The Novel Nuclear Targeting and BFRF1-Interacting Domains of BFLF2 Are Essential for Efficient Epstein-Barr Virus Virion Release.

Epstein-Barr virus (EBV) genomic DNA is replicated and packaged into procapsids in the nucleus to form nucleocapsids, which are then transported into the cytoplasm for tegumentation and final maturation. The process is facilitated by the coordination of the viral nuclear egress complex (NEC), which consists of BFLF2 and BFRF1. By expression alone, BFLF2 is distributed mainly in the nucleus. However, it colocalizes with BFRF1 at the nuclear rim and in cytoplasmic nuclear envelope-derived vesicles in coexpressing cells, suggesting temporal control of the interaction between BFLF2 and BFRF1 is critical for their proper function. The N-terminal sequence of BFLF2 is less conserved than that of alpha- and betaherpesvirus homologs. Here, we found that BFLF2 amino acids (aa) 2 to 102 are required for both nuclear targeting and its interaction with BFRF1. Coimmunoprecipitation and confocal analysis indicated that aa 82 to 106 of BFLF2 are important for its interaction with BFRF1. Three crucial amino acids (R47, K50, and R52) and several noncontinuous arginine and histidine residues within aa 59 to 80 function together as a noncanonical nuclear localization signal (NLS), which can be transferred onto yellow fluorescent protein (YFP)-LacZ for nuclear targeting in an importin ß-dependent manner. Virion secretion is defective in 293 cells harboring a BFLF2 knockout EBV bacmid upon lytic induction and is restored by trans-complementation of wild-type BFLF2, but not NLS or BFRF1-interacting defective mutants. In addition, multiple domains of BFRF1 were found to bind BFLF2, suggesting multiple contact regions within BFRF1 and BFLF2 are required for proper nuclear egress of EBV nucleocapsids.IMPORTANCE Although Epstein-Barr virus (EBV) BFRF1 and BFLF2 are homologs of conserved viral nuclear egress complex (NEC) in all human herpesviruses, unique amino acid sequences and functions were identified in both proteins. In this study, the nuclear targeting and BFRF1-interacting domains were found within the N terminus of BFLF2. We showed that amino acids (aa) 82 to 106 are the major region required for BFLF2 to interact with BFRF1. However, the coimmunoprecipitation (Co-IP) data and glutathione transferase (GST) pulldown experiments revealed that multiple regions of both proteins contribute to reciprocal interactions. Different from the canonical nuclear localization signal (NLS) in other herpes viral homologs, BFLF2 contains a novel importin-dependent nuclear localization signal, including R47, K50, and R52 and several neighboring discontinuous arginine and histidine residues. Using a bacmid complementation system, we show that both the nuclear targeting and the novel nuclear localization signal within aa 82 to 106 of BFLF2 are required for virion secretion.

The Ubiquitin Ligase Itch and Ubiquitination Regulate BFRF1-Mediated Nuclear Envelope Modification for Epstein-Barr Virus Maturation.

UNLABELLED: The cellular endosomal sorting complex required for transport (ESCRT) was recently found to mediate important morphogenesis processes at the nuclear envelope (NE). We previously showed that the Epstein-Barr virus (EBV) BFRF1 protein recruits the ESCRT-associated protein Alix to modulate NE structure and promote EBV nuclear egress. Here, we uncover new cellular factors and mechanisms involved in this process. BFRF1-induced NE vesicles are similar to those observed following EBV reactivation. BFRF1 is ubiquitinated, and elimination of possible ubiquitination by either lysine mutations or fusion of a deubiquitinase hampers NE-derived vesicle formation and virus maturation. While it interacts with multiple Nedd4-like ubiquitin ligases, BFRF1 preferentially binds Itch ligase. We show that Itch associates with Alix and BFRF1 and is required for BFRF1-induced NE vesicle formation. Our data demonstrate that Itch, ubiquitin, and Alix control the BFRF1-mediated modulation of the NE and EBV maturation, uncovering novel regulatory mechanisms of nuclear egress of viral nucleocapsids. IMPORTANCE: The nuclear envelope (NE) of eukaryotic cells not only serves as a transverse scaffold for cellular processes, but also as a natural barrier for most DNA viruses that assemble their nucleocapsids in the nucleus. Previously, we showed that the cellular endosomal sorting complex required for transport (ESCRT) machinery is required for the nuclear egress of EBV. Here, we further report the molecular interplay among viral BFRF1, the ESCRT adaptor Alix, and the ubiquitin ligase Itch. We found that BFRF1-induced NE vesicles are similar to those observed following EBV reactivation. The lysine residues and the ubiquitination of BFRF1 regulate the formation of BFRF1-induced NE-derived vesicles and EBV maturation. During the process, a ubiquitin ligase, Itch, preferably associates with BFRF1 and is required for BFRF1-induced NE vesicle formation. Therefore, our data indicate that Itch, ubiquitin, and Alix control the BFRF1-mediated modulation of the NE, suggesting novel regulatory mechanisms for ESCRT-mediated NE modulation.