Cytomegalovirus assemblin: the amino and carboxyl domains of the proteinase form active enzyme when separately cloned and coexpressed in eukaryotic cells

Enzymatic activities of human cytomegalovirus maturational protease assemblin and its precursor (pPR, pUL80a) are comparable: [corrected] maximal activity of pPR requires self-interaction through its scaffolding domain

Herpesviruses encode an essential, maturational serine protease whose catalytic domain, assemblin (28 kDa), is released by self-cleavage from a 74-kDa precursor (pPR, pUL80a). Although there is considerable information about the structure and enzymatic characteristics of assemblin, a potential pharmacologic target, comparatively little is known about these features of the precursor. To begin studying pPR, we introduced five point mutations that stabilize it against self-cleavage at its internal (I), cryptic (C), release (R), and maturational (M) sites and at a newly discovered "tail" (T) site. The resulting mutants, called ICRM-pPR and ICRMT-pPR, were expressed in bacteria, denatured in urea, purified by immobilized metal affinity chromatography, and renatured by a two-step dialysis procedure and by a new method of sedimentation into glycerol gradients. The enzymatic activities of the pPR mutants were indistinguishable from that of IC-assemblin prepared in parallel for comparison, as determined by using a fluorogenic peptide cleavage assay, and approximated rates previously reported for purified assemblin. The percentage of active enzyme in the preparations was also comparable, as determined by using a covalent-binding suicide substrate. An unexpected finding was that, in the absence of the kosmotrope Na2SO4, optimal activity of pPR requires interaction through its scaffolding domain. We conclude that although the enzymatic activities of assemblin and its precursor are comparable, there may be differences in how their catalytic sites become fully activated.

Cleavage of human cytomegalovirus protease pUL80a at internal and cryptic sites is not essential but enhances infectivity

The cytomegalovirus (CMV) maturational protease, assemblin, contains an "internal" (I) cleavage site absent from its homologs in other herpesviruses. Blocking this site for cleavage did not prevent replication of the resulting I(-) mutant virus. However, cells infected with the I(-) virus showed increased amounts of a fragment produced by cleavage at the nearby "cryptic" (C) site, suggesting that its replication may bypass the I-site block by using the C site as an alternate cleavage pathway. To test this and further examine the biological importance of these cleavages, we constructed two additional virus mutants-one blocked for C-site cleavage and another blocked for both I- and C-site cleavage. Infectivity comparisons with the parental wild-type virus showed that the I(-) mutant was the least affected for virus production, whereas infectivity of the C(-) mutant was reduced by approximately 40% and when both sites were blocked virus infectivity was reduced by nearly 90%, providing the first evidence that these cleavages have biological significance. We also present and discuss evidence suggesting that I-site cleavage destabilizes assemblin and its fragments, whereas C-site cleavage does not.

Chemical Rescue of I-site Cleavage in Living Cells and in Vitro Discriminates between the Cytomegalovirus Protease, Assemblin, and Its Precursor, pUL80a

Chemical rescue is an established approach that offers a directed strategy for designing mutant enzymes in which activity can be restored by supplying an appropriate exogenous compound. This method has been used successfully to study a broad range of enzymes in vitro, but its application to living systems has received less attention. We have investigated the feasibility of using chemical rescue to make a conditional-lethal mutant of the cytomegalovirus (CMV) maturational protease. The 28-kDa CMV serine protease, assemblin, has a Ser-His-His catalytic triad and an internal (I) cleavage site near its midpoint. We found that imidazole can restore I-site cleavage to mutants inactivated by replacing the critical active site His with Ala or with Gly, which rescued better. Comparable rescue was observed for counterpart mutants of the human and simian CMV assemblin homologs and occurred in both living cells and in vitro. Cleavage was established to be at the correct site by amino acid sequencing and proceeded at approximately 11%/h in bacteria and approximately 30%/h in vitro. The same mutations were unresponsive to chemical rescue in the context of the assemblin precursor, pUL80a. This catalytic difference distinguishes the two forms of the CMV protease.

Cytomegalovirus assemblin (pUL80a): cleavage at internal site not essential for virus growth; proteinase absent from virions

The human cytomegalovirus (HCMV) maturational proteinase is synthesized as an enzymatically active 74-kDa precursor that cleaves itself at four sites. Two of these, called the maturational (M) and release (R) sites, are conserved in the homologs of all herpesviruses. The other two, called the internal (I) and cryptic (C) sites, have recognized consensus sequences only among cytomegalovirus (CMV) homologs and are located in the 28-kDa proteolytic portion of the precursor, called assemblin. I-site cleavage cuts assemblin in half without detected effect on its enzymatic behavior in vitro. To investigate the requirement for this cleavage during virus infection, we used the CMV-bacterial artificial chromosome system (E. M. Borst, G. Hahn, U. H. Koszinowski, and M. Messerle, J. Virol. 73:8320-8329, 1999) to construct a virus encoding a mutant I site (Ala143 to Val) intended to be blocked for cleavage. Characterizations of the resulting mutant (i) confirmed the presence of the mutation in the viral genome and the inability of the mutant virus to effect I-site cleavage in infected cells; (ii) determined that the mutation has no gross effect on the rate of virus production or on the amounts of extracellular virions, noninfectious enveloped particles (NIEPs), and dense bodies; (iii) established that assemblin and its cleavage products are present in NIEPs but are absent from CMV virions, an apparent difference from what is found for virions of herpes simplex virus; and (iv) showed that the 23-kDa protein product of C-site cleavage is more abundant in mutant virus-than in wild-type virus-infected cells and NIEPs. We conclude that the production of infectious CMV requires neither I-site cleavage of assemblin nor the presence of assemblin in the mature virion.

The herpesvirus proteases as targets for antiviral chemotherapy

Viruses of the family Herpesviridae are responsible for a diverse set of human diseases. The available treatments are largely ineffective, with the exception of a few drugs for treatment of herpes simplex virus (HSV) infections. For several members of this DNA virus family, advances have been made recently in the biochemistry and structural biology of the essential viral protease, revealing common features that may be possible to exploit in the development of a new class of anti-herpesvirus agents. The herpesvirus proteases have been identified as belonging to a unique class of serine protease, with a Ser-His-His catalytic triad. A new, single domain protein fold has been determined by X-ray crystallography for the proteases of at least three different herpesviruses. Also unique for serine proteases, dimerization has been shown to be required for activity of the cytomegalovirus and HSV proteases. The dimerization requirement seriously impacts methods needed for productive, functional analysis and inhibitor discovery. The conserved functional and catalytic properties of the herpesvirus proteases lead to common considerations for this group of proteases in the early phases of inhibitor discovery. In general, classical serine protease inhibitors that react with active site residues do not readily inactivate the herpesvirus proteases. There has been progress however, with activated carbonyls that exploit the selective nucleophilicity of the active site serine. In addition, screening of chemical libraries has yielded novel structures as starting points for drug development. Recent crystal structures of the herpesvirus proteases now allow more direct interpretation of ligand structure-activity relationships. This review first describes basic functional aspects of herpesvirus protease biology and enzymology. Then we discuss inhibitors identified to date and the prospects for their future development.

Cytomegalovirus assembly protein precursor and proteinase precursor contain two nuclear localization signals that mediate their own nuclear translocation and that of the major capsid protein

The cytomegalovirus (CMV) assembly protein precursor (pAP) interacts with the major capsid protein (MCP), and this interaction is required for nuclear translocation of the MCP, which otherwise remains in the cytoplasm of transfected cells (L. J. Wood et al., J. Virol. 71:179-190, 1997). We have interpreted this finding to indicate that the CMV MCP lacks its own nuclear localization signal (NLS) and utilizes the pAP as an NLS-bearing escort into the nucleus. The CMV pAP amino acid sequence has two clusters of basic residues (e.g., KRRRER [NLS1] and KARKRLK [NLS2], for simian CMV) that resemble the simian virus 40 large-T-antigen NLS (D. Kalderon et al., Cell 39:499-509, 1984) and one of these (NLS1) has a counterpart in the pAP homologs of other herpesviruses. The work described here establishes that NLS1 and NLS2 are mutually independent NLS that can act (i) in cis to translocate pAP and the related proteinase precursor (pNP1) into the nucleus and (ii) in trans to transport MCP into the nucleus. By using combinations of NLS mutants and carboxy-terminal deletion constructs, we demonstrated a self-interaction of pAP and cytoplasmic interactions of pAP with pNP1 and of pNP1 with itself. The relevance of these findings to early steps in capsid assembly, the mechanism of MCP nuclear transport, and the possible cytoplasmic formation of protocapsomeric substructures is discussed.

Identification of a novel peptide substrate of HSV-1 protease using substrate phage display

The method of substrate phage display was used to select a preferred substrate from three monovalent display libraries using the HSV-1 protease. The display libraries consisted of four random amino acids, six random amino acids, and a biased library containing four amino acids from the P side of the HSV-1 maturation site followed by four random amino acids. A series of consensus peptides was synthesized based upon the results from these screens and tested in peptide cleavage assays. An eight amino acids consensus peptide (LVLASSSF) derived from the phage results was cleaved as efficiently as a 20-mer maturation site peptide. The selected amino acid sequences also allowed the design of a four amino acid paranitroanilide substrate for continuous assay of HSV-1 protease. Similar to HCMV protease, these results define P4 to P1 as a minimal substrate recognition domain for the HSV-1 protease and suggest that P4 to P1 is the minimal substrate domain which all herpesvirus proteases recognize.

Expression and characterization of recombinant murine cytomegalovirus protease

The protease domain of the murine cytomegalovirus (MCMV) M80 open reading frame was expressed in and purified from Escherichia coli. The recombinant enzyme was recovered as a mixture of active one- and two-chain forms. The two-chain enzyme was formed by internal cleavage of the one-chain enzyme at the I site. Activity measurements showed that MCMV protease cleaves R- and M-site peptide mimics with kinetics similar to those of recombinant human cytomegalovirus (HCMV) protease. Both the MCMV and HCMV proteases cleave I-site peptide substrates very poorly, but the crystal structure of the HCMV protease indicates that the cytomegalovirus I site likely resides on a solvent-exposed loop close to the active site.

Herpesvirus proteases: targets for novel antiviral drugs

Herpesvirus proteases have emerged as targets for the development of novel antiviral drugs. These enzymes, which are necessary for the replication of all herpesviruses, are serine proteases, but possess a unique structure as revealed by solution of the crystal structure of human cytomegalovirus protease. Many of the biochemical properties of these enzymes are now explained by the structure. Conventional serine protease inhibitors are not potent inhibitors of these enzymes and therefore the search for potent inhibitors possessing necessary features of an effective antiviral will require novel approaches. The three-dimensional structure serves as a milestone for continued endeavors towards this goal.

Assembly of herpes simplex virus capsids using the human cytomegalovirus scaffold protein: critical role of the C terminus

An essential step in assembly of herpes simplex virus (HSV) type 1 capsids involves interaction of the major capsid protein (VP5) with the C terminus of the scaffolding protein (encoded by the UL26.5 gene). The final 12 residues of the HSV scaffolding protein contains an A-X-X-F-V/A-X-Q-M-M-X-X-R motif which is conserved between scaffolding proteins found in other alphaherpesviruses but not in members of the beta- or gamma-herpesviruses. Previous studies have shown that the bovine herpesvirus 1 (alphaherpesvirus) UL26.5 homolog will functionally substitute for the HSV UL26.5 gene (E. J. Haanes et al., J. Virol. 69:7375-7379, 1995). The homolog of the UL26.5 gene in the human cytomegalovirus (HCMV) genome is the UL80.5 gene. In these studies, we tested whether the HCMV UL80.5 gene would substitute for the HSV UL26.5 gene in a baculovirus capsid assembly system that we have previously described (D. R. Thomsen et al., J. Virol. 68:2442-2457, 1994). The results demonstrate that (i) no intact capsids were assembled when the full-length or a truncated (missing the C-terminal 65 amino acids) UL80.5 protein was tested; (ii) when the C-terminal 65 amino acids of the UL80.5 protein were replaced with the C-terminal 25 amino acids of the UL26.5 protein, intact capsids were made and direct interaction of the UL80.5 protein with VP5 was detected; (iii) assembly of intact capsids was demonstrated when the sequence of the last 12 amino acids of the UL80.5 protein was changed from RRIFVA ALNKLE to RRIFVAAMMKLE; (iv) self-interaction of the scaffold proteins is mediated by sequences N terminal to the maturation cleavage site; and (v) the UL26.5 and UL80.5 proteins will not coassemble into scaffold structures. The results suggest that the UL26.5 and UL80.5 proteins form a scaffold by self-interaction via sequences in the N termini of the proteins and emphasize the importance of the C terminus for interaction of scaffold with the proteins that form the capsid shell.

Independently cloned halves of cytomegalovirus assemblin, An and Ac, can restore proteolytic activity to assemblin mutants by intermolecular complementation

Herpesviruses encode an essential serine proteinase called assemblin that is responsible for cleaving the precursor assembly protein during the process of capsid formation. In cytomegalovirus (CMV), assemblin undergoes autoproteolysis at an internal (I) site located near the middle of the molecule. I-site cleavage converts the enzyme to an active two-chain form consisting of the subunits An and Ac. We have recently shown that the recombinant An and Ac subunits can spontaneously associate within eukaryotic cells to yield active two-chain proteinase. This finding indicates that the subunits are able to independently assume their correct functional conformations and led us to test whether they are capable of intermolecular complementation. This was done by coexpressing inactive mutant (point, deletion, and insertion) forms of assemblin together with the wild-type subunit (either An or Ac) corresponding to the domain of assemblin that was mutated. Results of these experiments showed that both An and Ac are able to rescue the enzymatic activity of assemblin mutants. I-site cleavage of the mutated assemblin occurred during complementation but was not absolutely required, as shown by effective complementation of inactive assemblins with noncleavable I sites. We have also shown that intermolecular complementation can rescue the activity of an inactive mutant full-length proteinase precursor and can occur between different species of CMV (e.g., human CMV subunit can rescue activity of mutant simian CMV assemblin). These results indicate that assemblin is able to form active multimeric structures that may be of functional importance.

Cytomegalovirus "missing" capsid protein identified as heat-aggregable product of human cytomegalovirus UL46

Capsids of human and simian strains of cytomegalovirus (HCMV and SCMV, respectively) have identified counterparts for all but one of the protein components of herpes simplex virus (HSV) capsids. The open reading frames (ORFs) for the CMV and HSV counterpart proteins are positionally homologous in the two genomes. The HSV capsid protein without a recognized counterpart in CMV is VP19c, a 50-kDa element of the intercapsomeric "triplex." VP19c is encoded by HSV ORF UL38, whose positional homolog in the HCMV genome is UL46. The predicted protein product of HCMV UL4A6, however, has essentially no amino acid sequence similarity to HSV VP19c, is only two-thirds as long, and was not recognized as a component of CMV capsids. To identify and learn more about the protein encoded by HCMV UL46, we have expressed it in insect cells from a recombinant baculovirus and tested for its presence in CMV-infected human cells and virus particles with two UL4A6-specific antipeptide antisera. Results presented here show that this HCMV protein (i) has a size of approximately 30 kDa as expressed in both recombinant baculovirus-infected insect cells and HCMV-infected human cells; (ii) has a homolog in SCMV; (iii) is a capsid component and is present in a 1:2 molar ratio with the minor capsid protein (mCP), encoded by UL85; and (iv) interacts with the mCP, which is also shown to interact with itself as demonstrated by the GAL4 two-hybrid system; and (v) aggregates when heated and does not enter the resolving gel during sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), a characteristic that accounts for it eluding detection until now. We call this protein the mCP-binding protein, and on the basis of the characteristics that it shares with HSV VP19c, we conclude that the HCMV mCP-binding protein is the functional as well as genetic homolog of HSV VP19c.

Virus-specific interaction between the human cytomegalovirus major capsid protein and the C terminus of the assembly protein precursor

We previously identified a minimal 12-amino-acid domain in the C terminus of the herpes simplex virus type 1 (HSV-1) scaffolding protein which is required for interaction with the HSV-1 major capsid protein. An alpha-helical structure which maximizes the hydropathicity of the minimal domain is required for the interaction. To address whether cytomegalovirus (CMV) utilizes the same strategy for capsid assembly, several glutathione S-transferase fusion proteins to the C terminus of the CMV assembly protein precursor were produced and purified from bacterial cells. The study showed that the glutathione S-transferase fusion containing 16 amino acids near the C-terminal end was sufficient to interact with the major capsid protein. Interestingly, no cross-interaction between HSV-1 and CMV could be detected. Mutation analysis revealed that a three-amino-acid region at the N-terminal side of the central Phe residue of the CMV interaction domain played a role in determining the viral specificity of the interaction. When this region was converted so as to correspond to that of HSV-1, the CMV assembly protein domain lost its ability to interact with the CMV major capsid protein but gained full interaction with the HSV-1 major capsid protein. To address whether the minimal interaction domain of the CMV assembly protein forms an alpha-helical structure similar to that in HSV-1, peptide competition experiments were carried out. The results showed that a cyclic peptide derived from the interaction domain with a constrained (alpha-helical structure competed for interaction with the major capsid protein much more efficiently than the unconstrained linear peptide. In contrast, a cyclic peptide containing an Ala substitution for the critical Phe residue did not compete for the interaction at all. The results of this study suggest that (i) CMV may have developed a strategy similar to that of HSV-1 for capsid assembly; (ii) the minimal interaction motif in the CMV assembly protein requires an alpha-helix for efficient interaction with the major capsid protein; and (iii) the Phe residue in the CMV minimal interaction domain is critical for interaction with the major capsid protein.

Initial characterization of autoprocessing and active-center mutants of CMV proteinase

Human cytomegalovirus (CMV) encodes a unique serine proteinase that is required in the maturation of the viral capsid. The CMV proteinase can undergo autocatalytic activation and is subject to proteolytic self-inactivation. Mutant enzyme forms were prepared to eliminate the initial autoprocessing site and thus form an active single-chain protein for structure-function studies. Two mutants of CMV proteinase were cloned and expressed in Escherichia coli. The A143V mutant was a conservative substitution at the first internal cleavage site. The S132A mutant modified one of the triad of residues responsible for catalytic activity. Through the use of computer-controlled high-cell-density fermentations the mutant proteins were expressed in E. coli at approximately 170 mg/L as both soluble (approximately 40% of total) and inclusion-body forms (approximately 60% of total). The soluble enzyme was purified by standard methods; inclusion-body protein was isolated by standard methods after refolding and solubilization in guanidine or urea. Sedimentation equilibrium and sedimentation velocity analyses reveal that the enzyme undergoes concentration-dependent aggregation. It exhibits a monomer <==> dimer equilibrium (Kd = 1 microM) at low concentrations and remains dimeric at high concentrations (28 mg/ml). Differential scanning calorimetry data for protein thermal unfolding fit best to a non-two-state model with two components (Tm = 52.3 and 55.3 degrees C) which subsequently aggregate upon unfolding. Analysis of the short-UV circular dichroism spectra of protein forms resulting from expression as soluble molecules (not refolded) reveals that the two mutants have very similar secondary structures which comprise a mixed structural motif of 20% alpha-helix, 26% beta-sheet, and 53% random coil. Though soluble and active (A143V mutant only), CD analysis revealed that protein refolded from inclusion bodies did not exhibit spectra identical to that of protein expressed only in soluble form.