Prions: Beyond a Single Protein

Abstract

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

FIG 1

Schematic diagram of the nuclear magnetic resonance-derived structure, posttranslational modifications of human PrP^C, and epitopes of anti-PrP antibodies. Mature human PrP contains 209 amino acids. It consists of a flexible N-terminal domain containing four copper-binding octapeptide repeats and a folded C-terminal domain containing two β-sheets and three α-helical structures. The cysteines at positions 179 and 214 form a disulfide bond between the α2 and α3 domains. Two N-linked glycosylation sites are at residues 181 and 197, and the GPI anchor is linked to residue 231. The epitopes of anti-PrP antibodies 1E4 and 3F4 are located at residues 97 to 105 and 106 to 112, respectively.

FIG 2

Biosynthesis, trafficking, cleavage, and conversion of cellular PrP. PrP^C is synthesized and posttranslationally modified in the ER. It is then transported to the cell membrane after further modifications in the Golgi body. In the ER, the protein undergoes cleavage of the N- and C-terminal signal peptides, followed by the addition of N-linked glycan moieties at two sites, as well as the GPI anchor. Lastly, a single disulfide bond is formed. After reaching the cell membrane, some PrP^C is internalized into endosomes, while most of the PrP molecules are recycled to the cell membrane. A limited amount of the endocytosed PrP is subjected to cleavage at residue 110. The membrane-anchored PrP may be released into the extracellular space by cleavage within the GPI anchor. Conversion of PrP^C into PrP^Sc has been reported to occur in both cell membranes and endosomes or lysosomes. It is most likely that iPrP^C accumulates around the nucleus.

FIG 3

IPrP^C detected in ischemic-stroke brain tissue by IHC. (A) Healthy human brain sample. (B, C) Brain samples from ischemic-stroke patients. (D) Brain sample from a patient with sCJD. IHC was done with anti-PrP antibody 3F4.

FIG 4

Schematic diagrams of the conversion of four PrP glycoforms from VPSPr or fCJD^V180I and Western blot assays of four PrP glycoforms, including a PrP diglycosylated PrP (Diglyc), a PrP monoglycosylated at N197 (Mono197), a PrP monoglycosylated at N181 (Mono181), and an unglycosylated PrP (Unglyc) from cultured cells expressing human PrP^V180I or from the brains of VPSPr patients without PK treatment. In VPSPr or fCJD^V180I, only Mono197 and Unglyc are converted into a PrP^Sc that exhibits two bands by 3F4 and five bands by 1E4. In contrast, Diglyc and Mono181 are not converted into PrP^Sc.