Three-dimensional structure of a monomeric form of a retroviral protease

Unveiling the DHX15-G-patch interplay in retroviral RNA packaging

We explored how a simple retrovirus, Mason-Pfizer monkey virus (M-PMV) to facilitate its replication process, utilizes DHX15, a cellular RNA helicase, typically engaged in RNA processing. Through advanced genetic engineering techniques, we showed that M-PMV recruits DHX15 by mimicking cellular mechanisms, relocating it from the nucleus to the cytoplasm to aid in viral assembly. This interaction is essential for the correct packaging of the viral genome and critical for its infectivity. Our findings offer unique insights into the mechanisms of viral manipulation of host cellular processes, highlighting a sophisticated strategy that viruses employ to leverage cellular machinery for their replication. This study adds valuable knowledge to the understanding of viral-host interactions but also suggests a common evolutionary history between cellular processes and viral mechanisms. This finding opens a unique perspective on the export mechanism of intron-retaining mRNAs in the packaging of viral genetic information and potentially develop ways to stop it.

Precursors of Viral Proteases as Distinct Drug Targets

Viral proteases are indispensable for successful virion maturation, thus making them a prominent drug target. Their enzyme activity is tightly spatiotemporally regulated by expression in the precursor form with little or no activity, followed by activation via autoprocessing. These cleavage events are frequently triggered upon transportation to a specific compartment inside the host cell. Typically, precursor oligomerization or the presence of a co-factor is needed for activation. A detailed understanding of these mechanisms will allow ligands with non-canonical mechanisms of action to be designed, which would specifically modulate the initial irreversible steps of viral protease autoactivation. Binding sites exclusive to the precursor, including binding sites beyond the protease domain, can be exploited. Both inhibition and up-regulation of the proteolytic activity of viral proteases can be detrimental for the virus. All these possibilities are discussed using examples of medically relevant viruses including herpesviruses, adenoviruses, retroviruses, picornaviruses, caliciviruses, togaviruses, flaviviruses, and coronaviruses.

Crystal structures of inhibitor complexes of M-PMV protease with visible flap loops

Mason‐Pfizer monkey virus protease (PR) was crystallized in complex with two pepstatin‐based inhibitors in P1 space group. In both crystal structures, the extended flap loops that lock the inhibitor/substrate over the active site, are visible in the electron density either completely or with only small gaps, providing the first observation of the conformation of the flap loops in dimeric complex form of this retropepsin. The H‐bond network in the active site (with D26N mutation) differs from that reported for the P2₁ crystal structures and is similar to a rarely occurring system in HIV‐1 PR.

PDB Code(s): 7bgt and 7bgu;

Dimer Interface Organization is a Main Determinant of Intermonomeric Interactions and Correlates with Evolutionary Relationships of Retroviral and Retroviral-Like Ddi1 and Ddi2 Proteases

The life cycles of retroviruses rely on the limited proteolysis catalyzed by the viral protease. Numerous eukaryotic organisms also express endogenously such proteases, which originate from retrotransposons or retroviruses, including DNA damage-inducible 1 and 2 (Ddi1 and Ddi2, respectively) proteins. In this study, we performed a comparative analysis based on the structural data currently available in Protein Data Bank (PDB) and Structural summaries of PDB entries (PDBsum) databases, with a special emphasis on the regions involved in dimerization of retroviral and retroviral-like Ddi proteases. In addition to Ddi1 and Ddi2, at least one member of all seven genera of the Retroviridae family was included in this comparison. We found that the studied retroviral and non-viral proteases show differences in the mode of dimerization and density of intermonomeric contacts, and distribution of the structural characteristics is in agreement with their evolutionary relationships. Multiple sequence and structure alignments revealed that the interactions between the subunits depend mainly on the overall organization of the dimer interface. We think that better understanding of the general and specific features of proteases may support the characterization of retroviral-like proteases.

Inhibitor and substrate binding induced stability of HIV-1 protease against sequential dissociation and unfolding revealed by high pressure spectroscopy and kinetics

High-pressure methods have become an interesting tool of investigation of structural stability of proteins. They are used to study protein unfolding, but dissociation of oligomeric proteins can be addressed this way, too. HIV-1 protease, although an interesting object of biophysical experiments, has not been studied at high pressure yet. In this study HIV-1 protease is investigated by high pressure (up to 600 MPa) fluorescence spectroscopy of either the inherent tryptophan residues or external 8-anilino-1-naphtalenesulfonic acid at 25°C. A fast concentration-dependent structural transition is detected that corresponds to the dimer-monomer equilibrium. This transition is followed by a slow concentration independent transition that can be assigned to the monomer unfolding. In the presence of a tight-binding inhibitor none of these transitions are observed, which confirms the stabilizing effect of inhibitor. High-pressure enzyme kinetics (up to 350 MPa) also reveals the stabilizing effect of substrate. Unfolding of the protease can thus proceed only from the monomeric state after dimer dissociation and is unfavourable at atmospheric pressure. Dimer-destabilizing effect of high pressure is caused by negative volume change of dimer dissociation of -32.5 mL/mol. It helps us to determine the atmospheric pressure dimerization constant of 0.92 μM. High-pressure methods thus enable the investigation of structural phenomena that are difficult or impossible to measure at atmospheric pressure.

High-resolution structure of a retroviral protease folded as a monomer

Mason-Pfizer monkey virus (M-PMV), a D-type retrovirus assembling in the cytoplasm, causes simian acquired immunodeficiency syndrome (SAIDS) in rhesus monkeys. Its pepsin-like aspartic protease (retropepsin) is an integral part of the expressed retroviral polyproteins. As in all retroviral life cycles, release and dimerization of the protease (PR) is strictly required for polyprotein processing and virion maturation. Biophysical and NMR studies have indicated that in the absence of substrates or inhibitors M-PMV PR should fold into a stable monomer, but the crystal structure of this protein could not be solved by molecular replacement despite countless attempts. Ultimately, a solution was obtained in mr-rosetta using a model constructed by players of the online protein-folding game Foldit. The structure indeed shows a monomeric protein, with the N- and C-termini completely disordered. On the other hand, the flap loop, which normally gates access to the active site of homodimeric retropepsins, is clearly traceable in the electron density. The flap has an unusual curled shape and a different orientation from both the open and closed states known from dimeric retropepsins. The overall fold of the protein follows the retropepsin canon, but the C(α) deviations are large and the active-site 'DTG' loop (here NTG) deviates up to 2.7 Å from the standard conformation. This structure of a monomeric retropepsin determined at high resolution (1.6 Å) provides important extra information for the design of dimerization inhibitors that might be developed as drugs for the treatment of retroviral infections, including AIDS.

Crystal structure of a monomeric retroviral protease solved by protein folding game players

Following the failure of a wide range of attempts to solve the crystal structure of M-PMV retroviral protease by molecular replacement, we challenged players of the protein folding game Foldit to produce accurate models of the protein. Remarkably, Foldit players were able to generate models of sufficient quality for successful molecular replacement and subsequent structure determination. The refined structure provides new insights for the design of antiretroviral drugs.

Backbone (1)H, (13)C, and (15)N NMR assignment for the inactive form of the retroviral protease of the murine intracisternal A-type particle, inMIA-14 PR

Proteases play a crucial role in the retroviral infection but so far the mechanism of their regulation remains unclear. Protease MIA-14 from murine intracisternal A-type particles, containing a C-terminal domain rich in glycines (G-patch), is responsible for binding of single-stranded oligonucleotides (both RNA and DNA) without inhibiting the proteolytic activity. For investigations of untill now poorly characterized protease-oligonucleotide interactions, assignments of the observed NMR frequencies are mandatory. An almost complete assignments of the main chain and (13)C(beta) side chain resonances of the 34 kDa homo-dimeric inMIA-14 PR is presented in this study.

The solution structure of the simian foamy virus protease reveals a monomeric protein

In contrast to orthoretroviruses, foamy viruses (FVs) express their Pol polyprotein from a separate pol-specific transcript. Only the integrase domain is cleaved off, leading to a protease-reverse transcriptase (PR-RT) protein. We purified the separate PR domain (PRshort) of simian FV from macaques by expressing the recombinant gene in Escherichia coli. Sedimentation analyses and size exclusion chromatography indicate that PRshort is a stable monomer in solution. This allowed us to determine the structure of the PRshort monomer using 1426 experimental restraints derived from NMR spectroscopy. The superposition of 20 conformers resulted in a backbone atom rmsd of 0.55 A for residues Gln8-Leu93. Although the overall folds are similar, the macaque simian FV PRshort reveals significant differences in the dimerization interface relative to other retroviral PRs, such as HIV-1 (human immunodeficiency virus type 1) PR, which appear to be rather stable dimers. Especially the flap region and the N- and C-termini of PRshort are highly flexible. Neglecting these regions, the backbone atom rmsd drops to 0.32 A, highlighting the good definition of the central part of the protein. To exclude that the monomeric state of PRshort is due to cleaving off the RT, we purified the complete PR-RT and performed size exclusion chromatography. Our data show that PR-RT is also monomeric. We thus conclude adoption of a monomeric state of PR-RT to be a regulatory mechanism to inhibit PR activity before virus assembly in order to reduce packaging problems. Dimerization might therefore be triggered by additional viral or cellular factors.

The Role of the S-S Bridge in Retroviral Protease Function and Virion Maturation

Retroviral proteases are translated as a part of Gag-related polyproteins, and are released and activated during particle release. Mason-Pfizer monkey virus (M-PMV) Gag polyproteins assemble into immature capsids within the cytoplasm of the host cells; however, their processing occurs only after transport to the plasma membrane and subsequent release. Thus, the activity of M-PMV protease is expected to be highly regulated during the replication cycle. It has been proposed that reversible oxidation of protease cysteine residues might be responsible for such regulation. We show that cysteine residues in M-PMV protease can form an intramolecular S-S bridge. The disulfide bridge shifts the monomer/dimer equilibrium in favor of the dimer, and increases the proteolytic activity significantly. To investigate the role of this disulfide bridge in virus maturation and replication, we engineered an M-PMV clone in which both protease cysteine residues were replaced by alanine (M-PMV(PRC7A/C106A)). Surprisingly, the cysteine residues were dispensable for Gag polyprotein processing within the virus, indicating that even low levels of protease activity are sufficient for polyprotein processing during maturation. However, the long-term infectivity of M-PMV(PRC7A/C106A) was noticeably compromised. These results show clearly that the proposed redox mechanism does not rely solely on the formation of the stabilizing S-S bridge in the protease. Thus, in addition to the protease disulfide bridge, reversible oxidation of cysteine and/or methionine residues in other domains of the Gag polyprotein or in related cellular proteins must be involved in the regulation of maturation.

The RNA Binding G-patch Domain in Retroviral Protease Is Important for Infectivity and D-type Morphogenesis of Mason-Pfizer Monkey Virus

Retroviral proteases (PRs) cleave the viral polyprotein precursors into functional mature proteins late during particle release and are essential for viral replication. Unlike most retroviruses, beta-retroviruses, including Mason-Pfizer monkey virus (M-PMV), assemble immature capsids within the cytoplasm of the cell. The activation of beta-retroviral proteases must be highly regulated, because processing of the Gag-related polyprotein precursors occurs only after transport of immature capsids to the plasma membrane and budding. Several beta-retroviral proteases have unique C-terminal extension sequences, containing a glycine-rich motif (G-patch), which specifically binds in vitro to single-stranded nucleic acids. In M-PMV PR the G-patch is removed in vitro as well as in vivo by autoproteolytic processing to yield truncated active forms of PR. To investigate the role of the G-patch domain on the virus life cycle, we introduced mutations within the C-terminal domain of protease. We found that the G-patch domain of M-PMV PR is not required for the processing of viral polyproteins, but it significantly influences the infectivity of M-PMV, the activity of reverse transcriptase, and assembly of immature capsid within the cells. These results demonstrate for the first time that the G-patch domain of M-PMV PR is critical for the life cycle of beta-retroviruses, and its evolutionary conservation within members of this genus suggests its importance for retroviruses that display D-type morphology.

Proteinases of betaretroviruses bind single-stranded nucleic acids through a novel interaction module, the G-patch

Retroviral proteinases (PRs) are essential for retrovirus infectivity but the mechanism of their activity regulation is poorly understood. We investigated possible involvement in this process of the C-terminal domain (CTD) of betaretroviral PRs. We found that the presence of CTD attenuates proteolytic activity of Mason-Pfizer monkey virus PR, while it does not significantly affect the activity of mouse intracisternal A-particle retrovirus PR. However, both PRs bind single-stranded nucleic acids through their CTDs that contain a novel binding motif, the G-patch, whose function is dependent on a single conserved tyrosine residue. Oligonucleotide binding to both PRs does not inhibit their proteolytic activity.

The folding and dimerization of HIV-1 protease: evidence for a stable monomer from simulations

HIV-1 protease (PR) is a major drug target in combating AIDS, as it plays a key role in maturation and replication of the virus. Six FDA-approved drugs are currently in clinical use, all designed to inhibit enzyme activity by blocking the active site, which exists only in the dimer. An alternative inhibition mode would be required to overcome the emergence of drug-resistance through the accumulation of mutations. This might involve inhibiting the formation of the dimer itself. Here, the folding of HIV-1 PR dimer is studied with several simulation models appropriate for folding mechanism studies. Simulations with an off-lattice Gō-model, which corresponds to a perfectly funneled energy landscape, indicate that the enzyme is formed by association of structured monomers. All-atom molecular dynamics simulations strongly support the stability of an isolated monomer. The conjunction of results from a model that focuses on the protein topology and a detailed all-atom force-field model suggests, in contradiction to some reported equilibrium denaturation experiments, that monomer folding and dimerization are decoupled. The simulation result is, however, in agreement with the recent NMR detection of folded monomers of HIV-1 PR mutants with a destabilized interface. Accordingly, the design of dimerization inhibitors should not focus only on the flexible N and C termini that constitute most of the dimer interface, but also on other structured regions of the monomer. In particular, the relatively high phi values for residues 23-35 and 79-87 in both the folding and binding transition states, together with their proximity to the interface, highlight them as good targets for inhibitor design.