Fusion between virus and host membranes is critical for viruses to release genetic materials and to initiate infection. Whereas most viruses use a single protein for membrane fusion, poxviruses employ a multiprotein entry-fusion complex (EFC).

Poxviruses are known to encode a set of proteins that form an entry-fusion complex (EFC) to mediate virus entry. However, the diversity, evolution, and origin of these EFC proteins remain poorly understood. Here we identify the EFC protein homologs in poxviruses and other giant viruses of phylum Nucleocytoviricota. The 11 EFC genes are present in almost all poxviruses, with the two smallest, G3 and O3, absent in Entomopoxvirinae and basal lineages of Chordopoxvirinae. Five of the EFC genes are further grouped into two families, A16/G9/J5 and F9/L1, which are widely distributed across other major lineages of Nucleocytoviricota, including metagenome-assembled genomes, but are generally absent in viruses infecting algae or non-amoebozoan heterotrophic protists. The A16/G9/J5 and F9/L1 families co-occur, mostly as single copies, in 93% of the non-Poxviridae giant viruses that have at least one of them. Distribution and phylogenetic patterns suggest that both families originated in the ancestor of Nucleo-cytoviricota. In addition to the Poxviridae genes, homologs from each of the other Nucleocytoviricota families are largely clustered together, suggesting their ancient presence and vertical inheritance. Despite deep sequence divergences, we observed noticeable conservation of cysteine residues and predicted structures between EFC proteins of Poxviridae and other families. Overall, our study reveals widespread distribution of these EFC protein homologs beyond poxviruses, implies the existence of a conserved membrane fusion mechanism, and sheds light on host range and ancient evolution of Nucleocytoviricota.