ESCRT-III is recruited by human herpesvirus 6A nuclear egress complex to promote nuclear egress of the nucleocapsid.

Herpesviruses replicate their genomes and package them into capsids within the host cell nucleus. These capsids must then translocate from the nucleus to the cytoplasm through a process designated nuclear egress. The virus-encoded nuclear egress complex (NEC), consisting of a nuclear matrix protein and a nuclear membrane protein, plays a crucial role in this process. Although the role of NEC for nucleo-cytoplasmic transport of capsids is conserved in Herpesviridae, some of the binding partners of the NEC components are specific for individual viruses. The NECs of alpha- and gammaherpesviruses recruit the Endosomal Sorting Complex Required for Transport III (ESCRT-III) to the inner nuclear membrane for efficient nuclear egress of capsids. In contrast, the role of ESCRT-III for nuclear egress of betaherpesviruses, including human cytomegalovirus (HCMV) and human herpesvirus 6A (HHV-6A), has not been elucidated. Here, we show that ESCRT-III is recruited to the nuclear rim in cells expressing the NEC of HCMV or of HHV-6A. Inhibition of ESCRT-III impaired HHV-6A replication and nuclear egress of the capsids. Mechanistically, ESCRT-III adaptor ALIX interacts with HHV-6A NEC membrane protein U34 and thus contributes to HHV-6A replication. From these observations, we conclude that, like at least some alpha- and gammaherpesviruses, HHV-6A NEC recruits ESCRT-III through ALIX to promote viral capsid nuclear egress.IMPORTANCEESCRT-III performs reverse-topology scission involved in many diverse cellular processes, including cytokinesis, endosome maturation, autophagy, membrane repair, and viral budding. Nucleo-cytoplasmic transport of herpesvirus capsids requires scission at the inner nuclear membrane. In alpha- and gammaherpesviruses, this process requires ESCRT-III, but it is not known whether this is also the case for betaherpesviruses. Here, we show that ESCRT-III is also important for nuclear egress of capsids of the betaherpesvirus human herpesvirus 6A. These results imply that ESCRT-III-mediated inner nuclear membrane scission is a conserved feature in the virion maturation process of Herpesviridae. Our findings thus suggest that ESCRT-III is a potential therapeutic target also for betaherpesvirus infections.