Visualizing Viral Transport Through the Nuclear Pore by Cryo-Electron Microscopy and Cryo-Electron Tomography.

The nuclear envelope protects the host genome, yet many viruses must breach this barrier to access nuclear replication machinery. The nuclear pore complex (NPC), the sole gateway for nucleocytoplasmic transport, is therefore a central target for viral nuclear entry. For decades, the mechanisms by which large viral genomes and subviral assemblies traverse this selective channel remained unclear. This was due to limited accessibility to intact nuclear pores, the NPC's massive architecture, and the transient nature of viral nuclear entry. The advent of cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) has transformed this landscape, enabling visualization of NPC architecture and virus-NPC interactions at unprecedented detail. This review summarizes structural insights into NPC architecture and the strategies viruses employ to enter the nucleus. We examine how viruses engage canonical import pathways involving importins and phenylalanine-glycine-nucleoporins, as well as noncanonical mechanisms by which viral components mimic karyopherin. Finally, we offer perspectives on cryo-EM/cryo-ET capturing viral complexes during the entry process, revealing mechanisms of host-machinery exploitation by viruses.