Infectious proteins

Tanaka, M., Chien, P., Naber, N., Cooke, R., and Weissman,J. S. (2004). Conformational variations in an infectious protein determine prion strain differences. Nature 428, 323-328. 

The cause of AD is unknown although many hypotheses abound. The gene for one of the excessive amyloid proteins(b-type) has been associated with chromosome 21 at a point not far from a locus linked to some cases of familial Alzheimer s. Victims of Down s syndrome have an extra copy of chromosome 21 and usually fall prey to AD in their 4O s should they live to that age. However no all AD victims have a 21 mutation. Other causative theories involve accumulations of aluminum in the brain or the presence of a slow virus or an infectious protein substance called a prion. 

The prion protein (PrP) is an infectious protein that converts noninfectious PrP into the infectious form, which precipitates. PrP is implicated as the causative agent of the transmis sible spongiform encephalopathies, including Creutzfeld-Jakob disease. 

Jones GW, Tuite MF (2005) Chaperoning prions the cellular machinery for propagating an infectious protein Bioessays 27 823-832... 

We use the word prion to mean an infectious protein, a protein that is altered in some way so that it does not carry out its normal function properly, but has acquired the ability to convert the normal form of the protein to the same abnormal (nonfunctional) form. By this definition, three properties are expected of a prion that are in contrast to the corresponding properties of a nucleic acid replicon (Wickner, 1994). 

A prion is a potentially infectious protein found in multiple forms in mammals, often concentrated in nervous tissue. It is an abnormal form of a normal cellular protein. It tends to form plaques that destroy the nervous tissue. Prions have been found to be transmissible across species. 

There are morphologial similarities between amyloid protein and infectious protein in scrapie and Creutzfeld-Jakob disease, and protein deposits in Alzheimer s disease, all of which are associated with progressive and irreversible degeneration of nervous function, leading to death. [P.A.Merz etal. Nature 306 (1983) 474- 76 H.Diringer etal. ibid. 476-478] In Alzheimer s disease there is invariably a progressive accumulation of filamentous aggregates of amy-... 

Which mechanism could induce such a dramatic change of the 3D structure of a protein How could the a —> p conversion of PrP into PrP " account for the pathogenic properties of this protein In 1967 (25 years before Prusiner invented the term prion), a remarkably inspired mathematician, J. S. Griffith, proposed a simple but prophetic model of a disease caused by the replication of an infectious protein. One of his hypothesis was that the brain expressed a "normal" form of the protein, whereas the infectious particle contained an "abnormal" form of the same protein. The abnormal form of the protein was then supposed to bind to the normal brain protein and to convert it into a disease-causing protein. This simple mechanism... 

FIGURE 9.4 Griffith s view of the replication of infectious proteins. The infectious protein is symbolized by a red disk. The brain contains a full reservoir of healthy proteins (green squares) that are converted into infectious entities following contact with the infectious protein. The infectious protein has an abnormal form. Each contact of an infectious protein with a healthy one generates a new infectious protein so that an irreversible chain reaction gradually leads to the transformation of biUions of proteins that acquire an abnormal form and cause disease. 

This long quest was marked by milestones. As already discussed, the obtention of highly purified preparations of infectious scrapie particles was necessary to identify the PrP protein. Circular dichroism studies of PrP and PrP demonstrated the existence of two totally distinct conformations of PrP, one corresponding to the physiologically expressed brain protein and the other to the infectious protein. However, the masterly demonstration of the mechanism of prion replication is that PRNP° mice, which do not express the PrP protein, failed to propagate prion infectivity. Hence, without the brain reservoir of normal PrP proteins, infectious PrP proteins are harmless and unable to cause any disease. If we link this information with the respective structures of PrP and PrP , then we have a molecular mechanism accounting for the replication, by force, of prions invaders in the brain of healthy animals (Fig. 9.5). 

Proteins Involved in Brain Diseases Considered as Infectious Proteins... 

PROTEINS INVOLVED IN BRAIN DISEASES CONSIDERED AS INFECTIOUS PROTEINS ... 

The two partners are the membrane and the infectious protein (e.g., Alzheimer s -amyloid peptide Ap, Parkinson s associated a-synuclein, and HIV-1 gpl20) (Fig. 14.1A-C). Each of these proteins faces a complex membrane formed by a collection of lipids with wide biochemical diversity. Hopefully, among the myriad of brain membrane lipids, the protein generally selects a preferred target, most often located in a lipid raft microdomain. For instance, Ap peptides interact preferentially with ganglioside GMl, and a-synuclein with GM3. ... 

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