Figure 1.

[Het-s]/HET-S incompatibility. (A) When confronted on solid medium, [Het-s] and [Het-S] strains fuse and produce a barrage reaction (black arrowhead) indicative of the occurrence on an incompatibility reaction. In contrast, a contact between two [Het-S] strains is normal and shows no demarcation line (white arrowhead). (B) Diagram of the result of an [Het-s]/[Het-S] cell fusion: cytoplasmic mixing occurs, and the prion form of HET-s interacts with HET-S and leads to activation of a pore-forming activity. Upon activation, HET-S is relocated to the plasma membrane, exerts toxicity, and induces cell death. (C) When HET-s is in the soluble (nonprion) state ([Het-s*]), cell fusion with HET-S does not lead to incompatibility. (D) When [Het-s] fuses with [Het-s*], the prion form propagates within the [Het-s*] cells and converts them to the [Het-s] state.