Atkinsonella hypoxylon virus capsid structure highlights the diversity of capsid proteins among the Partitiviridae

<i>Atkinsonella hypoxylon</i> virus (AhV) is a fungi-infecting betapartitivirus and the typical member of the <i>Partitiviridae</i>, a family of persistent viruses that infect a broad range of organisms. Partitiviruses have been largely overlooked following their designation as cryptic viruses. However, evidence is accumulating that they play an important role in the ecology of their hosts. Since the capsid proteins of partitiviruses have been implicated in virus-host interactions, exploring their structural biology may give clues into the evolution, horizontal transmission and host adaptation of partitiviruses. The capsid of AhV shares the same organization of other partitiviruses with 60 dimeric capsid protein protomers arranged with <i>T</i>=1 icosahedral symmetry. The structure, determined by cryo-electron microscopy to 2.4 Å, shows that AhV has a unique iteration on the protrusion domain with an extensive network of hydrophobic interactions among equivalent interdigitating loops at the dimerization interface. AhV also shares a conserved helical core in the shell domain, which we extend to all genera of the recognized partitiviruses using protein structure prediction. The helical core appears to be a conserved element of the picobirnavirus lineage of capsid protein folds and provides a template onto which various elaborations of the protrusion domain have evolved. The involvement of the protrusion in virus-host interactions has previously been proposed, and our findings provide evidence of a structural device enabling capsid protein diversification during the evolution of the <i>Partitiviridae</i>.