The Ty1 Retrotransposon Restriction Factor p22 Targets Gag

Area of Science:

• Molecular Biology
• Genetics
• Virology

Background:

• The Saccharomyces genome maintains a low copy number of Ty1 retrovirus-like transposons through a novel copy number control (CNC) mechanism.
• Ty1 produces p22, an alternative transcript encoding a protein that inhibits Ty1 retrotransposition by interfering with Gag protein interactions during virus-like particle (VLP) assembly and maturation.

Purpose of the Study:

• To investigate the mechanism of Ty1 copy number control (CNC) and identify mutations conferring resistance to the Ty1 inhibitor p22.
• To understand how Gag protein interactions are modulated by p22 and how mutations can restore retrotransposition.

Main Methods:

• Forward genetic screen to identify CNC-resistant (CNCR) mutations in Ty1.
• Analysis of mutations in the Gag gene, including mapping to predicted domains (UBN2, CNCR domain) and conserved regions.
• Sequence alignments of Ty1-like Gag proteins and comparative analysis with related elements (e.g., Ty2).
• Co-immunoprecipitation assays to assess Gag-p22 binding and VLP composition.

Main Results:

• Identified missense mutations in Ty1 Gag that restore retrotransposition despite the presence of p22.
• Some CNCR mutations map to predicted helices within Gag, suggesting p22 disrupts a critical capsid function during VLP assembly.
• CNCR mutations do not affect Gag binding to p22 but exclude p22 from VLPs, enabling productive Gag-Gag interactions and VLP assembly.
• Ty2 elements, which contain non-Ty1 residues at CNCR sites, are resistant to p22 and do not exhibit Ty1-like CNC.

Conclusions:

• Ty1 CNC involves p22 inhibiting VLP assembly, and CNCR mutations restore retrotransposition by allowing Gag to exclude p22 from VLPs.
• This study reveals a novel strategy for resistance to Gag-like restriction factors, extending knowledge from retroviruses to retrotransposons.
• The findings highlight the importance of Gag-Gag interactions in VLP assembly and the evolutionary strategies of mobile genetic elements.