Wang, H.; Gianluca, M.; Munke, A. C.; Hassan, M. M.; Lalle, M.; Okamoto, K.

Giardia lamblia virus (GLV) is a non-enveloped icosahedral dsRNA virus and an endosymbiont virus infecting the zoonotic protozoan parasite Giardia duodenalis (syn. G. lamblia, G. intestinalis), a pathogen of mammals, including humans. Elucidating the transmission mechanism of GLV is crucial to an in-depth understanding of the virulence of the virus in G. duodenalis. GLV belongs to the family Totiviridae, which infects yeast and protozoa intracellularly; however, it also transmits extracellularly, similar to phylogenetically distantly related toti-like viruses that infect multicellular hosts. The GLV capsid structure is extensively involved in the longstanding discussion concerning the acquisition of extracellular transmission in Totiviridaeand toti-like viruses. Hence, this study constructed the first high-resolution comparative atomic models of two GLV strains, namely GLV-HP and GLV-CAT, which showed different intracellular localization and virulence phenotypes, using cryo-EM single-particle analysis. The atomic models of the GLV capsids presented swapped C-terminal extensions, extra surface loops, and a lack of cap-snatching pockets, similar to those of toti-like viruses. However, their open pores and lack of the extra crown protein (CrP) resemble those of other yeast and protozoan Totiviridae viruses, demonstrating the essential structures for acquiring extracellular cell-to-cell transmission. The intensive structural comparison between GLV-HP and GLV-CAT indicates the first evidence of critical structural motifs for the transmission and virulence of GLV in G. duodenalis.