Structure and mechanism of the HSV-1 origin-binding protein UL9

The herpesvirus DNA replication machinery comprises a battery of viral enzymes that orchestrate viral genome synthesis. In herpes simplex virus type 1 (HSV-1), the machinery consists of seven essential components, including the origin-binding protein UL9, the single-stranded DNA (ssDNA)-binding protein ICP8, the heterodimeric DNA polymerase complex UL30-UL42, and the heterotrimeric helicase-primase complex UL5-UL8-UL52. UL9, a superfamily 2 (SF2) helicase, functions as a dimer that specifically recognizes replication origins and unwinds duplex DNA to initiate replication. Furthermore, UL9 recruits the replication machinery through interactions with viral components and engages cellular proteins that regulate its function. However, the molecular mechanisms underlying the multifunctionality of UL9 remain incompletely understood due to the lack of structural information. Here, we present cryo-electron microscopy structures of UL9 in both apo and DNA-bound states. Together with biochemical and enzymatic assays, we elucidate the molecular basis of UL9 dimerization, origin recognition and allosteric regulation by ICP8.IMPORTANCEHerpes simplex virus 1 (HSV-1) is a widespread virus that causes lifelong infections, leading to periodic outbreaks ranging from common cold sores to life-threatening encephalitis, and no current treatment can eradicate the dormant virus. To multiply, HSV-1 relies on a protein-based molecular machine to replicate its genome, where the unwinding of double-stranded DNA at specific replication origins is coordinated by the viral origin-binding protein UL9. Here, we present the high-resolution structures of UL9, both alone and bound to DNA, revealing how it forms a stable homodimer to grab onto the origin. Combined with precise biochemical experiments, we further show how UL9 collaborates with another viral helper protein, ICP8, to unwind DNA efficiently. These discoveries solve a long-standing puzzle in herpesvirus biology and offer a vital structural blueprint for designing new antiviral drugs that can block viral replication at its very earliest stage.