Herpes Simplex Virus Type 1 DNA Polymerase: Eukaryotic Model Enzyme and Principal Target of Antiviral Therapy

Summary

As the major essential function of the viral DNA replication machinery, the herpes simplex virus (HSV) DNA polymerase (Pol) represents the ultimate target of many currently used antiherpetic drugs. Since development of antiviral drug resistance may occur for all of these drugs and has already become an important clinical issue, there is an apparent need for other targets which allow drugs to be designed which are to a lesser extent affected by drug resistance. Potential new targets are the recently identified novel interactions of HSV Pol and the viral replication proteins, which have been shown to be essential for viral DNA replication. This review tries to provide an insight into our present knowledge of the structural and functional analysis of HSV Pol, which has become an attractive model enzyme for studies of eukaryotic Pol. The extensive computer analysis carried out by several laboratories on the primary protein sequence of prokaryotic and eukaryotic Pol reveals that DNA-dependent and RNA-dependent Pol are similarly organized. Major replicative enzymes possess both a proofreading exonuclease and a polymerization activity, and according to the sequence comparison these catalytic functions are organized in analogous and separate protein entities, designated exonuclease and polymerization domain, as observed with the prokaryotic Escherichia coli Pol I. An updated sequence alignment indicates that amongst the cellular polymerase species, the HSV Pol is more related to the eukaryotic Pol δ, suggesting that the viral enzyme derives from an ancestor of the latter polymerase. Like this major DNA replicase, the HSV Pol has a proofreading function and is associated with an additional subunit. Furthermore, the close relationship is documented by the identical behavior to common replication inhibitors.

Genetic studies and the biochemical analysis of the HSV Pol have emphasized that the herpesviral enzyme may specify at least four distinct functions that are required for the faithful replication of herpesvirus DNA in vivo. Three of these functions can be correlated with known properties of the HSV enzyme such as polymerization activity, proofreading exonuclease, and association of a viral replication protein. The fourth function, which could correspond to a recently identified RNase H activity, is controversial.

Recent progress has been made in the analysis of the DNA template interaction of HSV Pol. Using a DNA-binding assay, it will be shown that many compounds known to inhibit the Pol activity exert a pronounced effect on the DNA-binding property of the enzyme. By employing a set of monospecific antibodies, directed against partially overlapping peptide domains of HSV-1 Pol of strain Angelotti, two antibodies which neutralized both catalytic activities of the enzyme were shown to interfere with DNA binding. The currently available information from structural modeling of E. coil Pol I and human immunodeficiency type 1 (HIV- l) reverse transcriptase, biochemical analysis, and antibody supershift experiments were combined into a working model for the HSV Pol holoenzyme. The usefulness of the identified protein-protein and protein-DNA interactions of HSV Pol and an auxiliary virus protein as possible novel targets for antiherpetic therapy are discussed.