Primary envelopment of Kaposi's sarcoma-associated herpesvirus at the nucleoplasmic reticulum.

Herpesvirus egress begins with primary envelopment of newly assembled capsids at the inner nuclear membrane (INM). Primary envelopment has been observed at the peripheral INM as well as nuclear infoldings. Nuclear infoldings from invaginations of the INM are Type-I nucleoplasmic reticulum (NR), whereas infoldings of both INM and outer nuclear membrane (ONM) are Type-II NR. Here, we report that Kaposi's sarcoma-associated herpesvirus (KSHV) reactivation from latency and lytic cycle progression correlates with increases in both types of NR, but primary envelopment is restricted to peripheral INM and Type-I NR. These Type-I NR structures co-localized with puncta containing CTP:phosphocholine cytidylyltransferase (CCTα), the enzyme that catalyzes the rate-limiting step in phosphatidylcholine (PtdCho) synthesis that drives the IMPORTANCE: Herpesvirus capsids assemble in the cell nucleus but are too large to exit via nuclear pores. Instead, they bud into the inner nuclear membrane to acquire a provisional lipid envelope that is shed through fusion with the outer nuclear membrane, delivering the capsid to the cytoplasm for subsequent steps in assembly and egress. These nuclear membranes are dynamic, with the ability to fold into invaginations that access the nuclear interior. Here, we demonstrate that during Kaposi's sarcoma-associated herpesvirus (KSHV) replication, nuclear membrane infolding increases, coinciding with recruitment of a host enzyme required for PtdCho synthesis at these sites. We observed accumulation of KSHV capsids at infoldings of the inner nuclear membrane and tracked the association and trafficking of fluorescent viral particles through these structures by live cell microscopy. This complements a more well-established mechanism of KSHV egress at the nuclear periphery and suggests versatility in nuclear egress mechanisms.