Crystal structure of varicella-zoster virus protease

Abstract

Members of the human herpes virus family are responsible for a variety of diseases from subclinical infections to fatal diseases in the immunocompromised or immunosuppressed. The family is divided into three subfamilies designated α, β, and γ. The α subfamily includes herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) and varicella-zoster virus (VZV); the β subfamily includes cytomegalovirus (CMV) and human herpes viruses 6 and 7; and the γ subfamily includes Epstein–Barr virus and human herpes virus 8. Viruses of the α subfamily are among those causing serious diseases. HSV-1 is the virus responsible for herpes labialis (cold sores), whereas HSV-2 causes genital herpes. VZV is a neurotropic α-herpes virus responsible for chickenpox, shingles, and postherpetic neuralgia: primary exposure to the virus results in chickenpox, reactivation of the virus after a period of latency gives rise to shingles, and postherpetic neuralgia is probably the result of nerve damage during the active replication phase of shingles (1).

An essential step in herpes virus assembly (2) is the proteolytic processing of an assemblin protein designated ICP35 in HSV-1 (3). Processing of the assemblin protein is catalyzed by a virally encoded serine protease that contains the assemblin protein at its C terminus (3). This protease catalyzes its own cleavage to produce an N-terminal domain having full catalytic activity (4, 5). Herpes protease domains show significant sequence homology within each subfamily, but only very limited homology between different subfamilies (Fig. ​(Fig.1,1, Table ​Table1).1). For example, the VZV protease shows 50% identity to HSV-1 and HSV-2 proteases, but only 26% to CMV protease. There is little sequence homology to other known proteins, including the absence of the conserved G-X-S/C-G-G sequence for chymotrypsin-like and G-T-S-M/A for subtilisin-like proteases. All human herpes virus proteases cleave a peptide bond between an alanine and a serine (6). Differences in substrate specificity exist. For example, HSV-1 protease cannot cleave the protein substrate of CMV protease, but the CMV protease can cleave that of HSV-1 protease (7).

Figure 1

The structure-assisted alignment of human herpes virus proteases. The secondary structure elements of CMV and VZV protease are underlined and labeled. Helical (AA-A7) regions (blue), strands (B1-B7) (red), and the conserved catalytic triad (green). CMV numbering is used.

Recently the crystal structure of CMV protease has been reported (8–11). The structure reveals a new fold that has not been reported for any other serine protease, and an active site consisting of a novel catalytic triad in which the third member of the triad is a histidine instead of an aspartic acid. The structure also suggests a catalytic tetrad composed of a serine, two histidines, and an aspartic acid. The limited sequence homology with CMV protease precluded detailed modeling of the VZV protease structure. Here we report the crystal structure of the serine protease from VZV, the first structure of the serine protease from an α-herpes virus. Comparison of the VZV and CMV protease structures should facilitate better understanding of the substrate specificity and catalytic mechanism of herpes virus proteases, and provide a structural basis for the rational design of antiviral agents.

Bold, from Fig. ​Fig.1;1; italic, from GCG, slightly different because  structure information was not used in sequence alignments.

Acknowledgments

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