Functional Identification and Characterization of the Nuclear Egress Complex of a Gammaherpesvirus

Abstract

Increasing amounts of knowledge indicate that the nuclear egress complex (NEC) is critical for the nuclear egress of herpesvirus capsids, which can be viewed as a vesicle-mediated transport pathway through the nuclear membrane. In this study, we identified open reading frame 67 (ORF67) and ORF69 as components of the NEC in murine gammaherpesvirus 68 (MHV-68) and demonstrated that they efficiently induce virion-like vesicles from the nuclear membrane in mammalian cells. This is the first time that the NEC of a gammaherpesvirus has been found to demonstrate such an essential characteristic. In addition, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 as well as nuclear egress. Notably, these amino acids are conserved in Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV), providing a structural basis to design antigammaherpesvirus drugs. ABSTRACT The herpesvirus nuclear egress complex (NEC) is composed of two viral proteins. They play key roles in mediating the translocation of capsids from the nucleus to the cytoplasm by facilitating the budding of capsids into the perinuclear space (PNS). The NEC of alphaherpesvirus can induce the formation of virion-like vesicles from the nuclear membrane in the absence of other viral proteins. However, whether the NEC of gammaherpesvirus harbors the ability to do so in mammalian cells remains to be determined. In this study, we first constructed open reading frame 67 (ORF67)-null and ORF69-null mutants of murine gammaherpesvirus 68 (MHV-68) and demonstrated that both ORF67 and ORF69 play critical roles in nuclear egress and hence viral lytic replication. Biochemical and bioimaging analyses showed that ORF67 and ORF69 interacted with each other and were sufficient to induce the formation of virion-like vesicles from the nuclear membrane in mammalian cells. Thus, we designated ORF67 and ORF69 components of MHV-68 NEC. Furthermore, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 through homology modeling and verified their function in nuclear egress, providing insights into the molecular basis of NEC formation in gammaherpesviruses. IMPORTANCE Increasing amounts of knowledge indicate that the nuclear egress complex (NEC) is critical for the nuclear egress of herpesvirus capsids, which can be viewed as a vesicle-mediated transport pathway through the nuclear membrane. In this study, we identified open reading frame 67 (ORF67) and ORF69 as components of the NEC in murine gammaherpesvirus 68 (MHV-68) and demonstrated that they efficiently induce virion-like vesicles from the nuclear membrane in mammalian cells. This is the first time that the NEC of a gammaherpesvirus has been found to demonstrate such an essential characteristic. In addition, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 as well as nuclear egress. Notably, these amino acids are conserved in Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV), providing a structural basis to design antigammaherpesvirus drugs.