Conserved tripartite tail proteins mediate virophage-host interactions through Synechococcus lipopolysaccharide...

Virophages are small double-stranded DNA viruses that parasitize giant viruses and play crucial roles in microbial ecosystem dynamics. In this study, we investigate three evolutionarily conserved tail proteins in Dishui Lake Virophage 2 (DSLV2) that exhibit significant structural homology to the T4 phage gp37 long tail fiber protein. Our integrated approach combining structural modeling, molecular dynamics simulations, and biochemical characterization reveals that these proteins form a stable heterotrimeric complex with a functional receptor-binding domain. Comprehensive genomic analysis of related virophages within the Aquatic Virophage 1 lineage demonstrates the widespread conservation of this tripartite tail architecture, suggesting its fundamental importance in virophage biology. Phylogenetic evidence indicates that these tail proteins of DSLV2 were evolutionarily related to those of Synechococcus phages, and Synechococcus cell surface lipopolysaccharides (LPSs) serve as receptors for the binding of the heterotrimeric tails. We propose a novel two-step infection mechanism wherein DSLV2 initially attaches to Synechococcus LPSs via its heterotrimeric tail complex, followed by subsequent entry into algal cells through bacterivory-mediated viral hitchhiking. These findings significantly advance our understanding of virophage-host interaction mechanisms in aquatic ecosystems.

This study significantly advances our understanding of virophage biology by elucidating key molecular mechanisms underlying their host interactions. The discovery of a conserved heterotrimeric tail complex in Dishui Lake Virophage 2, evolutionarily related to tail proteins of Synechococcus phages and specifically recognizing Synechococcus lipopolysaccharide (LPS), reveals an unexpected life cycle for virophages and provides the first structural basis for their attachment strategy. The proposed two-step infection mechanism involving initial LPS binding followed by bacterivory-mediated algal cell entry challenges conventional views of virophage entry and suggests that these viruses may play a more complex ecological role than previously recognized. These findings not only shed light on the evolutionary adaptation of virophages but also have important implications for understanding complex microbial interactions in aquatic ecosystems, particularly in terms of bacteria-mediated virophage cycling.