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

Structures of Substrate Complexes of Foamy Viral Protease-Reverse Transcriptase

Reverse transcriptases (RTs) use their DNA polymerase and RNase H activities to catalyze the conversion of single-stranded RNA to double-stranded DNA (dsDNA), a crucial process for the replication of retroviruses. Foamy viruses (FVs) possess a unique RT, which is a fusion with the protease (PR) domain. The mechanism of substrate binding by this enzyme has been unknown. Here, we report a crystal structure of monomeric full-length marmoset FV (MFV) PR-RT in complex with an RNA/DNA hybrid substrate. We also describe a structure of MFV PR-RT with an RNase H deletion in complex with a dsDNA substrate in which the enzyme forms an asymmetric homodimer. Cryo-electron microscopy reconstruction of the full-length MFV PR-RT–dsDNA complex confirmed the dimeric architecture. These findings represent the first structural description of nucleic acid binding by a foamy viral RT and demonstrate its ability to change its oligomeric state depending on the type of bound nucleic acid.

IMPORTANCE Reverse transcriptases (RTs) are intriguing enzymes converting single-stranded RNA to dsDNA. Their activity is essential for retroviruses, which are divided into two subfamilies differing significantly in their life cycles: Orthoretrovirinae and Spumaretrovirinae. The latter family is much more ancient and comprises five genera. A unique feature of foamy viral RTs is that they contain N-terminal protease (PR) domains, which are not present in orthoretroviral enzymes. So far, no structural information for full-length foamy viral PR-RT interacting with nucleic substrates has been reported. Here, we present crystal and cryo-electron microscopy structures of marmoset foamy virus (MFV) PR-RT. These structures revealed the mode of binding of RNA/DNA and dsDNA substrates. Moreover, unexpectedly, the structures and biochemical data showed that foamy viral PR-RT can adopt both a monomeric configuration, which is observed in our structures in the presence of an RNA/DNA hybrid, and an asymmetric dimer arrangement, which we observed in the presence of dsDNA.

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.

Structural and Functional Aspects of Foamy Virus Protease-Reverse Transcriptase

Reverse transcription describes the process of the transformation of single-stranded RNA into double-stranded DNA via an RNA/DNA duplex intermediate, and is catalyzed by the viral enzyme reverse transcriptase (RT). This event is a pivotal step in the life cycle of all retroviruses. In contrast to orthoretroviruses, the domain structure of the mature RT of foamy viruses is different, i.e., it harbors the protease (PR) domain at its N-terminus, thus being a PR-RT. This structural feature has consequences on PR activation, since the enzyme is monomeric in solution and retroviral PRs are only active as dimers. This review focuses on the structural and functional aspects of simian and prototype foamy virus reverse transcription and reverse transcriptase, as well as special features of reverse transcription that deviate from orthoretroviral processes, e.g., PR activation.

Related Endogenous Retrovirus-K Elements Harbor Distinct Protease Active Site Motifs

Background: Endogenous retrovirus-K is a group of related genomic elements descending from retroviral infections in human ancestors. HML2 is the clade of these viruses which contains the most intact provirus copies. These elements can be transcribed and translated in healthy and diseased tissues, and some of them produce active retroviral enzymes, such as protease. Retroviral gene products, including protease, contribute to illness in exogenous retroviral infections. There are ongoing efforts to test anti-retroviral regimens against endogenous retroviruses. Herein, we examine the potential activity and diversity of human endogenous retrovirus-K proteases, and their potential for impact on immunity and human disease.

Results: Sequences similar to the endogenous retrovirus-K HML2 protease and reverse transcriptase were identified in the human genome, classified by phylogenetic inference and compared to Repbase reference sequences. The topologies of trees inferred from protease and reverse transcriptase sequences were similar and agreed with the classification using reference sequences. Surprisingly, only 62/480 protease sequences identified by BLAST were classified as HML2; the remainder were classified as other HML groups, with the majority (216) classified as HML3. Variation in functionally significant protease motifs was explored, and two major active site variants were identified – the DTGAD variant is common in all groups, but the DTGVD motif appears limited to HML3, HML5, and HML6. Furthermore, distinct RNA expression patterns of protease variants are seen in disease states, such as amyotrophic lateral sclerosis, breast cancer, and prostate cancer.

Conclusion: Transcribed ERVK proteases exhibit a diversity which could impact immunity and inhibitor-based treatments, and these facets should be considered when designing therapeutic regimens.

Viral reverse transcriptases

Reverse transcriptases (RTs) play a major role in the replication of Retroviridae, Metaviridae, Pseudoviridae, Hepadnaviridae and Caulimoviridae. RTs are enzymes that are able to synthesize DNA using RNA or DNA as templates (DNA polymerase activity), and degrade RNA when forming RNA/DNA hybrids (ribonuclease H activity). In retroviruses and LTR retrotransposons (Metaviridae and Pseudoviridae), the coordinated action of both enzymatic activities converts single-stranded RNA into a double-stranded DNA that is flanked by identical sequences known as long terminal repeats (LTRs). RTs of retroviruses and LTR retrotransposons are active as monomers (e.g. murine leukemia virus RT), homodimers (e.g. Ty3 RT) or heterodimers (e.g. human immunodeficiency virus type 1 (HIV-1) RT). RTs lack proofreading activity and display high intrinsic error rates. Besides, high recombination rates observed in retroviruses are promoted by poor processivity that causes template switching, a hallmark of reverse transcription. HIV-1 RT inhibitors acting on its polymerase activity constitute the backbone of current antiretroviral therapies, although novel drugs, including ribonuclease H inhibitors, are still necessary to fight HIV infections. In Hepadnaviridae and Caulimoviridae, reverse transcription leads to the formation of nicked circular DNAs that will be converted into episomal DNA in the host cell nucleus. Structural and biochemical information on their polymerases is limited, although several drugs inhibiting HIV-1 RT are known to be effective against the human hepatitis B virus polymerase. In this review, we summarize current knowledge on reverse transcription in the five virus families and discuss available biochemical and structural information on RTs, including their biosynthesis, enzymatic activities, and potential inhibition.

Biochemical characterization of a multi-drug resistant HIV-1 subtype AG reverse transcriptase: antagonism of AZT discrimination and excision pathways and sensitivity to RNase H inhibitors

We analyzed a multi-drug resistant (MR) HIV-1 reverse transcriptase (RT), subcloned from a patient-derived subtype CRF02_AG, harboring 45 amino acid exchanges, amongst them four thymidine analog mutations (TAMs) relevant for high-level AZT (azidothymidine) resistance by AZTMP excision (M41L, D67N, T215Y, K219E) as well as four substitutions of the AZTTP discrimination pathway (A62V, V75I, F116Y and Q151M). In addition, K65R, known to antagonize AZTMP excision in HIV-1 subtype B was present. Although MR-RT harbored the most significant amino acid exchanges T215Y and Q151M of each pathway, it exclusively used AZTTP discrimination, indicating that the two mechanisms are mutually exclusive and that the Q151M pathway is obviously preferred since it confers resistance to most nucleoside inhibitors. A derivative was created, additionally harboring the TAM K70R and the reversions M151Q as well as R65K since K65R antagonizes excision. MR-R65K-K70R-M151Q was competent of AZTMP excision, whereas other combinations thereof with only one or two exchanges still promoted discrimination. To tackle the multi-drug resistance problem, we tested if the MR-RTs could still be inhibited by RNase H inhibitors. All MR-RTs exhibited similar sensitivity toward RNase H inhibitors belonging to different inhibitor classes, indicating the importance of developing RNase H inhibitors further as anti-HIV drugs.

AZT resistance alters enzymatic properties and creates an ATP-binding site in SFVmac reverse transcriptase

BACKGROUND

The replication of simian foamy virus from macaques can be inhibited by the nucleoside reverse transcriptase inhibitor azidothymidine (AZT, zidovudine). Four substitutions in the protease-reverse transcriptase (PR-RT) protein (K211I, I224T, S345T, E350K) are necessary to obtain highly AZT resistant and fully replication competent virus. AZT resistance is based on the excision of the incorporated AZTMP in the presence of ATP. I224T is a polymorphism which is not essential for AZT resistance per se, but is important for regaining efficient replication of the resistant virus.

RESULTS

We constructed PR-RT enzymes harboring one to four amino acid substitutions to analyze them biochemically and to determine their ability to remove the incorporated AZTMP. S345T is the only single substitution variant exhibiting significant AZTMP excision activity. Although K211I alone showed no AZTMP excision activity, excision efficiency doubled when K211I was present in combination with S345T and E350K. K211I also decreased nucleotide binding affinity and increased fidelity. NMR titration experiments revealed that a truncated version of the highly AZT resistant mt4 variant, comprising only the fingers-palm subdomains was able to bind ATP with a KD-value of ca. 7.6 mM, whereas no ATP binding could be detected in the corresponding wild type protein. We could show by NMR spectroscopy that S345T is responsible for ATP binding, probably by making a tryptophan residue accessible.

CONCLUSION

Although AZT resistance in SFVmac is based on excision of the incorporated AZTMP like in HIV-1, the functions of the resistance substitutions in SFVmac PR-RT appear to be different. No mutation resulting in an aromatic residue like F/Y215 in HIV, which is responsible for π-π-stacking interactions with ATP, is present in SFVmac. Instead, S345T is responsible for creating an ATP binding site, probably by making an already existing tryptophan more accessible, which in turn can interact with ATP. This is in contrast to HIV-1 RT, in which an ATP binding site is present in the WT RT but differs from that of the AZT resistant enzyme.

Inhibition of foamy virus reverse transcriptase by human immunodeficiency virus type 1 RNase H inhibitors

RNase H plays an essential role in the replication of human immunodeficiency virus type 1 (HIV-1). Therefore, it is a promising target for drug development. However, the identification of HIV-1 RNase H inhibitors (RHIs) has been hampered by the open morphology of its active site, the limited number of available RNase H crystal structures in complex with inhibitors, and the fact that, due to the high concentrations of Mg(2+) needed for protein stability, HIV-1 RNase H is not suitable for nuclear magnetic resonance (NMR) inhibitor studies. We recently showed that the RNase H domains of HIV-1 and prototype foamy virus (PFV) reverse transcriptases (RTs) exhibit a high degree of structural similarity. Thus, we examined whether PFV RNase H can serve as an HIV-1 RNase H model for inhibitor interaction studies. Five HIV-1 RHIs inhibited PFV RNase H activity at low-micromolar concentrations similar to those of HIV-1 RNase H, suggesting pocket similarity of the RNase H domains. NMR titration experiments with the PFV RNase H domain and the RHI RDS1643 (6-[1-(4-fluorophenyl)methyl-1H-pyrrol-2-yl)]-2,4-dioxo-5-hexenoic acid ethyl ester) were performed to determine its binding site. Based on these results and previous data, in silico docking analysis showed a putative RDS1643 binding region that reaches into the PFV RNase H active site. Structural overlays were performed with HIV-1 and PFV RNase H to propose the RDS1643 binding site in HIV-1 RNase H. Our results suggest that this approach can be used to establish PFV RNase H as a model system for HIV-1 RNase H in order to identify putative inhibitor binding sites in HIV-1 RNase H.

Determination of the protease cleavage site repertoire--the RNase H but not the RT domain is essential for foamy viral protease activity

In contrast to orthoretroviruses, the foamy virus protease is only active as a protease-reverse transcriptase fusion protein and requires viral RNA for activation. Maturation of foamy viral proteins seems to be restricted to a single cleavage site in Gag and Pol. We provide evidence that unprocessed Gag is required for optimal infectivity, which is unique among retroviruses. Analyses of the cleavage site sequences of the Gag and Pol cleavage sites revealed a high similarity compared to those of Lentiviruses. We show that positions P2׳ and P2 are invariant and that Gag and Pol cleavage sites are processed with similar efficiencies. The RNase H domain is essential for protease activity, but can functionally be substituted by RNase H domains of other retroviruses. Thus, the RNase H domain might be involved in the stabilization of the protease dimer, while the RT domain is essential for RNA dependent protease activation.

Foamy virus assembly with emphasis on pol encapsidation

Foamy viruses (FVs) differ from all other genera of retroviruses (orthoretroviruses) in many aspects of viral replication. In this review, we discuss FV assembly, with special emphasis on Pol incorporation. FV assembly takes place intracellularly, near the pericentriolar region, at a site similar to that used by betaretroviruses. The regions of Gag, Pol and genomic RNA required for viral assembly are described. In contrast to orthoretroviral Pol, which is synthesized as a Gag-Pol fusion protein and packaged through Gag-Gag interactions, FV Pol is synthesized from a spliced mRNA lacking all Gag sequences. Thus, encapsidation of FV Pol requires a different mechanism. We detail how WT Pol lacking Gag sequences is incorporated into virus particles. In addition, a mutant in which Pol is expressed as an orthoretroviral-like Gag-Pol fusion protein is discussed. We also discuss temporal regulation of the protease, reverse transcriptase and integrase activities of WT FV Pol.

Prototype foamy virus protease activity is essential for intraparticle reverse transcription initiation but not absolutely required for uncoating upon host cell entry

Foamy viruses (FVs) are unique among retroviruses in performing genome reverse transcription (RTr) late in replication, resulting in an infectious DNA genome, and also in their unusual Pol biosynthesis and encapsidation strategy. In addition, FVs display only very limited Gag and Pol processing by the viral protease (PR) during particle morphogenesis and disassembly, both thought to be crucial for viral infectivity. Here, we report the generation of functional prototype FV (PFV) particles from mature or partially processed viral capsid and enzymatic proteins with infectivity levels of up to 20% of the wild type. Analysis of protein and nucleic acid composition, as well as infectivity, of virions generated from different Gag and Pol combinations (including both expression-optimized and authentic PFV open reading frames [ORFs]) revealed that precursor processing of Gag, but not Pol, during particle assembly is essential for production of infectious virions. Surprisingly, when processed Gag (instead of Gag precursor) was provided together with PR-deficient Pol precursor during virus production, infectious, viral DNA-containing particles were obtained, even when different vector or proviral expression systems were used. Although virion infectivity was reduced to 0.5 to 2% relative to that of the respective parental constructs, this finding overturns the current dogma in the FV literature that viral PR activity is absolutely essential at some point during target cell entry. Furthermore, it demonstrates that viral PR-mediated Gag precursor processing during particle assembly initiates intraparticle RTr. Finally, it shows that reverse transcriptase (RT) and integrase are enzymatically active in the Pol precursor within the viral capsid, thus enabling productive host cell infection.

The prototype foamy virus protease is active independently of the integrase domain

Background

Recently, contradictory results on foamy virus protease activity were published. While our own results indicated that protease activity is regulated by the viral RNA, others suggested that the integrase is involved in the regulation of the protease.

Results

To solve this discrepancy we performed additional experiments showing that the protease-reverse transcriptase (PR-RT) exhibits protease activity in vitro and in vivo, which is independent of the integrase domain. In contrast, Pol incorporation, and therefore PR activity in the viral context, is dependent on the integrase domain. To further analyse the regulation of the protease, we incorporated Pol in viruses by expressing a GagPol fusion protein, which supported near wild-type like infectivity. A GagPR-RT fusion, lacking the integrase domain, also resulted in wild-type like Gag processing, indicating that the integrase is dispensable for viral Gag maturation. Furthermore, we demonstrate with a trans-complementation assays that the PR in the context of the PR-RT protein supports in trans both, viral maturation and infectivity.

Conclusion

We provide evidence that the FV integrase is required for Pol encapsidation and that the FV PR activity is integrase independent. We show that an active PR can be encapsidated in trans as a GagPR-RT fusion protein.

Insights into the structure and activity of prototype foamy virus RNase H

BACKGROUND

RNase H is an endonuclease that hydrolyzes the RNA strand in RNA/DNA hybrids. Retroviral reverse transcriptases harbor a C-terminal RNase H domain whose activity is essential for viral replication. The RNase H degrades the viral genomic RNA after the first DNA strand is synthesized. Here, we report the biophysical and enzymatic properties of the RNase H domain of prototype foamy virus (PFV) as an independently purified protein. Sequence comparisons with other retroviral RNases H indicated that PFV RNase H harbors a basic protrusion, including a basic loop and the so-called C-helix, which was suggested to be important for activity and substrate binding and is absent in the RNase H domain of human immunodeficiency virus. So far, no structure of a retroviral RNase H containing a C-helix is available.

RESULTS

RNase H activity assays demonstrate that the PFV RNase H domain is active, although its activity is about 200-fold reduced as compared to the full length protease-reverse transcriptase enzyme. Fluorescence equilibrium titrations with an RNA/DNA substrate revealed a KD for the RNase H domain in the low micromolar range which is about 4000-fold higher than that of the full-length protease-reverse transcriptase enzyme. Analysis of the RNase H cleavage pattern using a [32P]-labeled substrate indicates that the independent RNase H domain cleaves the substrate non-specifically. The purified RNase H domain exhibits a well defined three-dimensional structure in solution which is stabilized in the presence of Mg2+ ions.

CONCLUSIONS

Our data demonstrate that the independent PFV RNase H domain is structured and active. The presence of the C-helix in PFV RNase H could be confirmed by assigning the protein backbone and calculating the chemical shift index using NMR spectroscopy.

Fast mapping of biomolecular interfaces by Random Spin Labeling (RSL)

Random spin labeling (RSL) is a method for rapid mapping of biomolecular interaction surfaces using an interaction partner with SL and an interaction partner enriched in (13)C or (15)N nuclei for paramagnetic relaxation enhanced NMR-based detection. The SL reaction is conducted in a manner resulting in a heterogeneous reaction product consisting of different populations of the protein carrying a varying number of spin labels at different positions. Preparation of the paramagnetic probe is complete within a few hours and hence much faster than site selective SL. RSL is applicable to tightly interacting systems but shows its particular strength when applied to systems involving weak or transient contacts.

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.

Foamy virus biology and its application for vector development

Spuma- or foamy viruses (FV), endemic in most non-human primates, cats, cattle and horses, comprise a special type of retrovirus that has developed a replication strategy combining features of both retroviruses and hepadnaviruses. Unique features of FVs include an apparent apathogenicity in natural hosts as well as zoonotically infected humans, a reverse transcription of the packaged viral RNA genome late during viral replication resulting in an infectious DNA genome in released FV particles and a special particle release strategy depending capsid and glycoprotein coexpression and specific interaction between both components. In addition, particular features with respect to the integration profile into the host genomic DNA discriminate FV from orthoretroviruses. It appears that some inherent properties of FV vectors set them favorably apart from orthoretroviral vectors and ask for additional basic research on the viruses as well as on the application in Gene Therapy. This review will summarize the current knowledge of FV biology and the development as a gene transfer system.

Regulation of foamy virus protease activity by viral RNA: a novel and unique mechanism among retroviruses

Foamy viruses (FVs) synthesize the Pol precursor protein from a specific transcript. Thus, in contrast to what was found for orthoretroviruses, e.g., human immunodeficiency virus, no Gag-Pol precursor protein is synthesized. Foamy viral Pol consists of a protease (PR) domain, a reverse transcriptase domain, and an integrase domain and is processed into a mature protease-reverse transcriptase (PR-RT) fusion protein and the integrase. Protease activity has to be strictly regulated in order to avoid premature Gag and Pol processing before virus assembly. We have demonstrated recently that FV protease is an inactive monomer with a very weak dimerization tendency and postulated protease activation through dimerization. Here, we identify a specific protease-activating RNA motif (PARM) located in the pol region of viral RNA which stimulates PR activity in vitro and in vivo, revealing a novel and unique mechanism of retroviral protease activation. This mechanism is strikingly different to that of orthoretroviruses, where the protease can be activated even in the absence of viral RNA during the assembly of virus-like particles. Although it has been shown that the integrase domain is important for Pol uptake, activation of the foamy virus protease is integrase independent. We show that at least two foamy virus PR-RT molecules bind to the PARM and only RNAs containing the PARM result in significant activation of the protease. DNA harboring the PARM is not capable of protease activation. Structure determination of the PARM by selective 2' hydroxyl acylation analyzed by primer extension (SHAPE) revealed a distinct RNA folding, important for protease activation and thus virus maturation.

Crystal structure of XMRV protease differs from the structures of other retropepsins

Using energy and density guided Rosetta refinement to improve molecular replacement, we determined the crystal structure of the protease encoded by xenotropic murine leukemia virus-related virus (XMRV). Despite overall similarity of XMRV protease to other retropepsins, the topology of its dimer interface more closely resembles those of the monomeric, pepsin-like enzymes. Thus, XMRV protease may represent a distinct branch of the aspartic protease family.

Foamy retrovirus integrase contains a Pol dimerization domain required for protease activation

Unlike orthoretroviruses, foamy retroviruses (FV) synthesize Pol independently of Gag. The FV Pol precursor is cleaved only once between reverse transcriptase (RT) and integrase (IN) by the protease (PR), resulting in a PR-RT and an IN protein. Only the Pol precursor, not the cleaved subunits, is packaged into virions. Like orthoretroviral PRs, FV PR needs to dimerize to be active. Previously, we showed that a Pol mutant lacking IN has defects in PR activity and Pol packaging into virions. We now show that introduction of a leucine zipper (zip) dimerization motif in an IN truncation mutant can restore PR activity, leading to Pol processing in cells. However, these zip mutants neither cleave Gag nor incorporate Pol into virions. We propose that IN is required for Pol dimerization, which is necessary for the creation of a functional PR active site.

Molecular biology of foamy viruses

One of the most fascinating areas in retrovirology is the study of foamy viruses (FVs), because these viruses appear to do everything that is common to all other retroviruses differently. FVs have found a completely new way to propagate their genome. And they do this extremely successfully because most of wild non-human primates, felines, bovines, equines, and small ruminants are likely to be non-pathogenically infected. The success of FVs can also be viewed from a different angle, since they replicate very conservatively and do not need to shape their genotypic and phenotypic makeup every now and then. The elucidation of the underlying basic mechanisms of the FV replication strategy is the topic of this review.

Formation of transient dimers by a retroviral protease

Retroviral proteases have been shown previously to be only active as homodimers. They are essential to form the separate and active proteins from the viral precursors. Spumaretroviruses produce separate precursors for Gag and Pol, rather than a Gag and a Gag-Pol precursor. Nevertheless, processing of Pol into a PR (protease)-RT (reverse transcriptase) and integrase is essential in order to obtain infectious viral particles. We showed recently that the PR-RT from a simian foamy virus, as well as the separate PRshort (protease) domain, exhibit proteolytic activities, although only monomeric forms could be detected. In the present study, we demonstrate that PRshort and PR-RT can be inhibited by the putative dimerization inhibitor cholic acid. Various other inhibitors, including darunavir and tipranavir, known to prevent HIV-1 PR dimerization in cells, had no effect on foamy virus protease in vitro. 1H-15N HSQC (heteronuclear single quantum coherence) NMR analysis of PRshort indicates that cholic acid binds in the proposed PRshort dimerization interface and appears to impair formation of the correct dimer. NMR analysis by paramagnetic relaxation enhancement resulted in elevated transverse relaxation rates of those amino acids predicted to participate in dimer formation. Our results suggest transient PRshort homodimers are formed under native conditions but are only present as a minor transient species, which is not detectable by traditional methods.

Biophysical and enzymatic properties of the simian and prototype foamy virus reverse transcriptases

Background

The foamy virus Pol protein is translated independently from Gag using a separate mRNA. Thus, in contrast to orthoretroviruses no Gag-Pol precursor protein is synthesized. Only the integrase domain is cleaved off from Pol resulting in a mature reverse transcriptase harboring the protease domain at the N-terminus (PR-RT). Although the homology between the PR-RTs from simian foamy virus from macaques (SFVmac) and the prototype foamy virus (PFV), probably originating from chimpanzee, exceeds 90%, several differences in the biophysical and biochemical properties of the two enzymes have been reported (i.e. SFVmac develops resistance to the nucleoside inhibitor azidothymidine (AZT) whereas PFV remains AZT sensitive even if the resistance mutations from SFVmac PR-RT are introduced into the PFV PR-RT gene). Moreover, contradictory data on the monomer/dimer status of the foamy virus protease have been published.

Results

We set out to purify and directly compare the monomer/dimer status and the enzymatic behavior of the two wild type PR-RT enzymes from SFVmac and PFV in order to get a better understanding of the protein and enzyme functions. We determined kinetic parameters for the two enzymes, and we show that PFV PR-RT is also a monomeric protein.

Conclusions

Our data show that the PR-RTs from SFV and PFV are monomeric proteins with similar biochemical and biophysical properties that are in some aspects comparable with MLV RT, but differ from those of HIV-1 RT. These differences might be due to the different conditions the viruses are confronted with in dividing and non-dividing cells.

Bioinformatic flowchart and database to investigate the origins and diversity of clan AA peptidases

BACKGROUND

Clan AA of aspartic peptidases relates the family of pepsin monomers evolutionarily with all dimeric peptidases encoded by eukaryotic LTR retroelements. Recent findings describing various pools of single-domain nonviral host peptidases, in prokaryotes and eukaryotes, indicate that the diversity of clan AA is larger than previously thought. The ensuing approach to investigate this enzyme group is by studying its phylogeny. However, clan AA is a difficult case to study due to the low similarity and different rates of evolution. This work is an ongoing attempt to investigate the different clan AA families to understand the cause of their diversity.

RESULTS

In this paper, we describe in-progress database and bioinformatic flowchart designed to characterize the clan AA protein domain based on all possible protein families through ancestral reconstructions, sequence logos, and hidden markov models (HMMs). The flowchart includes the characterization of a major consensus sequence based on 6 amino acid patterns with correspondence with Andreeva's model, the structural template describing the clan AA peptidase fold. The set of tools is work in progress we have organized in a database within the GyDB project, referred to as Clan AA Reference Database http://gydb.uv.es/gydb/phylogeny.php?tree=caard.

CONCLUSION

The pre-existing classification combined with the evolutionary history of LTR retroelements permits a consistent taxonomical collection of sequence logos and HMMs. This set is useful for gene annotation but also a reference to evaluate the diversity of, and the relationships among, the different families. Comparisons among HMMs suggest a common ancestor for all dimeric clan AA peptidases that is halfway between single-domain nonviral peptidases and those coded by Ty3/Gypsy LTR retroelements. Sequence logos reveal how all clan AA families follow similar protein domain architecture related to the peptidase fold. In particular, each family nucleates a particular consensus motif in the sequence position related to the flap. The different motifs constitute a network where an alanine-asparagine-like variable motif predominates, instead of the canonical flap of the HIV-1 peptidase and closer relatives.