Structural mechanism of Borna disease virus 1 RNA polymerase autoinhibition and suramin-mediated inhibition

Borna disease virus 1 (BoDV-1) is a neurotropic pathogen that causes severe, often fatal encephalitis, yet effective treatments remain unavailable. As a nuclear-replicating mononegavirus, BoDV-1 employs a minimal L-P polymerase complex. Here, we report cryo-electron microscopy (cryo-EM) structures of the BoDV-1 polymerase in L-alone, apo-L-P, and inhibitor-bound L-P states, revealing the most compact L protein characterized among mononegaviruses. While the catalytic core is conserved, the C-terminal domains are degenerate, with the methyltransferase-like (MTase-like) domain lacking canonical functional motifs. We identify an N-terminal autoinhibitory element (AIE) that is positioned to physically block the template entry tunnel, suggesting an autoinhibition mechanism reminiscent of a "molecular plug." Furthermore, we demonstrate that the inhibitor suramin binds in a specific triple-molecule mode, potentially achieving inhibition by sterically occluding RNA access and allosterically restricting the catalytic core. These findings elucidate the architecture and regulation of the BoDV-1 polymerase, providing a structural framework for rational antiviral design.