Mapping of a Hypovirus p29 Protease Symptom Determinant Domain with Sequence Similarity to Potyvirus HC-Pro Protease

Abstract

Hypovirus infection of the chestnut blight fungus Cryphonectria parasitica results in a spectrum of phenotypic changes that can include alterations in colony morphology and significant reductions in pigmentation, asexual sporulation, and virulence (hypovirulence). Deletion of 88% [Phe(25) to Pro(243)] of the virus-encoded papain-like protease, p29, in the context of an infectious cDNA clone of the prototypic hypovirus CHV1-EP713 (recombinant virus Δp29) partially relieved virus-mediated suppression of pigmentation and sporulation without altering the level of hypovirulence. We now report mapping of the p29 symptom determinant domain to a region extending from Phe(25) through Gln(73) by a gain-of-function analysis following progressive repair of the Δp29 deletion mutant. This domain was previously shown to share sequence similarity [including conserved cysteine residues Cys(38), Cys(48), Cys(70), and Cys(72)] with the N-terminal portion of the potyvirus-encoded helper component-proteinase (HC-Pro), a multifunctional protein implicated in aphid-mediated transmission, genome amplification, polyprotein processing, long-distance movement, and suppression of posttranscriptional silencing. Substitution of a glycine residue for either Cys(38) or Cys(48) resulted in no qualitative or quantitative changes in virus-mediated symptoms. Unexpectedly, mutation of Cys(70) resulted in a very severe phenotype that included significantly reduced mycelial growth and profoundly altered colony morphology. In contrast, substitution for Cys(72) resulted in a less severe symptom phenotype approaching that observed for Δp29. The finding that p29-mediated symptom expression is influenced by two cysteine residues that are conserved in the potyvirus-encoded HC-Pro raises the possibility that these related viral-papain-like proteases function in their respective fungal and plant hosts by impacting ancestrally related regulatory pathways.