title: A trans-dominant form of Gag restricts Ty1 retrotransposition and mediates copy number control.
abstract: Saccharomyces cerevisiae and Saccharomyces paradoxus lack the conserved RNA interference pathway and utilize a novel form of copy number control (CNC) to inhibit Ty1 retrotransposition. Although noncoding transcripts have been implicated in CNC, here we present evidence that a truncated form of the Gag capsid protein (p22) or its processed form (p18) is necessary and sufficient for CNC and likely encoded by Ty1 internal transcripts. Coexpression of p22/p18 and Ty1 decreases mobility more than 30,000-fold. p22/p18 cofractionates with Ty1 virus-like particles (VLPs) and affects VLP yield, protein composition, and morphology. Although p22/p18 and Gag colocalize in the cytoplasm, p22/p18 disrupts sites used for VLP assembly. Glutathione S-transferase (GST) affinity pulldowns also suggest that p18 and Gag interact. Therefore, this intrinsic Gag-like restriction factor confers CNC by interfering with VLP assembly and function and expands the strategies used to limit retroelement propagation.
  
  
  OBJECTIVE
  Retrotransposons dominate the chromosomal landscape in many eukaryotes, can cause mutations by insertion or genome rearrangement, and are evolutionarily related to retroviruses such as HIV. Thus, understanding factors that limit transposition and retroviral replication is fundamentally important. The present work describes a retrotransposon-encoded restriction protein derived from the capsid gene of the yeast Ty1 element that disrupts virus-like particle assembly in a dose-dependent manner. This form of copy number control acts as a molecular rheostat, allowing high levels of retrotransposition when few Ty1 elements are present and inhibiting transposition as copy number increases. Thus, yeast and Ty1 have coevolved a form of copy number control that is beneficial to both "host and parasite." To our knowledge, this is the first Gag-like retrotransposon restriction factor described in the literature and expands the ways in which restriction proteins modulate retroelement replication.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia, USA.
authors.forename: Agniva
authors.initials: A
authors.lastname: Saha
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia, USA.
authors.forename: Jessica A
authors.initials: JA
authors.lastname: Mitchell
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia, USA.
authors.forename: Yuri
authors.initials: Y
authors.lastname: Nishida
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia, USA.
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Hildreth
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
authors.forename: Joshua A
authors.initials: JA
authors.lastname: Ariberre
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
authors.forename: Wendy V
authors.initials: WV
authors.lastname: Gilbert
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia, USA djgarf@bmb.uga.edu.
authors.forename: David J
authors.initials: DJ
authors.lastname: Garfinkel
mesh_terms: D015870:Gene Expression; D025941:Protein Interaction Mapping; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D011995:Recombination, Genetic; D018626:Retroelements; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D022701:Virosomes / Q000378:metabolism; D019065:Virus Assembly; D054301:gag Gene Products, Human Immunodeficiency Virus / Q000235:genetics / Q000378:metabolism*
chemical_list: D011994:Recombinant Proteins; D018626:Retroelements; D022701:Virosomes; D054301:gag Gene Products, Human Immunodeficiency Virus