Our previous work showed that C.a.
Ure2p can form a prion in S. cerevisiae,
but that C.g.
Ure2p cannot (40
). Here we find that C.a.
readily forms amyloid, but that C.g.
only forms a small amount of amyloid over a period of two years, providing a possible explanation for the inability of C.g.
Ure2p to be a prion. Of course, it is possible that there exists some other buffer condition under which C.g.
Ure2p would readily form amyloid. Whether amyloid forming ability (or lack of it) will be a general explanation of whether or not a protein can be a prion will require examination of a wide range of species.
The C.a.Ure2p1–90 amyloid formed is infectious for S. cerevisiae expressing C.a.Ure2p in place of the S.c.Ure2p, but we were not successful in inducing C.g.Ure2p to form [URE3] by introduction of the small amount of available C.g.Ure2p1–100 amyloid. Of course it remains possible that with larger amounts of C. glabrata amyloids, or different conditions of amyloid formation, infection might be observed. The infection produces a variety of [URE3alb] variants with varying degrees of strong or weak phenotype and stable or unstable inheritance. This array of prion variants presumably reflects an array of amyloid structures, and the wide peaks we observe in 2D solid state NMR experiments () are likely a reflection of such structural heterogeneity.
Our structural studies indicate that the amyloid fibers formed in vitro by C.a.Ure2p1–90 have an architecture like that of the other amyloid-based yeast prions, namely, an in-register parallel β-sheet. The 6 nm diameter of the filaments of C.a.Ure2p1–90 indicates that the sheet is multiply folded along its long axis, since a fully extended monomer would be approximately 30 nm in length. More detailed structural studies will be needed to ascertain the location of the folds and other details of the structure, but this will require development of a method to prepare structurally homogeneous filaments.
While mammalian prions are uniformly fatal, those of yeast and fungi are not, so that whether prions are diseases or of some adaptive value must be judged by other evidence. The conservation of prion formation among closely related species would not prove that it has a function for the host (any more than the conservation of occasional broken wings among birds suggests a benefit). However, the failure of conservation among close relatives would argue against a functional role for prion formation. Residues 10–39 of the S. cerevisiae
Ure2p prion domain is conserved among a range of yeasts (10
), and this led Harrison et al. to propose that this region is conserved to enable prion formation (59
). Ross et al. demonstrated that sequence is of little importance for prion formation by Ure2p or by Sup35p (35
), so the conserved part of the Ure2p prion domain is not likely conserved for prion formation: a need for prion formation would not result in conservation of the sequence. Moreover, C. glabrata
is closely related to S. cerevisiae
Ure2p has the conserved prion domain sequence (Fig. S1
), but does not form prions. In contrast, C. albicans
is more distantly related to S. cerevisiae
and its Ure2p lacks the conserved sequence (Fig. S1
), but we find that it readily forms prions. This suggests that the sequence is conserved for some other reason, perhaps for the stabilization of the full length protein by the prion domain as demonstrated for S. cerevisiae
While definitive data is scarce, the pattern seems to be that prion-forming ability of a given protein is scattered, rather than concentrated among close relatives. For example, the full length S. castellii
Ure2p cannot form a prion in S. cerevisiae
), but the Ure2p's of several other Saccharomyces
species can form [URE3]. One fourth of wild S. cerevisiae
strains tested had a large deletion in their Sup35 prion domains making them unable to form the [PSI+] prion (61
). Several N-terminal domains of Sup35 of other species fused to the S. cerevisiae
Sup35C have been found to be able to act as prion domains (16
). However, it is known that for Ure2p, Sup35p and HET-s, the prion domain is substantially stabilized by the remainder of the molecule (8
), so that the context of the full length molecule is important.
The absence of the [URE3] and [PSI+] prions in wild strains, the inability of some species closely related to S. cerevisiae
to form these prions, the fact that the 'prion domains' have clear-cut non-prion functions and the rapid evolution of the prion domains producing species barriers all indicate that these are yeast diseases (reviewed in (39
). The cells themselves signal their displeasure at infection with [URE3] and [PSI+] by undergoing the stress-response, inducing at least Hsp70s and Hsp104 (62
). Moreover, the existence of lethal and very pathological variants of [PSI+] and [URE3] further argue against an adaptive role for these prions (64
). The study of these yeast prion diseases is making important contributions to our understanding of mammalian prion diseases and amyloid diseases in general.