–Table of Contents–

<–more on CJD

We now know a good deal more about Creutzfeldt and Jakob Disease (CJD) and the category of diseases it is a part of, Transmissible Spongiform Encephalopathy (TSE) diseases, by way of the recent work done on the Prion Protein (PrP) itself. PrP is naturally a single-gene, simple protein that is expressed fairly ubiquitously in mammals with an increased prevalence in the Central Nervous System (CNS) (Collinge, 1994). PrP is an about 204 residues (after post-translational modification) long, extracellularly membrane anchored protein expressed throughout the body and encoded by the gene PRPNP located on chromosome 20. While several functions for PrP have been proposed throughout the body, there is still no definitive understanding for its functionality.

The interesting aspect about CJD, and other TSE diseases, is that the difference between the naturally occurring and perfectly harmless prion protein and the virulent prion that brings about rapid, grotesque, and unpreventable death is only a change in conformation (Ma, 2002). As a genetic disease, we would expect there to be some sort of genetic mutation or coding error when transcribing or translating the gene product, which would lead to some primary sequence mutation on the protein that ultimately effects that individually miss coded protein ability to function. However, there is no primary sequence difference between the two forms of the prion protein (Ma, 2002). The prion protein’s two different organizations of secondary structures are designated PrP cellular (C) and PrP scrapie (Sc) (Kupfer, 2009).

Current research has yet to fully describe all of the functions that PrP(C) performs in the body but there are several proposed functions for the protein depending on the tissue it is expressed in. As PrP is known to be expressed in most cells in relatively low concentrations throughout the body and seen to be upregulated in response to some infectious agents, PrP(C) is currently speculated to play some role in the innate immune system, although there is still not a large body of research around the function of PrP(C) in the periphery (Kupfer, 2009). Contemporary research is more focused around the role PrP(C) plays in neuroplasticity at the synapse, primarily in the CNS but also PNS (Mani, 2003).

PrP(C) is found anchored to the extracellular side of the membrane via a glycosylphosphatidylinositol (GPI) anchor molecule attached at its C-terminus. One specific function that the PrP(C) is proposed to perform is to bind and transport Cu2+ ions to the core protein of glypican-1 (GPC1), a heparan sulfate-substituted proteoglycan also anchored to the extracellular side of the membrane, to facilitate the binding of nitric oxide to a cystine residue in GPC1 making it “S-nitrosylated” and reducing the Cu2+ to Cu1+ in the reaction (Mani, 2003). The S-nitrosylated GPC1 then interacts with several important growth factors for the CNS to control brain development and overall functioning losing its nitric oxide in the process. PrP(C) has been shown to have a high affinity for metal ions like Cu2+, Zn2+, and Fe3+ with a metal binding domain of about 12 highly conserved residues between positions 61 and 87 on the protein (Mani, 2003). A knockout study of prion protein gene in mice showed minor brain development stunting which would be expected if this proposed mechanism was correct and a removal of PrP(C) would lead to a down regulation of GPC1 (Mani, 2003).

While the structure for the complete PrP has yet to be “solved” in either form, there is still a large body of research that allows for the approximation of the secondary structure. CyoEM and X-ray crystallography studies have primary separated the approximately 200 residue long, mature, PrP(C) prion protein into a N-domain spanning residues 23-~124 (after the N-terminus signaling domain is removed post-translationally) and a C-domain with residues~125-~231 (Zahn, 2000). Current imaging techniques have been unable to properly capture the more variable N-domain and have only been able to find the structure for the more conserved, globular C-domain. It is also the case that individual PrP(Sc) prion proteins have yet to be purified and visual “solved” with these types of techniques.

Other than the afore mentioned metal binding site on the N-domain of the PrP(C) structure there is also some conserved alpha helical structure but is otherwise considered fairly variable and not properly imaged with any particular motifs (Zahn, 2000). The secondary structure of the C-domain is far more understood with a loose, single pair antiparallel beta sheet formed between residues 128-131 and 161-164 along with three alpha helixes formed from residues 144-154, 173-194, and 200-228 (Zahn, 2000). The following image is a color-coded snapshot for the ribbon diagram of the PrP(C) C-domain from a paper by Zahn et al. (PDB: 1QLX)

In the image the sites of mutation associated with familial CJD are marked in orange, a Cys-Cys bond (179,214) is in red, and the two sites of glycosylation (181,197) in blue, exact attached carbohydrate not specified. Not pictured is the GPI anchored S230 C-terminus that holds the protein to the membrane as mentioned above (Zahn, 2000).

While research is still developing on the actual function of PrP, the main reason that TSE diseases are as well-known as they are now is because of its ability to transmit between different species as what the issue with “Mad Cow” or vCJD. This, as much as anything, proves that the prion protein, the transmission agent, must be fairly highly conserved at least amongst mammals which makes it seem like whatever functions that this protein is performing must be at least somewhat evolutionarily selected for, meaning that PrP(C) must still provide some sort of decently important role in the body.

more on what goes wrong–>

References:

Collinge, J., M. A. Whittington, K. C. Sidle, C. J. Smith, M. S. Palmer, A. R. Clarke, and J. G. Jefferys. “Prion Protein Is Necessary for Normal Synaptic Function.” Nature 370, no. 6487 (July 28, 1994): 295–97. https://doi.org/10.1038/370295a0.

Kupfer, L, W Hinrichs, and M.H Groschup. “Prion Protein Misfolding.” Current Molecular Medicine 9, no. 7 (September 2009): 826–35. https://doi.org/10.2174/156652409789105543.

Ma, Jiyan, and Susan Lindquist. “Conversion of PrP to a Self-Perpetuating PrPSc-like Conformation in the Cytosol.” Science 298, no. 5599 (November 29, 2002): 1785–88. https://doi.org/10.1126/science.1073619.

Mani, Katrin, Fang Cheng, Birgitta Havsmark, Mats Jönsson, Mattias Belting, and Lars-Åke Fransson. “Prion, Amyloid β-Derived Cu(II) Ions, or Free Zn(II) Ions Support S-Nitroso-Dependent Autocleavage of Glypican-1 Heparan Sulfate.” Journal of Biological Chemistry 278, no. 40 (October 3, 2003): 38956–65. https://doi.org/10.1074/jbc.M300394200.

Zahn, Ralph, Aizhuo Liu, Thorsten Lührs, Roland Riek, Christine von Schroetter, Francisco López García, Martin Billeter, Luigi Calzolai, Gerhard Wider, and Kurt Wüthrich. “NMR Solution Structure of the Human Prion Protein.” Proceedings of the National Academy of Sciences 97, no. 1 (January 4, 2000): 145–50. https://doi.org/10.1073/pnas.97.1.145.

4 thoughts on “The Peaceful Prion Protein”

  1. First of all, cool project. CJD, Kuru, BSE, and other prion diseases are very interesting and unique! I had a few questions for clarification on this section.

    1) Is CJD caused by PrP specifically, or another prion? Is PrP being discussed here as the actual infectious agent or an example of a prion we know more about?

    2) Why do you think resolving the N-domain of PrP has been harder than resolving the C-domain structure?

    3) You indicate that cross-species transmissibility means that prion protein is conserved among mammals. Why? You would not expect this if the infectious agent was a virus or bacteria. What is different about prions in this capacity?

    1. I couldn’t agree more about the uniqueness of the TSE disease. With regard to your questions:
      1. So PrP is the prion protein, the name comes from protease resistant prion, it is the sole protein associated with all TSE diseases and received its somewhat unfortunate name because it was discovered as the protein that would misfold to form the oligomeres that had be historically found in the brain of CJD victims. All prions are coded for by the PRNP gene without any isoforms so they are all considered the same protein, PrP, however there have been several polymorphism that have been associated with the development of different diseases and slightly differently structured protein plaques although the exact differences between these versions of PrP have yet to be fully understood.
      2. This is a good question. Historically it has been the membrane associated proteins that have been much harder to image than the more soluble ones which runs against have more success resolving the membrane associated C-domain than the more soluble N-domain. The primary explanation that I found in my research was that the N-domain lacks the same structurally rigidity as the more globular and secondary structure-centrist C-domain which leads to difficulty in forming a sound crystal.
      3. When it comes to the infective nature of prions compared to viruses or bacteria I would expect a similar capacity for cross-species transmission. My point in mentioning the conservation across species was more based in speculation that most viruses or bacteria primarily interact with binding sites on proteins and would only depend on how well conserved those sites remain but in the case of prion interactions several polymorphisms at the N and C domain interface have been shown to dramatically alter the PrP misfolding capability suggesting that in order for prion propagation to occur the misfolded prion must not only have a sufficiently similar site for the PrP(Sc) to interact with the host PrP(C) but the host PrP(C) must also share a similar enough structure throughout the entire protein to fold in the same way as the alien PrP(Sc).

  2. Hey Ethan! This is a really interesting topic. I have a question about the structure itself. You mention that there is a definitive metal binding domain as evidenced by a series of highly conserved residues. I did my project on a disease characterized by copper accumulation, and I am seeing some interesting parallels, especially with the transfer of copper ion and neurological symptoms. One question I have about the metal binding domain concerns its function here. You say in the page that a proposed function of the protein is to transfer copper to GPC1. Did you come across a proposed function of the metal binding domain itself in your research of the structure? I found in my work that the functions of these domains can be pretty variable, and the definitive functions of the domains in my protein of interest were still unclear. I am curious as to whether this domain mediates the transfer of the copper ion or if it is more regulatory and relies on something else to complete the copper transfer.

    1. Hey Alex, you are certainly correct about the variability present at the metal binding site on PrP. The source that described the use of copper and PrP’s relationship with GPC1 only did so in a functional sense and within the brain, it didn’t specifically work with the proposed binding site I mentioned, that was found from another source that considered a site for copper, zinc, and iron although it didn’t mention relative binding affinities. Because most total KO trials of PrP don’t lead to significant decreases in the quality of life for mice the actually necessity for the natrual protein is considered fairly low, that coupled with work that has shown that PrP is fairly ubiquitously expressed makes it quite possible that the regulation of PrP is fairly low which could mean that it responds to a variety of signals from several different metal ions.

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