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Abstract
Ty1 is a long terminal repeat (LTR) retrotransposon found in Saccharomyces cerevisiae with a life cycle similar to retroviruses. Ty1 encodes the structural capsid protein Gag which mediates assembly of virus-like particles (VLPs) through oligomerization and formation of sub-VLP structures called capsomeres. VLP assembly is necessary for transposition and the VLP serves as a compartment for the enzymatic activity of Ty1 protease, integrase, and reverse transcriptase. S. cerevisiae lacks transposable element (TE) restriction mechanisms commonly found in other organisms but maintains low TE copy numbers. Ty1 canonical (Ty1c), a subfamily of Ty1, maintains copy number control (CNC) through production of an N-terminally truncated Gag protein called p22. p22 is necessary and sufficient for Ty1c CNC and consists of the Gag capsid C-terminal domain (CA-CTD). This work focuses on the discovery and characterization of a domesticated restriction factor of Ty1’, another subfamily of Ty1, that is homologous to p22. Domesticated restriction of Ty1’ relic 2 (DRT2) arose through exaptation of the Ty1’ Gag CA-CTD. DRT2 was fixed early in the evolution of S. cerevisiae, is present in most strains, and has undergone three loss events. Drt2 from multiple strains restricts Ty1’ retromobility, suggesting it is active in all strains in which it is present. Drt2 associates with Ty1’ VLPs, providing evidence that it acts through a mechanism similar to the previously characterized p22 restriction factor which disrupts VLP assembly. The structure of Ty1c VLPs purified from yeast was determined by cryogenic electron tomography (Cryo-ET). Ty1c VLPs are heterogeneous in size with two major classes observed of 38 nm and 47 nm in diameter. Ty1c Gag forms pentameric and hexameric capsomeres with varied organizations that contribute to their overall heterogeneity. We determined a pentameric capsomere to ~9 Å and a hexameric capsomere to ~12 Å resolution. The x-ray crystal structure of Gag CA-CTD was fit into the average density maps, revealing their organization within the capsomere. The work presented here informs our understanding of TE-host co-evolution and the mechanisms of LTR retroelement particle assembly.