No abstract

Retrotransposition of the budding yeast long terminal repeat retrotransposon Ty3 is activated during mating. In this study, proteins that associate with Ty3 Gag3 capsid protein during virus-like particle (VLP) assembly were identified by mass spectrometry and screened for roles in mating-stimulated retrotransposition. Components of RNA processing bodies including DEAD box helicases Dhh1/DDX6 and Ded1/DDX3, Sm-like protein Lsm1, decapping protein Dcp2, and 5’ to 3’ exonuclease Xrn1 were among the proteins identified. These proteins associated with Ty3 proteins and RNA, and were required for formation of Ty3 VLP retrosome assembly factories and for retrotransposition. Specifically, Dhh1/DDX6 was required for normal levels of Ty3 genomic RNA, and Lsm1 and Xrn1 were required for association of Ty3 protein and RNA into retrosomes. This role for components of RNA processing bodies in promoting VLP assembly and retrotransposition during mating in a yeast that lacks RNA interference, contrasts with roles proposed for orthologous components in animal germ cell ribonucleoprotein granules in turnover and epigenetic suppression of retrotransposon RNAs.

Cells expressing the yeast retrotransposon Ty3 form concentrated foci of Ty3 proteins and RNA within which virus-like particle (VLP) assembly occurs. Gag3, the major structural protein of the Ty3 retrotransposon, is composed of capsid (CA), spacer (SP), and nucleocapsid (NC) domains analogous to retroviral domains. Unlike the known SP domains of retroviruses, Ty3 SP is highly acidic. The current studies investigated the role of this domain. Although deletion of Ty3 SP dramatically reduced retrotransposition, significant Gag3 processing and cDNA synthesis occurred. Mutations that interfered with cleavage at the SP-NC junction disrupted CA-SP processing, cDNA synthesis, and electron-dense core formation. Mutations that interfered with cleavage of CA-SP allowed cleavage of the SP-NC junction, production of electron-dense cores, and cDNA synthesis but blocked retrotransposition. A mutant in which acidic residues of SP were replaced with alanine failed to form both Gag3 foci and VLPs. We propose a speculative “spring” model for Gag3 during assembly. In the first phase during concentration of Gag3 into foci, intramolecular interactions between negatively charged SP and positively charged NC domains of Gag3 limit multimerization. In the second phase, the NC domain binds RNA, and the bound form is stabilized by intermolecular interactions with the SP domain. These interactions promote CA domain multimerization. In the third phase, a negatively charged SP domain destabilizes the remaining CA-SP shell for cDNA release.

Retroviruses and retrotransposons package genomic RNA into virus-like particles (VLPs) in a poorly understood process. Expression of the budding yeast retrotransposon Ty3 results in the formation of cytoplasmic Ty3 VLP assembly foci comprised of Ty3 RNA and proteins, and cellular factors associated with RNA processing body (PB) components, which modulate translation and effect nonsense-mediated decay (NMD). A series of Ty3 RNA variants were tested to understand the effects of read-through translation via programmed frameshifting on RNA localization and packaging into VLPs, and to identify the roles of coding and non-coding sequences in those processes. These experiments showed that a low level of read-through translation of the downstream open reading frame (as opposed to no translation or translation without frameshifting) is important for localization of full-length Ty3 RNA to foci. Ty3 RNA variants associated with PB components via independent determinants in the native Ty3 untranslated regions (UTRs) and in GAG3-POL3 sequences flanked by UTRs adapted from non-Ty3 transcripts. However, despite localization, RNAs containing GAG3-POL3 but lacking Ty3 UTRs were not packaged efficiently. Surprisingly, sequences within Ty3 UTRs, which bind the initiator tRNAMet proposed to provide the dimerization interface, were not required for packaging of full-length Ty3 RNA into VLPs. In summary, our results demonstrate that Gag3 is sufficient and required for localization and packaging of RNAs containing Ty3 UTRs and support a role for POL3 sequences, translation of which is attenuated by programmed frameshifting, in both localization and packaging of the Ty3 full-length gRNA.