Inhibition of Human Cytomegalovirus DNA Polymerase by C-Terminal Peptides from the UL54 Subunit

Abstract

In common with other herpesviruses, the human cytomegalovirus (HCMV) DNA polymerase contains a catalytic subunit (Pol or UL54) and an accessory protein (UL44) that is thought to increase the processivity of the enzyme. The observation that antisense inhibition of UL44 synthesis in HCMV-infected cells strongly inhibits viral DNA replication, together with the structural similarity predicted for the herpesvirus processivity subunits, highlights the importance of the accessory protein for virus growth and raises the possibility that the UL54/UL44 interaction might be a valid target for antiviral drugs. To investigate this possibility, overlapping peptides spanning residues 1161 to 1242 of UL54 were synthesized and tested for inhibition of the interaction between purified UL54 and UL44 proteins. A peptide, LPRRLHLEPAFLPYSVKAHECC, corresponding to residues 1221 to 1242 at the very C terminus of UL54, disrupted both the physical interaction between the two proteins and specifically inhibited the stimulation of UL54 by UL44. A mutant peptide lacking the two carboxy-terminal cysteines was markedly less inhibitory, suggesting a role for these residues in the UL54/UL44 interaction. Circular dichroism spectroscopy indicated that the UL54 C-terminal peptide can adopt a partially ␣-helical structure. Taken together, these results indicate that the two subunits of HCMV DNA polymerase most likely interact in a way which is analogous to that of the two subunits of herpes simplex virus DNA polymerase, even though there is no sequence homology in the binding site, and suggest that the UL54 peptide, or derivatives thereof, could form the basis for developing a new class of anti-HCMV inhibitors that act by disrupting the UL54/UL44 interaction.Human cytomegalovirus (HCMV) is a human pathogen responsible for a variety of severe diseases in immunocompromised patients, including pneumonia, gastrointestinal disease, and retinitis in transplant recipients and in patients with AIDS (7). HCMV is also a major cause of congenital defects in newborn children (51). Antiviral agents currently licensed for the treatment of HCMV infections include ganciclovir, foscarnet, and cidofovir, which all inhibit the HCMV DNA polymerase (21). Ganciclovir and cidofovir are nucleoside analogs which function as DNA chain terminators, whereas foscarnet inhibits viral DNA polymerase through binding to its pyrophosphate binding site (12). There is renewed interest in the search for new HCMV inhibitors because of the emergence of drug-resistant viral strains, particularly in immunocompromised patients (42), and because some of these antiviral agents, e.g., ganciclovir and foscarnet, have toxic side effects. Since the HCMV DNA polymerase represents a major target for antiviral chemotherapy, further studies on this enzyme could be important in developing novel drugs.HCMV DNA replication has not been as fully characterized as that of herpes simplex virus (HSV); however, HCMV homologs to six of the seven proteins essential for HSV type 1 (HSV-1) replication (53) have...

Crystal Structure of the Cytomegalovirus DNA Polymerase Subunit UL44 in Complex with the C Terminus from the Catalytic Subunit

“…Thus, this UL54 peptide is less ␣-helical than the analogous peptide derived from the HSV catalytic subunit, UL30 (16). This observation is consistent with circular dichroism studies demonstrating that the C-terminal peptide of HCMV UL54 is less ␣-helical in solution than similar peptides from HSV UL30 (29,30). The UL54 peptide makes several interactions with regions on the front face of UL44, which are detailed below.…”

“…The interaction between UL44 and UL54 would make an attractive drug target because it is specific and differs in important details from the PCNA-binding partner interactions and because both proteins are essential for viral replication (3,4,55). The interaction is known to be inhibited by peptides corresponding to the C-terminal 22 residues of UL54 (29), and the interaction has been shown to be sensitive to amino acid substitutions in either of the two subunits (12,19). A similar strategy has been utilized for their counterparts in HSV (30,31,56), and small molecules have been identified that block both the HSV UL42-UL30 interaction in vitro and viral replication (57).…”

“…Moreover, as is the case with HSV UL30, the C terminus of UL54 is both necessary and sufficient for binding to UL44 (12). This region displays some ␣-helical propensity in solution (29), although this tendency is less than that of corresponding peptides from the C terminus of HSV UL30 (30). However, unlike the HSV UL42-UL30 interaction, which is dominated by polar contacts (16,31), the strength of the UL44-UL54 interaction appears most dependent upon several specific hydrophobic residues (12,19).…”

Residues of Human Cytomegalovirus DNA Polymerase Catalytic Subunit UL54 That Are Necessary and Sufficient for Interaction with the Accessory Protein UL44

“…Recently, it was reported that a peptide corresponding to the C-terminal 22 residues of UL54 could both disrupt the physical interaction between UL54 and UL44 and specifically inhibit the stimulation of UL54 activity by the accessory protein (17). These findings suggested, but did not demonstrate, that UL54 interacts via its extreme C terminus with UL44.…”

“…The observations that both UL54 and UL44 are essential for HCMV DNA replication (21)(22)(23) and that the UL54-UL44 interaction is specific (17) make this interaction an attractive drug target. Recently, it was reported that a peptide corresponding to the C-terminal 22 residues of UL54 could both disrupt the physical interaction between UL54 and UL44 and specifically inhibit the stimulation of UL54 activity by the accessory protein (17).…”