Heritable activity: a prion that propagates by covalent autoactivation

Abstract

Known prions (infectious proteins) are self-propagating amyloids or conformationally altered proteins, but in theory an enzyme necessary for its own activation could also be a prion (or a gene composed of protein). We show that yeast protease B is such a prion, called [beta].[beta] is infectious, reversibly curable, and its de novo generation is induced by overexpression of the pro-protease. Present in normal cells but masked by the functionally redundant protease A, [beta] is advantageous during starvation and necessary for sporulation. We propose that other enzymes whose active, modified, form is necessary for their maturation might also be prions.

Figure 1.

Activation diagram of three vacuolar hydrolases. Arrows point from an activating enzyme toward the enzyme(s) it activates. Orange arrows indicate autoactivation in cis (intramolecular) or in trans (mature form activating a separate molecule of the pro-form) as indicated.

Figure 2.

(A) Indefinite propagation of the CpY⁺ phenotype. pep4Δ mutant progeny of a heterozygous diploid were germinated on YPG. Individual colonies were restreaked 10 times. Red colonies are CpY⁺; two colonies have become CpY^–. (B) CpY activity is rapidly lost on media that repress PRB1 transcription. A CpY⁺ pep4Δ mutant that had previously been cultured on YPG was streaked on YPAD. (C) pep4Δ mutants that have lost CpY activity generally remain CpY^– when returned to YPG; very rarely they may become CpY⁺ (data not shown). Contrast was adjusted in the images to make colony color differences more apparent in A–C. (D) PrB activity in crude lysates of CpY⁺ (i.e., [β]) and CpY^– (i.e., [β-o]) pep4Δ strains. pep4Δ prb1Δ (CpY^–): 4868-2C; pep4Δ CpY^–: TRY 250; pep4Δ CpY⁺: TRY 251; wild-type (CpY⁺): 4771-1A.

Figure 3.

Survival of strains during culture on dextrose minimal plates; (w.t.) PEP4⁺ PRB1⁺ (4771-1A) with pH392 (P_ADH1 vector); ([β]) pep4Δ prb1Δ (4868-2C) with P_ADH1 > PRB1 [β]; ([β-o]) pep4Δ prb1Δ (4868-2C) with P_ADH1 > PRB1 [β-o]; (prb1) pep4Δ prb1Δ (4868-2C) with P_ADH1 > prb1-519. Error bars encompass quartiles above and below the median.

Figure 4.

Diploids of the indicated genotypes were grown on sporulation media for 5 d, digested with glusulase, and visualized by differential interference contrast microscopy; (wild type) PEP4⁺ PRB1⁺; (pep4Δ/pep4Δ P_ADH1 > PRB1 [β]) TRY256; (pep4Δ/pep4Δ P_ADH1 > PRB1 [β-o]) TRY257; (pep4Δ/pep4Δ P_ADH1 > prb1-519) TRY258. Arrows indicate tetrads.