Tiffany Russell, Ben Bleasdale, Michael Hollinshead, Gillian Elliott

Despite differences in the pathogenesis and host range of alphaherpesviruses, many stages of their morphogenesis are thought to be conserved. Here, an ultrastructural study of bovine herpesvirus 1 (BoHV-1) envelopment revealed similar profiles to those previously found for HSV-1, with BoHV-1 capsids associating with endocytic tubules. Consistent with the similarity of their genomes and envelopment strategies, the proteomic composition of BoHV-1 and HSV-1 virions was also comparable. However, BoHV-1 morphogenesis exhibited a diversity in envelopment events. First, heterogeneous primary envelopment profiles were readily detectable at the inner nuclear membrane of BoHV-1 infected cells. Second, the BoHV-1 progeny comprised not just full virions, but also an abundance of capsidless, non-infectious light (L)-particles that were released from the infected cell in similar numbers to virions, and in the absence of DNA replication. Proteomic analysis of BoHV-1 L-particles and the much less abundant HSV-1 L-particles revealed that they contained the same complement of envelope proteins as virions but showed variations in tegument content. In the case of HSV-1, the UL46 tegument protein was reproducibly found to be more than 6-fold enriched in HSV-1 L-particles. More strikingly, the tegument proteins UL36, UL37, UL21 and UL16 were depleted in BoHV-1 but not HSV-1 L-particles. We propose that these combined differences reflect the presence of a truly segregated "inner" and "outer" tegument in BoHV-1, making it a critical system for studying the structure and process of tegumentation and envelopment.IMPORTANCE The alphaherpesvirus family includes viruses that infect humans and animals. Hence, not only do they have a significant impact on human health but also a substantial economic impact on the farming industry. While the pathogenic manifestations of the individual viruses differ from host to host, their relative genetic composition suggests similarity at the molecular level. This study provides a side-by-side comparison of the particle output from the major human pathogen HSV-1 and the veterinary pathogen BoHV-1. Ultrastructural and proteomic analyses have revealed that both viruses have broadly similar morphogenesis profiles and infectious virus composition. However, the demonstration that BoHV-1 has the capacity to generate vast numbers of capsidless enveloped particles that differ to those produced by HSV-1 in composition, imply a divergence in the cell biology of these viruses that impacts on our general understanding of alphaherpesvirus morphogenesis.