Prions
A prion is thought to be an infectious agent that is comprised entirely of a propagated, mis-folded (rouge) protein.
The rogue prion protein has been implicated in a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt-Jakob disease (CJD) in humans.
All hypothesized prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and are always fatal.
PrPC refers to the normal cellular protein, which is found in a multitude of tissues, while PrPSC refers to the prion form of PrPC, and is responsible for clogging brain cells, and causing cells to die eventually.
Hypothetically, Prions infect and propagate by refolding converting normal molecules of the protein into the abnormally structured form.
All known prions induce the formation of tightly packed beta sheets. This altered structure is extremely stable and accumulates in infected tissue, causing cell death and tissue damage.
This stability means that prions are resistant to denaturation by chemical and physical agents, making disposal and containment of these particles difficult.
Mechanism of Prion Propagation
Prion Protein Structure
How Prions Arise
Prions Madcow mechanism
Viriods
Viroids are infectious agents composed of a single piece of circular single stranded RNA which has some double-stranded regions.
Because of their simplified structures both prions and viroids are sometimes called subviral particles.
Viroids mainly cause plant diseases but have recently been reported to cause a human disease (Hepatitis D)
Human diseases caused by Viroids
The only human disease known to be caused by a viroid is hepatitis D. For hepatitis D to occur there must be simultaneous infection of a cell with both the hepatitis B virus and the hepatitis D viroid (co-infection)
There is extensive sequence complementarity between the hepatitis D viroid RNA and human liver cell 7S RNA, a small cytoplasmic RNA that is a component of the signal recognition particle, the structure involved in the translocation of secretory and membrane-associated particles.
The hepatitis D viroid causes liver cell death via sequestering this 7S RNA and/or cleaving it.
Transmission
The hepatitis D viroid can only enter a human liver cell if it is enclosed in a capsid that contains a binding protein. It obtains this from the hepatitis B virus.
The viroid then enters the blood stream and can be transmitted via blood or serum transfusions.
Virusoids
Virusoids are circular single-stranded RNAs that are dependent on plant viruses(Viroids) for replication and encapsidation.
The genome of virusoids consist of several hundred nucleotides and only encodes structural proteins.
Virusoids are not considered as viruses but as subviral particles. Since they depend on helper viruses, they are classified as satellites
Done By: Alyssa Huang