Structures of the Foamy virus fusion protein reveal an unexpected link with the F protein of paramyxo- and pneumoviruses

Foamy viruses (FVs) constitute a subfamily of retroviruses. Their envelope glycoprotein (Env) drives the merger of viral and cellular membranes during entry into cells. The only available structures of retroviral Envs are those from human and simian immunodeficiency viruses from the subfamily of orthoretroviruses, which are only distantly related to the FVs. We report here the cryo-EM structures of the FV Env ectodomain in the pre- and post-fusion states, which demonstrate structural similarity with the fusion protein (F) of paramyxo- and pneumoviruses, implying an evolutionary link between the two viral fusogens. Based on the structural information on the FV Env in two states, we propose a mechanistic model for its conformational change, highlighting how the interplay of its structural elements could drive the structural rearrangement. The structural knowledge on the FV Env now provides a framework for functional investigations such as the FV cell tropism and molecular features controlling the Env fusogenicity, which can benefit the design of FV Env variants with improved features for use as gene therapy vectors.