This study provides evidence that proteasomal activity is required at multiple steps in human cytomegalovirus replication. Electron microscopy revealed that no viral particles were assembled in the presence of proteasome inhibitor MG132. Immunofluorescence and Western blot analyses using MG132 demonstrated that immediate early gene expression was suppressed at low but not high MOI. In contrast, expression of late proteins was completely blocked independent of MOI. Additionally, pulsed-field gel electrophoresis demonstrated that MG132 interferes with cleavage of HCMV DNA. Bromodeoxyuridine incorporation studies showed that de novo viral DNA synthesis is reduced in the presence of MG132. Furthermore, in contrast to previous hypotheses we demonstrated that neither the ND10 components PML and hDaxx nor NFjB activation represent the target for MG132.

DNA packaging is the key step in viral maturation and involves binding and cleavage of viral DNA containing specific DNA-packaging motifs. This process is mediated by a group of specific enzymes called terminases. We previously demonstrated that the human cytomegalovirus (HCMV) terminase is composed of the large subunit pUL56 and the small subunit pUL89. While the large subunit mediates sequence-specific DNA binding and ATP hydrolysis, pUL89 is required only for duplex nicking. An excellent inhibitor targeting HCMV terminase is 2-bromo-5,6-dichloro-1-(beta-d-ribofuranosyl)benzimidazole (BDCRB), but it was not developed as an antiviral drug due to its metabolic cleavage in experimental animals. We now have tested several new benzimidazole d-ribonucleosides in order to determine whether these compounds represent new, potent inhibitors. Analysis by bioluminometric ATPase activity assays identified two of the new compounds with a high inhibitory effect, 2-bromo-4,5,6-trichloro-1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl) benzimidazole (BTCRB) and 2,4,5,6-tetrachloro-1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl benzimidazole (Cl(4)RB). By using viral plaque formation, viral yield, and viral growth kinetics, we demonstrated that the two compounds BTCRB and Cl(4)RB had antiviral activities similar to that of BDCRB. Interestingly, BTCRB retained its inhibitory activity after preincubation with HFF cells. By use of electron microscopy, we observed an increase of B capsids and a lack of cytoplasmic capsids in the presence of the compounds that correlated with the virus yield. Furthermore, cleavage of concatenated DNA was inhibited by both compounds, and inhibition by BTCRB was shown to be dose dependent. These results demonstrate that the new compounds are highly active against HCMV and act by mechanisms similar but not identical to those of BDCRB.

Recently we characterized two inhibitors targeting the human cytomegalovirus (HCMV) terminase, 2-bromo-4,5,6-trichloro-1-(2,3,5-tri-O-acetyl-␤-D-ribofuranosyl) benzimidazole (BTCRB) and 2,4,5,6-tetrachloro-1-(2,3,5-tri-O-acetyl-␤-D-ribofuranosyl) benzimidazole (Cl 4 RB). The terminase consists of the ATP-hydrolyzing subunit pUL56 and the subunit pUL89 required for duplex nicking. Because mammalian cell DNA replication does not involve cleavage of concatemeric DNA by a terminase, these compounds represent attractive alternative HCMV antivirals. We now have tested these previously identified benzimidazole ribonucleosides in order to determine if they are active against HCMV clinical isolates as well as those of herpes simplex virus type 1, mouse cytomegalovirus, rat cytomegalovirus (RCMV), and varicella-zoster virus (VZV). Antiviral activity was quantified by measurement of viral plaque formation (plaque reduction) and by viral growth kinetics. Interestingly, both BTCRB and Cl 4 RB had an inhibitory effect in ganciclovir (GCV)-sensitive and GCV-resistant clinical isolates, with the best effect produced by Cl 4 RB. Electron microscopy revealed that in cells infected with GCV-sensitive or GCV-resistant isolates, B capsids and dense bodies were formed mainly. Furthermore, pulsed-field gel electrophoresis showed that cleavage of concatenated DNA was inhibited in clinical isolates. In addition, the antiviral effect on other herpesviruses was determined. Interestingly, in plaque reduction assays, BTCRB was active against all tested herpesviruses. The best effects were observed on VZV-and RCMV-infected cells. These results demonstrate that the new compounds are highly active against GCV-resistant and GCVsensitive clinical isolates and slightly active against other herpesviruses.Human cytomegalovirus (HCMV) is one of eight human herpesviruses and is a serious, life-threatening, opportunistic pathogen in immunocompromised patients (organ recipients or AIDS patients) (8,19). HCMV is widespread, with a seroprevalence throughout the world of up to 100% in adults. To date, nearly all anti-HCMV drugs for systemic treatment are inhibitors of the viral DNA polymerase (2,13,21,22). Due to the low bioavailability, number of side effects, dose-dependent toxicity, and appearance of resistances caused by the current available drugs, development of new antiviral compounds which have a different mode of action is needed. Consequently, to broaden therapy of HCMV infections and to circumvent current mechanisms of drug resistance, an inhibitor of HCMV terminase would be of great value, because it would act subsequently to DNA synthesis and block the first steps in viral maturation.Viral replication includes cleavage of newly synthesized, concatemeric DNA into unit-length genomes and packaging into preformed procaspids. These processes occur in or in close proximity to replication centers in the nucleus. Enzymes involved in the packaging process are responsible for duplex nicking and insertion of the DNA into the procapsids (1, 4, 9, 10), t...

a b s t r a c tIn this study we used the fungal antibiotic brefeldin A (BFA) to analyze its effect on viral replication. Analysis by electron microscopy demonstrated that no viral particles were observed in cells treated before the onset of viral replication. In the presence of BFA expression of IE2, MCP, pUL104, pUL56 and pUL89 were reduced, while no or slight effect was observed on expression of pp65, pUL44 and pUL57. Strikingly, real time PCR revealed that de novo viral DNA synthesis is reduced but not completely abolished in the presence of BFA. These results indicated that BFA represents a multi-functional compound leading to inhibition of several steps of viral maturation such as expression of viral DNA packaging proteins and capsid formation.