Yukihiko Sugita and his colleagues at Kyoto University have finally solved one of virology's most stubborn puzzles. Using cryo-electron microscopy, the team captured the three-dimensional structure of Borna disease virus 1's nucleoprotein-RNA complex — the molecular machinery that shields the virus's genetic material and drives its replication. The findings, published in Science Advances, mark the first detailed structural description of this complex in the entire Bornaviridae family.
Borna disease virus 1 is mercifully rare in humans, but for those who develop illness, the consequences are devastating: almost all cases result in fatal encephalitis, a brutal inflammation of the brain. The virus belongs to the order Mononegavirales, a notorious group that also includes the culprits behind Ebola, measles, and rabies. Understanding the precise architecture of its nucleoprotein-RNA complex has been a missing piece in the virology puzzle — until now.
"Bornaviruses are less well known than many other human RNA viruses, yet they represent the last major unresolved case for nucleoprotein-RNA structural analysis among human-infecting mononegaviruses," said Sugita. "Closing this long-standing gap and connecting structural biology with virological function were major motivations for our team."
The researchers from Kyoto University collaborated with teams at Osaka Dental University and Osaka Metropolitan University to image the complex at near-atomic resolution. Their analysis revealed ring-like assemblies with viral RNA nestled in an inner groove. Perhaps most striking: each nucleoprotein subunit cradles exactly eight RNA nucleotides, a binding arrangement distinct from anything seen in related viruses.
The team also made an unexpected discovery about assembly. Mutations that disrupted RNA binding crippled viral replication, yet the nucleoprotein pieces could still assemble without any RNA present. This suggests that assembly and RNA engagement are separate but coordinated processes — a finding that rewires how scientists understand these viruses putting themselves together.
Beyond filling a genuine gap in scientific knowledge, the work opens practical doors. The molecular blueprint they produced could guide future antiviral research, offering new targets for drugs designed to interrupt viral replication at its foundation.