The [Het-s] Prion, an Amyloid Fold as a Cell Death Activation Trigger

Prions are self-propagating amyloids. Amyloids are proteinpolymers with a cross-b structure, in which short b-strands fromthe monomers stack one on top of each other to make up a fibrillarpolymer [1,2]. These amyloids act as templates that convertmonomers to the amyloid polymerised state. Spontaneous orchaperone-assisted fragmentation of this amyloid fibril allowsmultiplication of the prion particle by generating novel fibril endswhere templating occurs.Prions have initially been identified in the context of mamma-lian spongiform encephalopathies such as scrapie in sheep,Creutzfeld-Jacob disease in humans, or bovine spongiformencephalopathy (BSE) in cattle [3]. In these diseases, the hostencoded GPI-anchored PrP prion protein turns into prionaggregates, leading to incurable lethal neurodegenerative diseases.The prion phenomenon is not restricted to this sole example. Inparticular, nine prion proteins have been identified in yeast andcorrespond to proteins with a wide range of cellular functions [4].Generally, prion formation leads to loss of the cellular function ofthe protein. Yeast prions are thus detected as non-Mendeliangenetic elements, leading to infectious and inheritable proteininactivation.More than 30 human diseases, including Alzheimer disease, arecaused by accumulation of amyloid aggregates of various proteinsand peptides in the brain or systemic locations [2]. Many proteinscan form amyloids, and the amyloid fold has been envisioned as adefault conformational state that is formed when the native state ofthe protein cannot be efficiently attained or maintained. This viewis, for instance, illustrated by the finding that inclusion bodiesformed during recombinant protein over-expression in E. coli havean amyloid-like structure [5]. But in nature, the amyloid fold is notonly found in abnormal or pathological situations. The specificphysico-chemical properties of this fold have also been exploited toperform various biological functions. Amyloids constitute variouscell surface structures in bacteria, fungi, and insects. Amyloidsallow storage and delayed release of various peptide hormones inmammals [6]. Then, it has been proposed that this ability of aprotein to exist under two states (a functional and a self-inactivating form) represents an epigenetic mechanism of generegulation [7]. In that perspective, yeast prions have beensuggested to represent a benefit [8], but this view remains debated[9].