UL88 Mediates the Incorporation of a Subset of Proteins into the Virion Tegument

Abstract

Little is known about the human cytomegalovirus (HCMV) tegument protein UL88. Large-scale genomic studies have reported disparate results for UL88-null viruses, reporting both no phenotype and a >1-log decrease in virus titers. UL88 has also been reported to interact with UL69 and UL48, but the functional relevance of this interaction is unknown. Here, we report that UL88, which is conserved among different viral strains, is dispensable for production of infectious HCMV virions in multiple HCMV strains and cell types. However, the specific infectivity of HCMV virions suffers in the absence of UL88, as more genomes are required per PFU. This may be a result of altered virion tegument protein composition, as Western blot analysis shows a significant reduction in the tegument levels of pp71, UL47, and UL48 in viruses lacking UL88. While an interaction between UL88 and UL48 has previously been reported, we show that UL88 can interact with UL47; however, UL88 does not appear to be part of a stable complex consisting of UL47 and UL48. These findings identify an important role for UL88 in incorporating the viral proteins UL47 and UL48 into the virion tegument layer. IMPORTANCE A better understanding of the role and functions of tegument proteins in HCMV, many of which remain uncharacterized, will contribute to our understanding of the biology of HCMV. The virus has a large genome, greater than 230 kb, and functional annotation of these genes is important for identifying novel targets for improving therapeutic intervention. This study identifies a role for a viral tegument protein with unknown function, UL88, in maintaining the proper tegument composition of HCMV virions. Virions produced in the absence of UL88 exhibit decreased fitness and require more genomes per infectious unit.

UL49 is an essential subunit of the viral pre-initiation complex that regulates human cytomegalovirus gene transcription

“…Some tegument genes were also found to be vPIC-regulated, including the capsid-associated UL32 ( Yu et al., 2017 ), the early acting virion cargo tegument UL48 ( Kwon et al., 2017 ; Wang et al., 2006 ), and envelopment and egress-associated UL99 and UL103 ( Figure 5 C) ( Ahlqvist and Mocarski, 2011 ; Sanchez et al., 2000 ; Seo and Britt, 2006 ; Silva et al., 2003 ). In contrast, we found that tegument genes UL25 ( Battista et al., 1999 ), UL26 ( Mathers et al., 2014 ; Munger et al., 2006 ), UL83 ( Kalejta, 2008 ), and UL88 ( Kumar et al., 2020 ) were not regulated by the vPIC ( Figure S2 D). Moreover, UL26, UL83, UL86/MCP, and UL99 exhibit similar late expression kinetics over the course of infection ( Weekes et al., 2014 ), but only UL86 and UL99 are regulated by the vPIC ( Figures 2 B, 3 A-3G, 5 C and S2 D).…”

“…A similar analysis of genes in cluster III revealed that 29% are classified as late (class 5) ( Figures 5 E and S2 D), and only 36% of genes encode virion cargo ( Figures 5 F and S2 D). Notable examples of late genes that are not regulated by the vPIC include UL25 ( Battista et al., 1999 ), UL26 ( Mathers et al., 2014 ; Munger et al., 2006 ), the multifunctional UL83 ( Kalejta, 2008 ) and UL88 ( Kumar et al., 2020 ) ( Figure S2 D).…”

UL34 Deletion Restricts Human Cytomegalovirus Capsid Formation and Maturation

“…Cluster VI was enriched with non-essential tegument proteins with class five expression (late) and high virion enrichment. These included UL25 [ 42 ], UL82 [ 43 ], UL83 [ 13 ], and UL88 [ 44 ] ( Figure 1 A). Clusters IV and V grouped augmenting gene products with a diverse set of known functions ( Figure 1 A).…”

Spatial, Quantitative and Functional Deconstruction of Virus and Host Protein Interactions Inside Intact Cytomegalovirus Particles

“…Only the smallest capsid protein UL48A, for which we detected relatively few cross-links, co-clusters with the tegument group of cluster 1. Besides UL48A, this subcluster contained typical inner tegument proteins like UL32 and UL48, that are characterized by their tight association to the nucleocapsid 3 as well as the UL48-associated protein UL47 27 ,UL82 (also known as pp71), the UL82 interactors UL35 28 and UL88, which are important for recruiting UL47, UL48 and UL82 into virions 29 . As UL82 and UL35 proteins traffic independently from the nucleocapsid upon infection 30, 31 , their allocation to the nucleocapsid-proximal tegument layer indicates that proximity to the nucleocapsid does not necessarily reflect tight attachment upon cell entry.…”