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Protein Phosphatase 2A

Protein Phosphatases are general class of enzymes that are responsible for dephosphorylating other molecules or proteins. A kinase enzyme could attach a phosphate group while the phosphatases could remove the group. The process of phosphorylation occurs in many metabolic processes and are one of the most common post transcriptional modifications. Protein phosphatases typically use three different amino acids to phosphorylate a protein. The three most common are serine, threonine and tyrosine. Protein phosphatase 2A uses serine and threonine to phosphorylate a compound. Protein Phosphatases are involved in many biological pathways. This is due to the fact that they have broad substrate specificity. Protein phosphatases are involved in process such as cell-cycle regulation, cell growth and cell development. Protein phosphatases are also involved in signal transduction and can be involved with the phosphorylation of signal proteins such as Raf and MEK. ^[1] The function and mechanisms of protein phosphatase is unclear to the present date. PP2A is a three subunit holoenzyme that features a HEAT repeat, Huntington-elongation-A subunit-TOR, which is also found in the huntingtin gene. The following sections will discuss the PP2A enzyme and the importance of understanding HEAT repeats.

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Structure and Function

The (or scaffolding subunit) along with the catalytic subunit makes up the core enzyme. Aα is composed of fifteen HEAT repeats that form a double layer of antiparallel alpha helices. The arrangement of the alpha helices causes the subunit to adopt a half circle shape. When in this conformation the Aα subunit is able to interact with the catalytic subunit via four-conserved HEAT repeats ^[2], causing a conformational activation of the catalytic subunit and creation of the core enzyme

The (Catalytic subunit) associates with the A subunit to form a heterodimer protein. The catalytic subunit (in conjunction with the scaffolding and regulatory subunits) can have many functions and is a particularly important factor in cell growth regulation, signal transduction regulation, and cellular development as well as playing a key role in suppressing uncontrolled cell proliferation. It is thought that the B subunit is largely responsible for the specific function of the A-C dimer.

In the case of PP2A holoenzyme the (or regulatory subunit) is not part of the core enzyme but rather a coenzyme that associates with the Aα and Cα subunits to form the holoenzyme. This subunit structurally resembles the Aα scaffolding subunit; it contains eighteen alpha helixes stacked antiparallel to each other to cause a crescent structure. Of these eighteen alpha helixes eight of them resemble HEAT repeat motifs that are similar to repeats found on the Aα subunit. These conserved repeats are located on the, largely hydrophobic, convex side of the B subunit. The hydrophobic residues create a groove to which the Aα subunit loosely associates with, mainly via van der Waal interaction. The concave side of the B subunit consists of a variety of negatively charged amino acid residues, which create an isolated acidic environment. The acid nature of the concave side promotes multiple hydrogen bonds with residues from the Cα subunit. These hydrogen bonds coupled with a multitude of van der Waal interactions results in strong B -Cα subunit association. ^[3]

HEAT Repeat Motif

PP2A belongs in the HEAT Repeat Motif or Huntington-elongation-A subunit-TOR. The main backbone of PP2A is 15 repeating units or , each composed of a conserved 39-residue sequence.The HEAT Motif has a signature sequence of conserved residues asparagine at position 19 and arginine at position 25. Each HEAT unit consists of a pair of antiparallel alpha-helices. These antiparallel helices assemble in a L-shaped fashion resulting in an overall shape of a double layer of alpha helices. These HEAT units are responsible for a majority of the overall protein packing and structure by interactions of the ridges of each HEAT unit. ^[4]

Huntington's Disease

One unique feature of the Huntington's Disease protein, Htt, is that it features a HEAT repeat like that of PP2A. Continued understanding of how PP2A heat repeat operates could provide valuable insight in understanding the mechanisms of Huntington's Disease function. Huntington's Disease is caused by the huntingin protein, Htt, which is found in all mammalian cells. The protein is known to be involved with numerous functions in the body such as signal transduction, transcription and intercellular transporting. ^[5] Huntington's Disease is thought to be a result of toxic function of the Htt protein, mHtt, instead of a lack or excess of the protein. ^[6] Modifications of the Htt protein result in either mHtt proteins high in glutamine or protein fragments from cleavage that are high in glutamine. Glutamine is plentiful in the mHtt protein due to the fact that it is coded for by the cytosine-adenine-guanine series in the HTT gene that is expanded in the mutation. The polar nature of glutamine causes the protein to form clumps which center around the axons and dendrites of neurons. These clumps result in lack of normal nerve function leading to the visible symptoms of Huntington's Disease. ^[7] The following link will provide an image of a neuron that is affected by mHtt.

Huntington's Disease is a genetically inherited disease that is dominant in nature. This means an infected parent will have a 50% chance of transmission to their child. It is classified as a neurodegenerative disorder that will result in death. The disease can strike as early as childhood but usually starts to affect the individual from 35 to 45 years of age. Initial signs consist of mood changes for the first few years followed by disruption of motor skills called chorea. Chorea is a term to describe jerky uncontrolled movements of the extremities followed by the entire body as the disease progresses. Chorea like conditions are also present in individuals with Parkinson's Disease. Huntington's Disease continues to affect the mind as well as the body. The later stages of the disease will result in a individual that could be described as being schizophrenic with dementia and Alzheimer's disease that also has Parkinson's Disease. The individual will fade both mentally and physically until death comes in the common form of heart failure or pneumonia from the weakened condition. This disease creates a lot of controversy in the genetic counseling community due to the fact that it can now be detected as early as in-vitro fertilization. The ramifications from a positive diagnosis would be considered by most as a life shattering event. There is a quote from a Campbell biology textbook that poses the question of 'At what point would it be beneficial to know that the individual has a incurable fatal disease that will generally result in death in the early fifties'? ^[8] The answer to this question becomes apparent when one considers family planning. Recent advancements in in-vitro technology allow scientists to fertilize multiple eggs and select embryos free from Huntington's Disease without informing the donor of the egg or sperm if they are a carrier of the disease. Understanding how HEAT repeat mechanisms work in proteins like PP2A and how to disrupt their function could yield valuable insight in battling Huntington's Disease.

References

1. ↑ Bradford D. "Protein Phosphatases." Current opinions in structural biology (1995): 728-734.
2. ↑ Xing, Yongna, Yanhui Xu, Yu Chen, Philip D. Jeffrey, Yang Chao, Zheng Lin, Zhu Li, Stefan Strack, Jeffry B. Stock, and Yigong Shi. "Structure of Protein Phosphatase 2A Core Enzyme Bound to Tumor-Inducing Toxins." Cell (2006): 341-53.
3. ↑ Xu, Y., Xing, Y., Chen, Y., Chao, Y., Lin, Z., Fan, E., Yu, J.W., Strack, S., Jeffery, P.D., Shi, Y. 2006. Structure of the protein phosphatase 2a holoenzyme. Cell (127) 1239-1251. DOI 10/1016/j.cell.2006.11.033
4. ↑ Groves MR, Hanlon N, Turowski P, Hemmings BA, Barford D. The structure of the protein phosphatase 2A PR65/A subunit reveals the conformation of its 15 tandemly repeated HEAT motifs. Cell. 1999 Jan 8;96(1):99-110. PMID:9989501
5. ↑ Harjes P, Wanker EE (2003). "The hunt for huntingtin function: interaction partners tell many different stories". Trends Biochem. Sci. 28 (8): 425–33. doi:10.1016/S0968-0004(03)00168-3
6. ↑ Walker FO (2007). "Huntington's disease". Lancet 369 (9557): 218–28 [221]. doi:10.1016/S0140-6736(07)60111-1
7. ↑ "Huntingtin Protein and Protein Aggregation | HOPES – A guide to the science of Huntington's disease"
8. ↑ Campbell, N.A., Reece J.B., Urry, L.A., Cain, M.L., Waserman, S.A., Minorsky, P.V., Jackson, R.B. 2008. Biology, eighth edition. Pearson Benjamin Cummings, San Francisco