What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Prion Protein: Structure, Function, and Disease Mechanisms

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Prion Protein

What is a Prion?

A prion is an infectious agent composed entirely of protein, with no associated nucleic acid genome. The term "prion" derives from "proteinaceous infectious particle," coined by Stanley Prusiner in 1982 to describe the agent responsible for transmissible spongiform encephalopathies (TSEs). This discovery was revolutionary because it challenged the central dogma of molecular biology, which held that genetic information flows from nucleic acids to proteins. Prions replicate by converting a normal cellular protein into an abnormal conformation, a mechanism fundamentally different from the replication strategies of viruses, bacteria, fungi, or parasites.

Prions are distinguished from all other infectious agents by three properties: they lack nucleic acids, they are resistant to treatments that inactivate viruses and bacteria (including UV radiation, nucleases, and standard autoclaving), and they propagate by inducing conformational change in a host-encoded protein. The prion protein (PrP) is encoded by the PRNP gene on human chromosome 20, and it exists in two major isoforms: the normal cellular form (PrPC) and the disease-associated scrapie form (PrPSc).

The Prion Hypothesis

The prion hypothesis states that a misfolded protein can act as an infectious agent by templating its abnormal conformation onto the normal cellular protein. This hypothesis was initially met with skepticism because it contradicted the established understanding that infectious agents require nucleic acids for replication. However, decades of experimental evidence have validated this model. Key supporting observations include:

Prion diseases can be transmitted between animals by inoculation with protein-only preparations that lack detectable nucleic acids.
Mutations in the PRNP gene cause inherited forms of prion disease, demonstrating that alterations in the protein sequence alone can trigger disease.
Transgenic mice expressing mutant PrP develop prion disease spontaneously, and their brain homogenates can transmit disease to wild-type mice.
Recombinant PrP, when converted to the β-sheet-rich conformation in vitro, can generate infectious prions that cause disease when inoculated into animals.

The prion hypothesis has broader implications beyond infectious disease. It establishes a paradigm for understanding other neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases, which involve the propagation of misfolded protein aggregates through similar templating mechanisms.

Structure and Isoforms of Prion Protein

PrPC: Normal Cellular Form

The human PRNP gene encodes a 253-amino-acid precursor protein. Following translation, the N-terminal 22-amino-acid signal peptide is cleaved in the endoplasmic reticulum, and the C-terminal 23-amino-acid peptide is removed upon addition of a glycosylphosphatidylinositol (GPI) anchor. The mature PrPC protein is therefore approximately 208 amino acids long and is tethered to the outer leaflet of the plasma membrane via its GPI anchor.

PrPC has a distinctive domain architecture:

N-terminal region (residues 23–120): This region is intrinsically disordered and contains a series of five octapeptide repeats with the consensus sequence PHGGGWGQ. These repeats are the primary copper-binding sites of PrPC. The N-terminus also contains a positively charged region (residues 23–28) that may interact with membrane phospholipids and other proteins.
Globular C-terminal domain (residues 121–231): This domain adopts a well-defined three-dimensional structure consisting of three α-helices (designated H1, H2, and H3) and a short two-stranded antiparallel β-sheet. The structure is stabilized by a single disulfide bond between cysteine residues 179 and 214.
GPI anchor: The C-terminus is covalently attached to a GPI lipid anchor that inserts into the plasma membrane, localizing PrPC to cholesterol-rich lipid rafts.

PrPC is expressed throughout the body but is most abundant in the central nervous system, particularly in neurons. It is also found in immune cells, skeletal muscle, and other tissues. The protein undergoes N-linked glycosylation at two sites (asparagine residues 181 and 197), resulting in three glycoforms: unglycosylated, monoglycosylated, and diglycosylated. The glycosylation pattern influences PrP trafficking, localization, and conversion efficiency.

PrPSc: Infectious Scrapie Form

PrPSc is the disease-associated isoform of the prion protein. It has the same primary amino acid sequence as PrPC but adopts a fundamentally different three-dimensional conformation. The most striking structural difference is the secondary structure content:

Property	PrPC	PrPSc
Secondary structure	~42% α-helix, ~3% β-sheet	~30% α-helix, ~43% β-sheet
Solubility	Soluble in non-ionic detergents	Insoluble; forms aggregates
Protease sensitivity	Fully sensitive to proteinase K	Partially resistant (N-terminus cleaved)
Membrane association	GPI-anchored to cell surface	Membrane-associated and extracellular aggregates
Normal function	Present	Absent (pathological)

The precise atomic structure of PrPSc has been difficult to determine because of its insolubility and heterogeneous nature. However, cryo-electron microscopy (cryo-EM) studies of prion fibrils have revealed that PrPSc forms amyloid fibrils in which the C-terminal domain refolds into a parallel in-register β-sheet architecture. In this arrangement, β-strands stack perpendicular to the fibril axis, stabilized by hydrogen bonds between backbone atoms of adjacent molecules. The N-terminal region remains partially flexible and is protease-sensitive, which explains why proteinase K digestion of PrPSc produces a resistant core of approximately 27–30 kDa (termed PrP27-30).

The conversion from PrPC to PrPSc is a post-translational event; no covalent modifications distinguish the two isoforms. The conformational change involves the refolding of the predominantly α-helical C-terminal domain into a β-sheet-rich structure, accompanied by the formation of oligomers and eventually amyloid fibrils.

Normal Function of Prion Protein

Despite decades of research, the physiological function of PrPC remains incompletely understood. However, several well-characterized activities have been established:

Copper binding and antioxidant defense. The octapeptide repeats in the N-terminal domain bind copper ions (Cu²⁺) with sub-micromolar affinity. Each repeat can coordinate one copper ion, and the full-length protein can bind up to five copper atoms. Copper binding induces a conformational change in the N-terminus, promoting a more structured state. PrPC has been proposed to function as a copper transporter or buffer at the synapse, where copper is released during synaptic transmission. Copper-bound PrPC also exhibits superoxide dismutase-like activity, suggesting a role in protecting neurons from oxidative stress.

Neuroprotection and anti-apoptotic signaling. PrPC has been shown to protect cells against various apoptotic stimuli, including serum withdrawal, oxidative stress, and treatment with the toxic peptide Aβ (amyloid beta) in Alzheimer's disease models. This protective effect is mediated through multiple signaling pathways. PrPC interacts with the stress-inducible protein 1 (STI1), activating the PI3K/Akt and MAPK/ERK pathways, which promote cell survival. PrPC also modulates the activity of the NR2D subunit of NMDA receptors, protecting neurons from excitotoxicity.

Signal transduction. PrPC is localized to lipid rafts, where it interacts with various signaling molecules. It binds to the neural cell adhesion molecule (NCAM), promoting neurite outgrowth and neuronal differentiation. PrPC also interacts with the laminin receptor precursor (LRP/LR), which mediates cell adhesion and survival signals. Additionally, PrPC has been implicated in the regulation of the Src-family kinase Fyn, which plays roles in synaptic plasticity and memory formation.

Synaptic function and neuroplasticity. PrPC knockout mice exhibit altered synaptic transmission, particularly in the hippocampus. These mice show impaired long-term potentiation (LTP), a cellular correlate of learning and memory, and deficits in spatial learning tasks. PrPC appears to modulate synaptic function through interactions with synaptic proteins, including the AMPA receptor subunit GluA1 and the scaffolding protein PSD-95.

Cellular prion protein as a receptor. PrPC has been identified as a receptor for several ligands, including the toxic oligomeric forms of β-amyloid (Aβ oligomers) in Alzheimer's disease. This interaction mediates the neurotoxic effects of Aβ oligomers, suggesting that PrPC may play a role in the pathogenesis of Alzheimer's disease. PrPC also serves as a receptor for the viral protein gp120 of HIV-1, mediating viral entry into neurons.

The redundancy of these functions and the relatively mild phenotype of PrPC knockout mice suggest that PrPC may be a multifunctional scaffold protein that coordinates multiple signaling pathways rather than having a single essential function.

Mechanism of Prion Conversion and Propagation

Template-Directed Conversion

The central event in prion pathogenesis is the conversion of PrPC to PrPSc. This process follows a template-directed model in which PrPSc acts as a template that induces the refolding of PrPC into the PrPSc conformation. The conversion is a nucleated polymerization reaction with the following characteristics:

Nucleation phase (lag phase): The conversion begins with the formation of a small oligomeric nucleus of PrPSc. This step is thermodynamically unfavorable because it requires the association of multiple misfolded monomers. The lag phase can be shortened by the addition of pre-formed PrPSc seeds, which bypass the nucleation step.

Elongation phase: Once a stable nucleus is formed, monomeric PrPC molecules are recruited to the growing fibril ends. The binding of PrPC to the fibril end induces its conversion to the PrPSc conformation, extending the fibril. This process is driven by the higher thermodynamic stability of the PrPSc fibril compared to soluble PrPC.

Fragmentation: The growing fibrils can fragment, generating new fibril ends that can recruit additional PrPC monomers. Fragmentation is a critical step for prion propagation because it increases the number of active fibril ends and creates new infectious units.

The conversion reaction can be described kinetically as a first-order process with respect to PrPSc concentration, meaning that the rate of conversion is proportional to the amount of PrPSc present. This autocatalytic amplification explains the exponential growth of prions during infection.

Several factors influence the efficiency of conversion:

Species barrier: The amino acid sequence of PrP differs between species, and conversion is most efficient when the PrPC and PrPSc are from the same species. This species barrier explains why bovine spongiform encephalopathy (BSE) prions transmit poorly to humans, although the emergence of variant Creutzfeldt-Jakob disease (vCJD) demonstrated that transmission can occur.
Polymorphisms: In humans, a common polymorphism at codon 129 of PRNP (methionine or valine) influences susceptibility to prion disease. Homozygosity at this position (MM or VV) is associated with increased susceptibility to sporadic CJD, while heterozygosity (MV) is protective.
Chaperone proteins: Molecular chaperones, such as Hsp70 and Hsp104, can influence prion conversion. In yeast, the chaperone Hsp104 is essential for prion propagation because it fragments prion fibrils, generating new seeds. In mammals, chaperones may play a role in either promoting or inhibiting conversion depending on the context. For more on these cellular helpers, see Chaperone Protein.

Amyloid Fibril Formation

The end product of prion conversion is the formation of amyloid fibrils. Amyloid fibrils are highly ordered protein aggregates characterized by a cross-β structure, in which β-strands run perpendicular to the fibril axis and are stacked in parallel β-sheets. This arrangement creates a dense hydrogen-bonding network along the fibril axis, conferring remarkable stability.

The structure of prion fibrils has been elucidated by cryo-EM. In the case of the most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD), the fibrils consist of two intertwined protofilaments. Each protofilament contains the C-terminal domain of PrP (residues ~95–231) folded into a parallel in-register β-sheet architecture. The N-terminal region (residues 23–94) remains disordered and is not part of the fibril core.

Amyloid fibrils exhibit several properties that are relevant to prion pathogenesis:

Protease resistance: The tightly packed β-sheet core is resistant to proteolytic digestion, which is why PrPSc can be distinguished from PrPC by proteinase K treatment.
Thermodynamic stability: Amyloid fibrils are extremely stable, with melting temperatures often exceeding 90°C. This stability explains the resistance of prions to heat inactivation.
Cytotoxicity: Oligomeric intermediates formed during fibril assembly are thought to be the most toxic species, causing neuronal dysfunction and death through membrane disruption, calcium dysregulation, and activation of apoptotic pathways.

The accumulation of PrPSc in the brain leads to the characteristic neuropathological features of prion disease: spongiform degeneration (vacuolation of the neuropil), neuronal loss, astrogliosis, and microglial activation. The precise mechanism of neuronal death is not fully understood, but it likely involves both gain-of-function toxicity (from PrPSc aggregates) and loss-of-function effects (from depletion of PrPC).

Prion Diseases: Pathogenesis and Clinical Features

Human Prion Diseases

Human prion diseases are rare, affecting approximately 1–2 people per million per year worldwide. They are classified into three etiological categories:

Sporadic prion diseases (~85% of cases). Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common human prion disease. It occurs without any known cause, likely arising from spontaneous conversion of PrPC to PrPSc or from somatic mutations in the PRNP gene. The disease typically presents in patients over 60 years of age with rapidly progressive dementia, myoclonus, visual disturbances, and cerebellar ataxia. The clinical course is rapid, with a median survival of approximately 4–6 months from symptom onset.

Genetic prion diseases (~10–15% of cases). Inherited prion diseases are caused by mutations in the PRNP gene. Over 40 pathogenic mutations have been identified, including point mutations and octapeptide repeat insertions. These mutations are inherited in an autosomal dominant pattern with high penetrance. The major genetic prion diseases include:

Familial CJD (fCJD): Caused by point mutations such as E200K, V210I, and D178N.
Gerstmann-Sträussler-Scheinker syndrome (GSS): A slowly progressive ataxic disorder caused by mutations such as P102L and A117V.
Fatal familial insomnia (FFI): Caused by the D178N mutation in cis with methionine at codon 129. FFI is characterized by severe insomnia, autonomic dysfunction, and endocrine abnormalities.

Acquired prion diseases (<1% of cases). Acquired prion diseases result from transmission of prions from an external source:

Kuru: A disease transmitted through ritualistic cannibalism among the Fore people of Papua New Guinea. Kuru is now virtually eliminated following the cessation of cannibalism.
Iatrogenic CJD: Transmitted through contaminated surgical instruments, dura mater grafts, corneal transplants, or human growth hormone derived from cadaveric pituitary glands.
Variant CJD (vCJD): Caused by consumption of BSE-contaminated beef products. vCJD differs from sCJD in its younger age of onset, longer duration, and prominent psychiatric symptoms. The disease is associated with the prion strain that causes BSE in cattle.

Animal Prion Diseases

Several prion diseases affect animals:

Scrapie: The first described prion disease, affecting sheep and goats. Scrapie has been recognized for over 200 years and is transmitted horizontally between animals, likely through contact with placenta or other contaminated materials.
Bovine spongiform encephalopathy (BSE): Also known as "mad cow disease," BSE emerged in the United Kingdom in the 1980s as a result of feeding cattle with meat-and-bone meal contaminated with scrapie prions. The disease reached epidemic proportions, with over 180,000 confirmed cases in cattle.
Chronic wasting disease (CWD): A prion disease affecting deer, elk, and moose in North America. CWD is highly contagious and is spreading through wild and farmed cervid populations.
Feline spongiform encephalopathy and transmissible mink encephalopathy: Rare prion diseases affecting cats and farmed mink, respectively.

The neuropathological hallmarks of prion diseases are consistent across species: spongiform vacuolation of the neuropil, neuronal loss, reactive astrogliosis, and microglial activation. The vacuoles are membrane-bound vesicles within neuronal processes and are thought to result from the accumulation of PrPSc and the disruption of cellular trafficking.

Methods to Study Prion Protein

PMCA and RT-QuIC

The study of prion biology has been revolutionized by the development of in vitro amplification techniques that exploit the autocatalytic nature of prion conversion.

Protein misfolding cyclic amplification (PMCA). PMCA mimics prion replication in vitro by incubating PrPSc seeds with excess PrPC substrate (usually brain homogenate from a susceptible animal) in the presence of conversion buffer. The reaction mixture is subjected to cycles of incubation and sonication:

Incubation phase: PrPSc seeds recruit and convert PrPC to PrPSc, extending the fibrils. This phase typically lasts 30 minutes at 37°C.
Sonication phase: Ultrasonic pulses fragment the fibrils, generating new seeds. This phase typically lasts 1 minute.

The cycle is repeated (typically 48–96 cycles) to achieve exponential amplification. PMCA can detect as few as a single prion seed and is used for prion detection, strain typing, and studying the molecular mechanisms of conversion.

Real-time quaking-induced conversion (RT-QuIC). RT-QuIC is a diagnostic assay that detects prion seeds in biological fluids. The assay uses recombinant PrPC as a substrate and measures the aggregation of PrPSc by the fluorescence of the amyloid-binding dye thioflavin T (ThT). The reaction is performed in a 96-well plate with shaking (quaking) to fragment fibrils, and ThT fluorescence is measured at regular intervals. RT-QuIC has high sensitivity and specificity for diagnosing prion disease from cerebrospinal fluid (CSF) and other tissues. For more on laboratory approaches to prion diagnosis, see Prion Disease Lab Test.

Animal Models and Cell Assays

Transgenic mouse models. Mice expressing human PrP or mutant PrP are essential for studying prion disease. Transgenic mice expressing human PrP with the codon 129 methionine polymorphism are susceptible to vCJD prions, while mice expressing valine at this position are more resistant. Knock-in mice expressing mutant PrP (e.g., P102L) develop spontaneous prion disease, providing models for genetic forms.

Cell culture models. Several cell lines support prion infection, including the mouse neuroblastoma cell line N2a and the rabbit kidney epithelial cell line RK13. These cells can be persistently infected with prions and are used to study the cellular mechanisms of prion propagation, screen for anti-prion compounds, and investigate the role of cellular factors in conversion.

Structural biology methods. The determination of PrP structures has relied on multiple techniques:

NMR spectroscopy: Used to determine the solution structure of recombinant PrPC. NMR is suitable for the monomeric, soluble form of the protein.
X-ray crystallography: Used to determine crystal structures of PrPC and antibody-bound PrP fragments. The first high-resolution structure of human PrPC was determined by NMR in 1996.
Cryo-electron microscopy (cryo-EM): Used to determine the structures of PrPSc fibrils. Cryo-EM has revealed the parallel in-register β-sheet architecture of prion fibrils at near-atomic resolution.
Hydrogen/deuterium exchange mass spectrometry: Used to map the regions of PrP that are protected in the fibril core.

Biochemical methods. The standard method for detecting PrPSc is limited proteolysis followed by western blotting. PrPSc is resistant to proteinase K digestion, while PrPC is completely degraded. The protease-resistant fragment (PrP27-30) can be detected with anti-PrP antibodies. This assay is the basis for the "conformational" distinction between PrPC and PrPSc.

Common Pitfalls and Misconceptions

Prions vs. Viruses

A common misconception is that prions are a type of virus. This is incorrect. Viruses contain nucleic acids (DNA or RNA) that encode their structural proteins and enzymes. Prions contain no nucleic acids and are composed entirely of protein. This distinction has important practical implications:

Inactivation: Viruses are inactivated by treatments that damage nucleic acids (UV radiation, nucleases) or denature proteins (heat, detergents). Prions are resistant to UV radiation and nucleases but are also resistant to many protein-denaturing treatments because of their highly stable amyloid structure.
Replication mechanism: Viruses replicate by hijacking the host's translational machinery to produce viral proteins from viral nucleic acids. Prions replicate by templating their conformation onto the host's normal PrPC protein.
Evolution: Viruses evolve through mutation and selection of nucleic acid sequences. Prions can exhibit strain variation, but this is encoded in the conformation of the protein, not in a nucleic acid sequence.

Infectivity and Strains

Another misconception is that PrPSc is a single, uniform entity. In reality, prions exist as multiple strains that differ in their biological properties:

Incubation period: Different strains cause disease after different incubation periods in the same host.
Neuropathological lesion profile: Different strains target different brain regions, producing distinct patterns of spongiform degeneration.
Biochemical properties: Different strains produce PrPSc with different protease cleavage sites, glycosylation patterns, and conformational stabilities.

The strain phenomenon is explained by the conformational hypothesis: different strains represent different PrPSc conformations that template their specific structure onto PrPC. The same PrP amino acid sequence can adopt multiple misfolded conformations, each associated with a distinct strain.

A related misconception is that all PrPSc is infectious. Some forms of misfolded PrP may be non-infectious or have low infectivity. The relationship between PrPSc accumulation and infectivity is complex, and the most infectious species may be oligomers rather than large fibrils.

Summary and Key Takeaways

Prion diseases represent a unique class of infectious disorders in which the infectious agent is a misfolded protein. The conversion of the normal cellular prion protein (PrPC) to the disease-associated isoform (PrPSc) is the central event in pathogenesis. This conversion is a template-directed process in which PrPSc induces the refolding of PrPC into the β-sheet-rich, amyloid-forming conformation.

The study of prions has fundamentally changed our understanding of protein biology and disease. It has established that proteins can encode biological information through their conformation, not just their amino acid sequence. This paradigm has implications for other neurodegenerative diseases, including Alzheimer's and Parkinson's diseases, which involve the propagation of misfolded protein aggregates.

Frequently Asked Questions

Is prion a protein?

Yes, a prion is a protein. The term "prion" is derived from "proteinaceous infectious particle" and refers to an infectious agent composed entirely of protein with no nucleic acid genome. The prion protein (PrP) exists in two forms: the normal cellular form (PrPC) and the disease-associated form (PrPSc).

Is a prion a protein?

Yes, a prion is a protein. Specifically, it is the misfolded, infectious form of the prion protein (PrPSc). The infectious agent is not a virus, bacterium, or other organism; it is a protein that propagates by converting the normal cellular prion protein (PrPC) into the misfolded conformation.

What is prion protein?

Prion protein (PrP) is a cell-surface glycoprotein encoded by the PRNP gene. It exists in two major isoforms: the normal cellular form (PrPC), which is α-helical, GPI-anchored, and protease-sensitive, and the disease-associated form (PrPSc), which is β-sheet-rich, aggregated, and partially protease-resistant. PrPC is expressed throughout the body and has proposed functions in copper binding, neuroprotection, and signal transduction.

What are prion proteins?

Prion proteins are proteins that can adopt two distinct conformations: a normal cellular form (PrPC) and a disease-associated form (PrPSc). The term "prion proteins" can refer to the PrP family of proteins across species, including human PrP, mouse PrP, and sheep PrP. The conversion of PrPC to PrPSc is the molecular basis of prion diseases.

How do prions cause disease?

Prions cause disease by converting the normal cellular prion protein (PrPC) into the misfolded, disease-associated form (PrPSc). This conversion is a template-directed process in which PrPSc acts as a template, inducing PrPC to refold into the β-sheet-rich PrPSc conformation. The accumulation of PrPSc in the brain leads to spongiform degeneration, neuronal loss, and gliosis, resulting in the clinical features of prion disease.

Are prions alive?

No, prions are not alive. They are infectious proteins that lack the characteristics of living organisms, including cellular structure, metabolism, and the ability to reproduce independently. Prions replicate by converting the host's normal prion protein into the misfolded form, a process that does not involve nucleic acid replication or protein synthesis.

Key Takeaways

Prions are infectious agents composed entirely of protein, with no nucleic acid genome, and they cause transmissible spongiform encephalopathies.
The prion protein (PrP) exists as a normal cellular form (PrPC) that is α-helical, GPI-anchored, and protease-sensitive, and a disease-associated form (PrPSc) that is β-sheet-rich, aggregated, and partially protease-resistant.
PrPC has physiological functions in copper binding, neuroprotection, signal transduction, and synaptic plasticity, though its precise role remains incompletely defined.
Prion conversion follows a template-directed, nucleated polymerization model in which PrPSc induces the refolding of PrPC into the misfolded conformation, leading to amyloid fibril formation.
Human prion diseases include sporadic CJD, genetic forms (familial CJD, GSS, FFI), and acquired forms (kuru, iatrogenic CJD, vCJD), all characterized by spongiform degeneration and neuronal loss.
Prion strains represent distinct PrPSc conformations that template their specific structure, explaining variations in incubation period, lesion profile, and biochemical properties.
The prion paradigm has broader implications for understanding protein misfolding diseases, including Alzheimer's and Parkinson's diseases, where similar templated conformational changes occur.