Translocation of the papillomavirus L2/vDNA complex across the limiting membrane requires the onset of mitosis

The human papillomavirus type 16 (HPV16) L2 protein acts as a chaperone to ensure that the viral genome (vDNA) traffics from endosomes to the trans-Golgi network (TGN) and eventually the nucleus, where HPV replication occurs. En route to the nucleus, the L2/vDNA complex must translocate across limiting intracellular membranes. The details of this critical process remain poorly characterized. We have developed a system based on subcellular compartmentalization of the enzyme BirA and its cognate substrate to detect membrane translocation of L2-BirA from incoming virions. We find that L2 translocation requires transport to the TGN and is strictly dependent on entry into mitosis, coinciding with mitotic entry in synchronized cells. Cell cycle arrest causes retention of L2/vDNA at the TGN; only release and progression past G2/M enables translocation across the limiting membrane and subsequent infection. Microscopy of EdU-labeled vDNA reveals a rapid and dramatic shift in vDNA localization during early mitosis. At late G2/early prophase vDNA egresses from the TGN to a pericentriolar location, accumulating there through prometaphase where it begins to associate with condensed chromosomes. By metaphase and throughout anaphase the vDNA is seen bound to the mitotic chromosomes, ensuring distribution into both daughter nuclei. Mutations in a newly defined chromatin binding region of L2 potently blocked translocation, suggesting that translocation is dependent on chromatin binding during prometaphase. This represents the first time a virus has been shown to functionally couple the penetration of limiting membranes to cellular mitosis, explaining in part the tropism of HPV for mitotic basal keratinocytes.
Author summary Human papillomaviruses (HPVs) are the most prevalent sexually transmitted infections and are responsible for about 5% of total human cancers worldwide. These small DNA viruses must enter cells and deliver their genomes to the host cell nucleus to successfully establish an infection. A major barrier that HPVs and other non-enveloped viruses must overcome is the limiting membrane—the host cellular membrane through which the viral genome must be transported en route to the nucleus. Here we have developed a novel enzyme-dependent assay to study what is called penetration or translocation of the limiting membrane by the HPV16 minor capsid protein L2. We find that translocation is strictly dependent on host cell division, specifically progression into mitosis, and occurs post trans-Golgi-network localization. After exiting the trans-Golgi network the viral genomes are seen decorating the segregating mitotic chromosomes, to ensure the equal partitioning into and infection of the daughter cells. The remarkable timing with, and absolute dependence of, this event on mitosis likely dictates the natural tropism of these viruses for the replicating basal keratinocytes of differentiating epithelial tissues. Further work will be necessary to fully understand the molecular mechanisms of this extraordinary mitosis-dependent phenomenon.