Subviral Particles: Prions and Viroids (2B) - MCAT Biological and Biochemical Foundations of Living Systems

Viroids lack a capsid; viruses have a protein capsid. Viroids are naked RNA molecules, whereas viruses package their genomes inside a protein capsid for protection and infectivity.

PrP$^C$. PrP$^C$ is the endogenous, properly folded prion protein expressed in healthy cells, particularly in neurons.

Circular single-stranded RNA. Viroid genomes are compact, circular ssRNA structures that enable efficient replication in host cells.

Template-induced misfolding of PrP$^C$ into PrP$^{Sc}$. Prions replicate by acting as templates that convert normal PrP$^C$ proteins into the pathogenic PrP$^{Sc}$ form through direct interaction.

Rolling-circle replication. Viroids use a rolling-circle mechanism to generate multimeric RNA intermediates, which are processed into monomeric units.

Prion. UV irradiation damages nucleic acids, inactivating viroids but not prions, which lack any genome.

Prion: protein only; viroid: RNA only. Prions transmit via misfolded protein without nucleic acids, while viroids use RNA as their infectious genetic material.

Neither prions nor viroids have an envelope. Both agents lack lipid envelopes, consisting only of protein (prions) or RNA (viroids) without additional structures.

RNA-mediated disruption of host gene expression (e.g., RNA silencing). Viroid RNA interferes with host RNA processing or triggers silencing pathways, disrupting normal gene expression without protein products.

Host RNA polymerase (typically RNA polymerase II). Viroids hijack the host's RNA polymerase II to transcribe their RNA, as they encode no replicative enzymes.

Viroids do not encode proteins. Viroids lack open reading frames and rely entirely on host machinery, without translating any proteins.

Infectious misfolded protein; no nucleic acid genome. Prions consist solely of misfolded proteins that propagate by inducing conformational changes in normal proteins, without any nucleic acid component.

Small circular single-stranded RNA; no protein coat. Viroids are composed of short, circular ssRNA molecules that infect plants without any protective protein structure.

They lack nucleic acids yet can transmit disease. Prions are subviral because they transmit infectivity through protein alone, defying the need for nucleic acids in replication.

They are naked RNA without a capsid or envelope. Viroids are subviral as they consist only of RNA, lacking the protein capsid that encapsulates viral genomes.

Increase in $eta$-pleated sheet content. The pathogenic PrP$^{Sc}$ form has higher $eta$-sheet structure, making it insoluble and aggregation-prone compared to the $alpha$-helical PrP$^C$.

Minimal or absent adaptive immune response. Prions, being self-proteins, evade strong adaptive immunity, leading to little inflammation or antibody production.

Central nervous system. Prion diseases primarily target the brain, where PrP$^{Sc}$ accumulation causes neuronal damage and spongiform changes.

Spongiform encephalopathy (vacuolization of brain tissue). Prion-induced misfolding leads to vacuole formation in neurons, creating a sponge-like appearance in brain tissue.

Progressive neurodegeneration leading to death. Prion diseases are relentlessly progressive, with accumulating PrP$^{Sc}$ causing irreversible neuronal loss and fatal outcomes.

Highly resistant to heat, radiation, and proteases. The stable $eta$-sheet structure of PrP$^{Sc}$ confers resistance to denaturation by physical and chemical agents.

No genome replication; propagation occurs by protein conformational conversion. Unlike viruses that replicate genomes, prions propagate solely by inducing host protein misfolding without nucleic acid involvement.

Prions: none; viroids: RNA genome only. Prions lack any nucleic acid, relying on protein for infectivity, while viroids use RNA as their sole genetic material.

Plants. Viroids primarily infect plant hosts, where they replicate and cause diseases like potato spindle tuber disease.