Human cytomegalovirus forms phase-separated compartments at viral genomes to facilitate viral replication

Herpesvirus Replication Compartments: Dynamic Biomolecular Condensates?

“…Concerning lytic herpesvirus replication and bona fide RCs, we recently found that the HCMV proteins UL112-113 undergo LLPS to create a pro-replicative environment around incoming genomes to ensure their replication [ 42 ]. Newly formed pre-replication compartments (pre-RCs) marked by UL112-113 appear spherical and recover sphericity after fusion, indicating surface tension.…”

“…At early times post-infection, they also exhibit fast fluorescence recovery after photobleaching (FRAP) within seconds, indicating fluidity. Moreover, they are highly sensitive to inhibitors of weak molecular interactions, such as 1,6-Hexanediol or 1,2-Propanediol, indicating that condensate formation is mediated by weak interaction of the UL112-113 IDRs [ 42 ]. Clustering the UL112-113 IDRs is sufficient for the formation of liquid condensates.…”

“…It is, therefore, likely that UL112-113 acts as a scaffold forming pre-RCs. Importantly, the essential viral polymerase processivity factor UL44 cannot form condensates on its own but is recruited as a client into pre-RCs by its IDR, thereby ensuring that replication factors are concentrated at viral genomes [ 42 ]. In contrast, a study aimed at analyzing why RNA Polymerase II (RNApolII) is sequestered out of cellular chromatin and enriched in HSV-1 RCs came to seemingly different conclusions [ 43 ].…”

“…Although pre-RCs are likely homogenous biomolecular condensates, their properties change with ongoing viral infection. We found that HCMV RCs lose their spherical shape around 30 hpi, indicating that intra-RC interactions overcome surface tension at this point [ 42 ] ( Figure 1 C). Moreover, RCs expand dramatically during infection, at the end filling almost the complete host nucleus.…”

“…Mature RCs are much more resistant to inhibitors of weak interaction, such as 1,6-Hexanediol or 1,2-Propanediol, indicating that stronger molecular interactions stabilize the condensate. Moreover, mature RCs, show less motility and slower recovery after photobleaching and a lower fraction of mobile molecules [ 42 ]. Importantly, PAA, an inhibitor of viral DNA replication, abrogates condensate maturation and reverses most of the changes, indicating that viral genomes likely play a role in the maturation of RCs as a condensate [ 42 ].…”