Amyloid Prions in Fungi

Natural situations of prion propagation in fungi. (A) Prion propagation after hyphal anastomosis in a filamentous fungus (for instance, [Het-s] in Podospora anserina). The prion form is transmitted from a donor-infected strain (right) to a recipient strain (left). The prion form then converts the entire mycelium to the prion state due to cytoplasmic continuity throughout the thallus. Prion transmission also occurs in meiotic crosses with maternal inheritance (not depicted here). (B) Prion propagation during mitotic cell divisions in yeast. Prion seeds are transmitted from mother to daughter cells during budding. (C) Prion transmission during sexual crosses. In a cross between a [PRION+] (left) and a [prion –] strain (right), the resulting diploid is [PRION+] and there is non-Mendelian segregation of the [PRION+] character (often, but not always, with 4:0 segregation as in the example depicted here).

Structure of HET-s prion-forming domain. (A) Lateral view of a trimer of HET-s(218-289) in the prion amyloid conformation. Each monomer bears a different color, after pdb 2KJ3. (B) View from the fibril axis of one HET-s(218-289) monomer; the N- and C-terminal ends are marked, after pdb 2KJ3. (C) Structure of the two individual repeats of HET-s(218-289) marked R1 (position 226 to 246) and R2 (position 262 to 282) as well as the C-terminal semihydrophobic loop (position 283 to 289), after pdb 2KJ3. Amino acids are coded by chemical property (G in light gray, polar in green, hydrophobic in yellow, positively charged in red, negatively charged in blue, and aromatic in magenta). The sequence of HET-s(218-289) is given below with the same color coding in R1, R2 (underlined), and the C-terminal loop.