The spongiform encephalopathies are normally thought of as occuring in mammals. However, homologues of the prion gene is found much more widely in nature and its study in other organisms may yield insight into its normal biological function and evolution.
The warble fly and hay mites have been mentioned as possible insect vectors for transmitting BSE. However, these species are not convienent experimentally, in contrast to the well-studied fruit fly. The study here shows that a mammalian prion gene can be expressed in the fruit fly; while surprisingly it gets covalently anchored correctly to the cell membrane, its outer surface is incorrectly formed.
Molecular biologists didn't believe in infectious prion proteins -- the "not-invented-here" syndrome -- until a comparable situation [involving a non-homologous protein] was found. It is far easier to do detailed biochemical genetic studies in yeast than in a mammal. The entire yeast chromosome has been sequenced. Possibly some other yeast gene is weakly homologous to mammalian prion protein.
The best understood of all animal nervous systems is the nematode. Every neuron and every neural connection has been mapped by electron microscopy. Fifty per cent of the chromosome has been sequenced and many neurological mutations have been located and studied. If a prion analogue exists, it could be most rapidly studied in this animal model.
Chickens are of interest because rendered beef offal has been diverted to them as a feedstuff, over 100,000 tons a year in California alone. One might suppose there would be an insurmountable species barrier -- however, 3 zoo ostriches in Germany have died from a spongiform encephalopathy, possibly from their feed.
Expression of prion protein in transgenic Drosophila melanogasterRaeber AJ; Muramoto T; Kornberg TB; Prusiner SB
Mech Dev 51: 317-27 (1995)
To evaluate the fruit fly as a model for studying neurodegenerative diseases caused by prions, transgenic flies were generated by introducing the Syrian hamster prion protein (SHaPrP) gene into Drosophila melanogaster. Nine transgenic lines were isolated; induction of transgenes that had been placed under the control of the Drosophila heat shock promoter resulted in the synthesis of full-length SHaPrP. The relative molecular weight of the recombinant protein was lower than that of authentic SHaPrP due to incomplete processing of Asn-linked carbohydrates.
Cellular localization: heat shock induced SHaPrP was anchored to the surface of cells by a glycolipid, demonstrating that the carboxy-terminal glyco-lipidation signal of hamster prion protein is recognized by this evolutionarily distant host. No difference in fruitfly lifespans compared with controls was detected. No disease phenotype or protease-resistant product was associated with expression of hamster prion protein over the entire lifespan of transgenic flies.
Prion-like protein in yeastMasison DC; Wickner RB
Science 270: 93-5 (1995)
The genetic properties of the [URE3] non-Mendelian element of Saccharomyces cerevisiae suggest that it is a prion (infectious protein) form of Ure2p, a regulator of nitrogen catabolism.
In extracts from [URE3] strains, Ure2p was partially resistant to proteinase K compared with Ure2p from wild-type extracts. Overexpression of Ure2p in wild-type strains induced a 20- to 200-fold increase in the frequency with which [URE3] arose. Overexpression of just the amino-terminal 65 residues of Ure2p increased the frequency of [URE3] induction 6000-fold.
Without this "prion-inducing domain" the carboxyl-terminal domain performed the nitrogen regulation function of Ure2p, but could not be changed to the [URE3] prion state. Thus, this domain induced the prion state in trans, whereas in cis it conferred susceptibility of the adjoining nitrogen regulatory domain to prion infections.
A gene homologue in Caenorhabditis elegans to prion proteinIwasaki,M., Okumura,K., Kondo,Y., Igarashi,H. and Tanaka,T.
Nucleic Acids Res. 20, 4001-4007 (1992)
One letter amino acid codes: M = methionine, T = threonine, V = valine, etc.
MTDVEIKAENGSGDASLEPENLRKIFVGGLTSNTTDDLMREFYS QFGEITDIIVMRDPTTKRSRGFGFVTFSGKTEVDAAMKQRPHIIDGKTVDPKRAVPRD DKNRSESNVSTKRLYVSGVREDHTEDMLTEYFTKYGTVTKSEIILDKATQKPRGFGFV TFDDHDSVDQCVLQKSHMVNGHRCDVRKGLSKDEMSKAQMNRDRETRGGRSRDGQRGG YNGGGGGGGGWGGPAQRGGPGAYGGPGGGGQGGYGGDYGGGWGQQGGGGQGGWGGPQQ QQGGGGWGQQGGGGQGGWGGPQQQQQGGWGGPQQGGGGGGWGGQGQQQGGWGGQSGAQ
Amino acid sequence of chicken prion proteinGabriel,J.-M., Oesch,B., Kretzschmar,H., Scott,M. and Prusiner,S.B.
Proc. Natl. Acad. Sci. U.S.A. 89, 9097-9101 (1992)
MARLLTTCCLLALLLAACTDVALSKKGKGKPSGGGWGAGSHRQP SYPRQPGYPHNPGYPHNPGYPHNPGYPHNPGYPHNPGYPQNPGYPHNPGYPGWGQGYN PSSGGSYHNQKPWKPPKTNFKHVAGAAAAGAVVGGLGGYAMGRVMSGMNYHFDSPDEY RWWSENSARYPNRVYYRDYSSPVPQDVFVADCFNITVTEYSIGPAAKKNTSEAVAAAN QTEVEMENKVVTKVIREMCVQQYREYRLASGIQLHPADTWLAVLLLLLTTLFAMH