Recombinant full-length human cytomegalovirus protease has lower activity than recombinant processed protease domain in purified enzyme and cell-based assays

Cytomegalovirus capsid protease: biological substrates are cleaved more efficiently by full-length enzyme (pUL80a) than by the catalytic domain (assemblin)

We compared the full-length capsid maturational protease (pPR, pUL80a) of human cytomegalovirus with its proteolytic domain (assemblin) for the ability to cleave two biological substrates, and we found that pPR is more efficient with both. Affinity-purified, refolded enzymes and substrates were combined under defined reaction conditions, and cleavage was monitored and quantified following staining of the resulting electrophoretically separated fragments. The enzymes were stabilized against self-cleavage by a single point mutation in each cleavage site (ICRMT-pPR and IC-assemblin). The substrates were pPR itself, inactivated by replacing its catalytic nucleophile (S132A-pPR), and the sequence-related assembly protein precursor (pAP, pUL80.5). Our results showed that (i) ICRMT-pPR is 5- to 10-fold more efficient than assemblin for all cleavages measured (i.e., the M site of pAP and the M, R, and I sites of S132A-pPR). (ii) Cleavage of substrate S132A-pPR proceeded M>R>I for both enzymes. (iii) Na(2)SO(4) reduced M- and R-site cleavage efficiency by ICRMT-pPR, in contrast to its enhancing effect for both enzymes on I site and small peptide cleavage. (iv) Disrupting oligomerization of either the pPR enzyme or substrate by mutating Leu382 in the amino-conserved domain reduced cleavage efficiency two- to fourfold. (v) Finally, ICRMT-pPR mutants that include the amino-conserved domain, but terminate with Pro481 or Tyr469, retain the enzymatic characteristics that distinguish pPR from assemblin. These findings show that the scaffolding portion of pPR increases its enzymatic activity on biologically relevant protein substrates and provide an additional link between the structure of this essential viral enzyme and its biological mechanism.

Sequential autoprocessing of Marek's disease herpesvirus protease differs from that of other herpesviruses

Herpesviruses encode a unique serine protease essential for viral capsid maturation. This protease undergoes autoprocessing at two sites, R and M, at the consensus sequence (V, L, I)(P3)-X(P2)-A(P1)/S(P1') (where X is a polar amino acid). We observed complete autoprocessing at the R and M sites of Marek's disease virus (MDV) protease following production of the polyprotein in Escherichia coli. Site-directed mutagenesis confirmed the predicted sequence of the R and M sites, with the M site sequence being nonconsensual: M(P3)-N(P2)-A(P1)/S(P1'). Mutagenesis and expression kinetics studies suggested that the atypical MDV M site was cleaved exclusively by the processed short protease, a feature making MDV unique among herpesviruses.

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.

Kinetics, inhibition and oligomerization of Epstein-Barr virus protease

Epstein-Barr virus (EBV) is an omnipresent human virus causing infectious mononucleosis and EBV associated cancers. Its protease is a possible target for antiviral therapy. We studied its dimerization and enzyme kinetics with two enzyme assays based either on the release of paranitroaniline or 7-amino-4-methylcoumarin from labeled pentapeptide (Ac-KLVQA) substrates. The protease is in a monomer-dimer equilibrium where only dimers are active. In absence of citrate the K(d) is 20 microM and drops to 0.2 microM in presence of 0.5M citrate. Citrate increases additionally the activity of the catalytic sites. The inhibitory constants of different substrate derived peptides and alpha-keto-amide based inhibitors, which have at best a K(i) of 4 microM, have also been evaluated.

Novel yeast cell-based assay to screen for inhibitors of human cytomegalovirus protease in a high-throughput format

The protease encoded by the human cytomegalovirus (HCMV) is an attractive target for antiviral drug development because of its essential function in viral replication. We describe here a cellular assay in the yeast Saccharomyces cerevisiae for the identification of small molecule inhibitors of HCMV protease by conditional growth in selective medium. In this system, the protease cleavage sequence is inserted into the N-(5'-phosphoribosyl)anthranilate isomerase (Trp1p), a yeast protein essential for cell proliferation in the absence of tryptophan. Coexpression of HCMV protease with the engineered Trp1p substrate in yeast cells results in site-specific cleavage and functional inactivation of the Trp1p enzyme, thereby leading to an arrest of cell proliferation. This growth arrest can be suppressed by the addition of validated HCMV protease inhibitors. The growth selection system presented here provides the basis for a high-throughput screen to identify HCMV protease inhibitors that are active in eukaryotic cells.

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.

The crystal structure of the Epstein-Barr virus protease shows rearrangement of the processed C terminus

Epstein-Barr virus (EBV) belongs to the gamma-herpesvirinae subfamily of the Herpesviridae. The protease domain of the assemblin protein of herpesviruses forms a monomer-dimer equilibrium in solution. The protease domain of EBV was expressed in Escherichia coli and its structure was solved by X-ray crystallography to 2.3A resolution after inhibition with diisopropyl-fluorophosphate (DFP). The overall structure confirms the conservation of the homodimer and its structure throughout the alpha, beta, and gamma-herpesvirinae. The substrate recognition could be modelled using information from the DFP binding, from a crystal contact, suggesting that the substrate forms an antiparallel beta-strand extending strand beta5, and from the comparison with the structure of a peptidomimetic inhibitor bound to cytomegalovirus protease. The long insert between beta-strands 1 and 2, which was disordered in the KSHV protease structure, was found to be ordered in the EBV protease and shows the same conformation as observed for proteases in the alpha and beta-herpesvirus families. In contrast to previous structures, the long loop located between beta-strands 5 and 6 is partially ordered, probably due to DFP inhibition and a crystal contact. It also contributes to substrate recognition. The protease shows a specific recognition of its own C terminus in a binding pocket involving residue Phe210 of the other monomer interacting across the dimer interface. This suggests conformational changes of the protease domain after its release from the assemblin precursor followed by burial of the new C terminus and a possible effect onto the monomer-dimer equilibrium. The importance of the processed C terminus was confirmed using a mutant protease carrying a C-terminal extension and a mutated release site, which shows different solution properties and a strongly reduced enzymatic activity.