Retroviruses evolved from long terminal repeat (LTR) retrotransposons by acquisition of envelope functions, and subsequently reinvaded host genomes. Together, endogenous retroviruses and LTR retrotransposons represent major components of animal, plant, and fungal genomes. Sequences from these elements have been exapted to perform essential host functions, including placental development, synaptic communication, and transcriptional regulation. They encode a Gag polypeptide, the capsid domains of which can oligomerize to form a virus-like particle. The structures of retroviral capsids have been extensively described. They assemble an immature viral particle through oligomerization of full-length Gag. Proteolytic cleavage of Gag results in a mature, infectious particle. In contrast, the absence of structural data on LTR retrotransposon capsids hinders our understanding of their function and evolutionary relationships. Here, we report the capsid morphology and structure of the archetypal Gypsy retrotransposon Ty3. We performed electron tomography (ET) of immature and mature Ty3 particles within cells. We found that, in contrast to retroviruses, these do not change size or shape upon maturation. Cryo-ET and cryo-electron microscopy of purified, immature Ty3 particles revealed an irregular fullerene geometry previously described for mature retrovirus core particles and a tertiary and quaternary arrangement of the capsid (CA) C-terminal domain within the assembled capsid that is conserved with mature HIV-1. These findings provide a structural basis for studying retrotransposon capsids, including those domesticated in higher organisms. They suggest that assembly via a structurally distinct immature capsid is a later retroviral adaptation, while the structure of mature assembled capsids is conserved between LTR retrotransposons and retroviruses.
