In-cell structure and snapshots ofcopiaretrotransposons in intact tissue by cryo-electron tomography

Abstract

Long terminal repeat (LTR) retrotransposons belong to the transposable elements (TE), autonomously replicating genetic elements that integrate into the host’s genome. LTR retrotransposons represent a major component of genomes across the tree of life; some derived sequences have even been domesticated by the host to perform cellular functions in essential processes such as development. Among animals,Drosophila melanogasterserves as an important model organism for TE research, harboring several LTR retrotransposons, including the Ty1-copiafamily, which is evolutionarily related to retroviruses and forms virus-like particles (VLPs). The architectural organization ofcopiaVLPsin situhas remained unknown. In this study, we use cryo-FIB milling and lift-out approaches to visualizecopiaVLPs in isolated ovarian cells and intact egg chambers and resolve thein situ copiacapsid structure to 7.7 Å resolution by cryo-ET. While cytosoliccopiaVLPs vary in size, nuclear VLPs are homogenous and form densely packed clusters, supporting a model in which nuclear import acts as a size selector. By analyzing flies deficient in the TE-suppressing PIWI-piRNA pathway, we observe a change incopialocalization from cytosolic to nuclear during spermatogenesis in testes. Our findings provide insights into the cellular structural biology of an active LTR retrotransposon and shed light on the replication cycle ofcopiain the context of host gametogenesis.