A prion of yeast metacaspase homolog (Mca1p) detected by a genetic screen.

Saccharomyces cerevisiae can be infected with four amyloid-based prions: [URE3], [PSI(+)], [PIN(+)], and [SWI(+)], due to self-propagating aggregation of Ure2p, Sup35p, Rnq1p and Swi1p, respectively. We searched for new prions of yeast by fusing random segments of yeast DNA to SUP35MC, encoding the Sup35 protein lacking its own prion domain, selecting clones in which Sup35MC function was impaired. Three different clones contained parts of the Q/N-rich amino-terminal domain of Mca1p/Yca1p with the Sup35 part of the fusion protein partially inactive. This inactivity was dominant, segregated 4:0 in meiosis, and was efficiently transferred by cytoplasmic mixing. The inactivity was cured by overexpression of Hsp104, but the prion could arise again in the cured strain (reversible curing). Overproduction of the Mca1 N-terminal domain induced the de novo appearance of the prion form of the fusion. The prion state, which we name [MCA], was transmitted to the chromosomally encoded Mca1p based on genetic, cytological and biochemical tests.