The Molecular Mechanism of Cellular Attachment for an Archaeal Virus

Abstract

Sulfolobus turreted icosahedral virus (STIV) is a model archaeal virus and member of the PRD1-Adenovirus lineage. While it is known that STIV employs pyramidal lysis structures to exit the host, knowledge of the viral entry process is lacking. We therefore initiated studies on STIV attachment and entry. Negative stain and cryo-EM micrographs showed virion attachment to pili-like structures emanating from the Sulfolobus solfataricus host. Tomographic reconstruction and sub-tomogram averaging revealed pili recognition by the STIV C381 turret protein. Specifically, the triple jelly-roll structure of C381 determined by X-ray crystallography shows that pilus recognition is mediated by conserved surface residues in the 2nd and 3rd domains. Additionally, the STIV petal protein (C557), when present, occludes the pili binding site, suggesting it functions as a maturation protein. Combined, these results demonstrate a role for the namesake STIV turrets in initial cellular attachment and provide the first molecular model for viral attachment in the archaeal domain of life.