Signal, 14(1):1-9.
6. Huang Y, Roelink H, McKnight GS (2002). Protein kinase A deficiency causes axially localized neural tube defects in mice. J. Biol. Chem., 277(22):19889-19896.