trans-Complementation rescue of cyclophilin A-deficient viruses reveals that the requirement for cyclophilin A in human immunodeficiency virus type 1 replication is independent of its isomerase activity

Fluorescent protein-tagged Vpr dissociates from HIV-1 core after viral fusion and rapidly enters the cell nucleus

Background

HIV-1 Vpr is recruited into virions during assembly and appears to remain associated with the viral core after the reverse transcription and uncoating steps of entry. This feature has prompted the use of fluorescently labeled Vpr to visualize viral particles and to follow trafficking of post-fusion HIV-1 cores in the cytoplasm.

Results

Here, we tracked single pseudovirus entry and fusion and observed that fluorescently tagged Vpr gradually dissociates from post-fusion viral cores over the course of several minutes and accumulates in the nucleus. Kinetics measurements showed that fluorescent Vpr released from the cores very rapidly entered the cell nucleus. More than 10,000 Vpr molecules can be delivered into the cell nucleus within 45 min of infection by HIV-1 particles pseudotyped with the avian sarcoma and leukosis virus envelope glycoprotein. The fraction of Vpr from cell-bound viruses that accumulated in the nucleus was proportional to the extent of virus-cell fusion and was fully blocked by viral fusion inhibitors. Entry of virus-derived Vpr into the nucleus occurred independently of envelope glycoproteins or target cells. Fluorescence correlation spectroscopy revealed two forms of nuclear Vpr—monomers and very large complexes, likely involving host factors. The kinetics of viral Vpr entering the nucleus after fusion was not affected by point mutations in the capsid protein that alter the stability of the viral core.

Conclusions

The independence of Vpr shedding of capsid stability and its relatively rapid dissociation from post-fusion cores suggest that this process may precede capsid uncoating, which appears to occur on a slower time scale. Our results thus demonstrate that a bulk of fluorescently labeled Vpr incorporated into HIV-1 particles is released shortly after fusion. Future studies will address the question whether the quick and efficient nuclear delivery of Vpr derived from incoming viruses can regulate subsequent steps of HIV-1 infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0215-z) contains supplementary material, which is available to authorized users.

The next generation of HIV/AIDS drugs: novel and developmental antiHIV drugs and targets

There are presently 42 million people worldwide living with HIV/AIDS, the majority of which have limited access to antiretrovirals. Even if worldwide penetration was possible, our current chemotherapeutic strategies still suffer from issues of cost, patient compliance, deleterious acute and chronic side effects, emerging single and multidrug resistance, and generalized treatment and economic issues. Even our best antiretroviral therapeutic strategy, highly active antiretroviral therapy (HAART), falls short of completely suppressing HIV replication. Therefore, expansion of current therapeutic options by discovering new antiretrovirals and targets will be critical in the coming years. This review addresses the current status of reverse transcriptase and protease inhibitor development, and summarizes the progress in emerging classes of HIV inhibitors, including entry (T-20, T-1249), coreceptor (SCH-C, SCH-D), integrase (beta-Diketos) and p7 nucleocapsid Zn finger inhibitors (thioesters and PATEs). In addition, the processes of virus entry, PIC transport to the nucleus, HIV interaction with nuclear pores, Tat function, Rev function and virus budding (Tsg101 and ubiquitination) are examined, and proof of concept inhibitors and potential antiviral targets discussed.

The host-pathogen interaction of human cyclophilin A and HIV-1 Vpr requires specific N-terminal and novel C-terminal domains

BACKGROUND

Cyclophilin A (CypA) represents a potential key molecule in future antiretroviral therapy since inhibition of CypA suppresses human immunodeficiency virus type 1 (HIV-1) replication. CypA interacts with the virus proteins Capsid (CA) and Vpr, however, the mechanism through which CypA influences HIV-1 infectivity still remains unclear.

RESULTS

Here the interaction of full-length HIV-1 Vpr with the host cellular factor CypA has been characterized and quantified by surface plasmon resonance spectroscopy. A C-terminal region of Vpr, comprising the 16 residues 75GCRHSRIGVTRQRRAR90, with high binding affinity for CypA has been identified. This region of Vpr does not contain any proline residues but binds much more strongly to CypA than the previously characterized N-terminal binding domain of Vpr, and is thus the first protein binding domain to CypA described involving no proline residues. The fact that the mutant peptide Vpr75-90 R80A binds more weakly to CypA than the wild-type peptide confirms that Arg-80 is a key residue in the C-terminal binding domain. The N- and C-terminal binding regions of full-length Vpr bind cooperatively to CypA and have allowed a model of the complex to be created. The dissociation constant of full-length Vpr to CypA was determined to be approximately 320 nM, indicating that the binding may be stronger than that of the well characterized interaction of HIV-1 CA with CypA.

CONCLUSIONS

For the first time the interaction of full-length Vpr and CypA has been characterized and quantified. A non-proline-containing 16-residue region of C-terminal Vpr which binds specifically to CypA with similar high affinity as full-length Vpr has been identified. The fact that this is the first non-proline containing binding motif of any protein found to bind to CypA, changes the view on how CypA is able to interact with other proteins. It is interesting to note that several previously reported key functions of HIV-1 Vpr are associated with the identified N- and C-terminal binding domains of the protein to CypA.

Dissecting the microscopic steps of the cyclophilin A enzymatic cycle on the biological HIV-1 capsid substrate by NMR

Peptidyl-prolyl isomerases (PPIases) are emerging as key regulators of many diverse biological processes. Elucidating the role of PPIase activity in vivo has been challenging because mutagenesis of active-site residues not only reduces the catalytic activity of these enzymes but also dramatically affects substrate binding. Employing the cyclophilin A PPIase together with its biologically relevant and natively folded substrate, the N-terminal domain of the human immunodeficiency virus type 1 capsid (CA(N)) protein, we demonstrate here how to dissect residue-specific contributions to PPIase catalysis versus substrate binding utilizing NMR spectroscopy. Surprisingly, a number of cyclophilin A active-site mutants previously assumed to be strongly diminished in activity toward biological substrates based only on a peptide assay catalyze the human immunodeficiency virus capsid with wild-type activity but with a change in the rate-limiting step of the enzymatic cycle. The results illustrate that a quantitative analysis of catalysis using the biological substrates is critical when interpreting the effects of PPIase mutations in biological assays.

Target cell type-dependent modulation of human immunodeficiency virus type 1 capsid disassembly by cyclophilin A

The binding of cyclophilin A (CypA) to the human immunodeficiency virus type 1 (HIV-1) capsid protein (CA protein) is required soon after virus entry into natural target cells. In Jurkat T lymphocytes, disrupting CypA-CA interaction either by cyclosporine (Cs) treatment or by alteration (e.g., P90A) of the CA inhibits HIV-1 infection. In HeLa cells, however, treatment with Cs or Cs analogues minimally inhibits the early phase of HIV-1 infection but selects for a Cs-dependent virus with a change (A92E) in CA. To understand these phenomena, we examined the effects of the P90A and A92E changes in the HIV-1 CA protein on the stability of capsid complexes assembled in vitro and on capsid disassembly in the cytosol of virus-exposed target cells. The A92E change impaired CA-CA interactions in vitro and decreased the amount of particulate capsids in the cytosol of HeLa target cells. Reducing the binding of CypA to the A92E mutant capsid, either by Cs treatment or by an additional P90A change in the CA protein, increased the amount of particulate capsids and viral infectivity in HeLa cells. In contrast, reduction of the binding of CypA to HIV-1 capsids in Jurkat T lymphocytes resulted in a decrease in the amount of particulate capsids and infectivity. Thus, depending on the capsid and the target cell, CypA-CA binding either stabilized or destabilized the capsid, indicating that CypA modulates HIV-1 capsid disassembly. In both cell types examined, decreased stability of the capsid was associated with a decrease in the efficiency of HIV-1 infection.

Proteomic Analysis of Human Immunodeficiency Virus Using Liquid Chromatography/Tandem Mass Spectrometry Effectively Distinguishes Specific Incorporated Host Proteins

A major challenge to studying virus-incorporated host proteins is the fact that they are not encoded by the viral genome. We used Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS) on whole virions to obtain a snapshot of the HIV-1 proteome. We identified known viral and host-cellular proteins and also identified novel components of HIV-1 and confirm these by traditional biochemical methods. Our comparison of wild-type and mutant viruses demonstrates that LC-MS/MS has the specificity to distinguish the presence/absence of a single host protein in intact virions.

Enzymes: An integrated view of structure, dynamics and function

Microbes utilize enzymes to perform a variety of functions. Enzymes are biocatalysts working as highly efficient machines at the molecular level. In the past, enzymes have been viewed as static entities and their function has been explained on the basis of direct structural interactions between the enzyme and the substrate. A variety of experimental and computational techniques, however, continue to reveal that proteins are dynamically active machines, with various parts exhibiting internal motions at a wide range of time-scales. Increasing evidence also indicates that these internal protein motions play a role in promoting protein function such as enzyme catalysis. Moreover, the thermodynamical fluctuations of the solvent, surrounding the protein, have an impact on internal protein motions and, therefore, on enzyme function. In this review, we describe recent biochemical and theoretical investigations of internal protein dynamics linked to enzyme catalysis. In the enzyme cyclophilin A, investigations have lead to the discovery of a network of protein vibrations promoting catalysis. Cyclophilin A catalyzes peptidyl-prolyl cis/trans isomerization in a variety of peptide and protein substrates. Recent studies of cyclophilin A are discussed in detail and other enzymes (dihydrofolate reductase and liver alcohol dehydrogenase) where similar discoveries have been reported are also briefly discussed. The detailed characterization of the discovered networks indicates that protein dynamics plays a role in rate-enhancement achieved by enzymes. An integrated view of enzyme structure, dynamics and function have wide implications in understanding allosteric and co-operative effects, as well as protein engineering of more efficient enzymes and novel drug design.

Pharmacological targeting of catalyzed protein folding: the example of peptide bond cis/trans isomerases

Peptide bond isomerases are involved in important physiological processes that can be targeted in order to treat neurodegenerative disease, cancer, diseases of the immune system, allergies, and many others. The folding helper enzyme class of Peptidyl-Prolyl-cis/trans Isomerases (PPIases) contains the three enzyme families of cyclophilins (Cyps), FK506 binding proteins (FKBPs), and parvulins (Pars). Although they are structurally unrelated, all PPIases catalyze the cis/trans isomerization of the peptide bond preceding the proline in a polypeptide chain. This process not only plays an important role in de novo protein folding, but also in isomerization of native proteins. The native state isomerization plays a role in physiological processes by influencing receptor ligand recognition or isomer-specific enzyme reaction or by regulating protein function by catalyzing the switch between native isomers differing in their activity, e.g., ion channel regulation. Therefore elucidating PPIase involvement in physiological processes and development of specific inhibitors will be a suitable attempt to design therapies for fatal and deadly diseases.

[Multiple isoforms of cyclophilin A associated with human immunodeficiency virus type 1]

It is well-known that a peptidyl-prolyl cis-trans isomerase cyclophilin A (CyPA) is incorporated into Human immunodeficiency virus type 1 (HIV-1) particle. The proteome analysis of the purified HIV-1 strain LAV-1 (HIV-1(LAV-1)) reveals that three isoforms of CyPA with an isoelectric point (pI) of 6.00, 6.40, and 6.53 are inside the viral membrane and another isoform with a pI of 6.88 is outside the viral membrane; and that the CyPA isoform with a pI of 6.53 is N-acetylated. The mechanisms that permit the redistribution of CyPA with a pI of 6.88 on the viral surface have not yet been clarified, but it penetrates the viral membrane after budding.

Cis/trans isomerization in HIV-1 capsid protein catalyzed by cyclophilin A: insights from computational and theoretical studies

A network of protein vibrations has recently been identified in the enzyme cyclophilin A (CypA) that is associated with its peptidyl-prolyl cis/trans isomerization activity of small peptide substrates. It has been suggested that this network may have a role in promoting the catalytic step during the isomerization reaction. This work presents the results from the characterization of this network during the isomerization of the Gly89-Pro90 peptide bond in the N-terminal domain of the capsid protein (CA(N)) from human immunodeficiency virus type 1 (HIV-1), which is a naturally occurring, biologically relevant protein substrate for CypA. A variety of computational and theoretical studies are utilized to investigate the protein dynamics of the CypA-CA(N) complex, at multiple time scales, during the isomerization step. The results provide insights into the detailed mechanism of isomerization and confirm the presence of previously reported network of protein vibrations coupled to the reaction. Conserved CypA residues at the complex interface and at positions distal to the interface form parts of this network. There is HIV-1 related medical interest in CypA; incorporation of CypA, complexed with the capsid protein, into the virion is required for the infectious activity of HIV-1. Interaction energy and dynamical cross-correlation calculations are used for a detailed investigation of the protein-protein interactions in the CypA-CA(N) complex. The results show that CA(N) residues His87-Ala-Gly-Pro-Ile-Ala92 form the majority of the interactions with CypA residues. New protein-protein interactions distal to the active site (CypA Arg148-CA(N) Gln95 and CypA Arg148-CA(N) Asn121) are also identified.

Cyclophilin A functions as an endogenous inhibitor for membrane-bound guanylate cyclase-A

Cyclophilin A (CypA), a receptor for the immunosuppressive agent cyclosporin A, is a cis-trans-peptidyl-prolyl isomerase (PPIase). It accelerates the cis-trans isomerization of prolyl-peptide bonds. CypA binds and regulates the activity of a variety of proteins. Atrial natriuretic factor (ANF) and its receptor membrane-bound guanylate cyclase-A (GC-A) are involved in the regulation of blood pressure. We examined whether CypA affects the activation of GC-A by ANF. The results showed that CypA associated with GC-A. Interestingly, binding of ANF to GC-A released CypA. Transfection of CypA inhibited ANF-stimulated GC-A activity, indicating that CypA functions as an endogenous inhibitor for GC-A activation. CypA also inhibits the activity of guanylate cyclase-C (GC-c), the catalytic domain of GC-A, indicating that CypA interacts with the catalytic domain of GC-A. In contrast, transfection of CypA R55A, a CypA mutant expressing low PPIase activity, did not significantly attenuate the activity of GC-c and the activation of GC-A. Inhibition of PPIase activity of CypA with cyclosporin A also blocks the inhibitory effect of CypA on GC-c activity. These results demonstrate that PPIase activity is required for CypA to inhibit GC-c activity and GC-A activation by ANF. Furthermore, mutation of Pro 822, 902, or 958 in GC-c abolished its activity. Therefore, it is likely that CypA binds to GC-A and catalyzes the cis-trans isomerization of Pro 822, 902, or 958, which keeps GC-A in the inactive state, and that binding of ANF to GC-A alters the conformation of the catalytic domain that releases CypA from GC-A leading to enzyme activation.

Mutation of Phe50 to Ser50 in the 126/183-kDa proteins of Odontoglossum ringspot virus abolishes virus replication but can be complemented and restored by exact reversion

Sequence comparison of a non-biologically active full-length cDNA clone of Odontoglossum ringspot virus (ORSV) pOT1 with a biologically active ORSV cDNA clone pOT2 revealed a single nucleotide change of T-->C at position 211. This resulted in the change of Phe50 in OT2 to Ser50 in OT1. It was not the nucleotide but the amino acid change of Phe50 that was responsible for the inability of OT1 to replicate. Time-course experiments showed that no minus-strand RNA synthesis was detected in mutants with a Phe50 substitution. Corresponding mutants in Tobacco mosaic virus (TMV) showed identical results, suggesting that Phe50 may play an important role in replication in all tobamoviruses. Complementation of a full-length mutant OT1 was demonstrated in a co-infected local-lesion host, a systemic host and protoplasts by replication-competent mutants tORSV.GFP or tORSV.GFPm, and further confirmed by co-inoculation using tOT1.GFP+tORSV (TTC), suggesting that ORSV contains no RNA sequence inhibitory to replication in trans. Surprisingly, a small number of exact revertants were detected in plants inoculated with tOT1+tORSV.GFPm or tOT1.GFP+tORSV (TTC). No recombination was detected after screening of silent markers in virus progeny extracted from total RNA or viral RNA from inoculated and upper non-inoculated leaves as well as from transfected protoplasts. Exact reversion from TCT (OT1) to TTT (OT2), rather than recombination, restored its replication function in co-inoculated leaves of Nicotiana benthamiana.

Reconstitution of retroviral fusion and uncoating in a cell-free system

The molecular events underlying the immediate steps of retroviral uncoating, occurring after membrane fusion and leading to the formation of an active reverse transcription complex, are not known. To better understand these processes, we have developed a cell-free system that recapitulates these early steps of retroviral replication by using avian sarcoma and leukosis virus as a model retrovirus. The substrates used in this system are viral particles that are trapped before completing membrane fusion. These virions are induced to fuse out of endosomes and the viral cores are released into solution where they are amenable to biochemical manipulation. This system revealed that membrane fusion is not sufficient to stimulate the formation of a reverse transcription complex. Instead, ATP hydrolysis and cellular factors >5 kDa in size are required. Furthermore, later steps of avian sarcoma and leukosis virus reverse transcription were stimulated by nuclear factors. The cell-free system should now allow for the definition of retroviral uncoating mechanisms and facilitate the identification and characterization of the cellular factors involved.

Chimeric human immunodeficiency virus type 1 (HIV-1) virions containing HIV-2 or simian immunodeficiency virus Nef are resistant to cyclosporine treatment

The viral protein Nef and the cellular factor cyclophilin A are both required for full infectivity of human immunodeficiency virus type 1 (HIV-1) virions. In contrast, HIV-2 and simian immunodeficiency virus (SIV) do not incorporate cyclophilin A into virions or need it for full infectivity. Since Nef and cyclophilin A appear to act in similar ways on postentry events, we determined whether chimeric HIV-1 virions that contained either HIV-2 or SIV Nef would have a direct effect on cyclophilin A dependence. Our results show that chimeric HIV-1 virions containing either HIV-2 or SIV Nef are resistant to treatment by cyclosporine and enhance the infectivity of virions with mutations in the cyclophilin A binding loop of Gag. Amino acids at the C terminus of HIV-2 and SIV are necessary for inducing cyclosporine resistance. However, transferring these amino acids to the C terminus of HIV-1 Nef is insufficient to induce cyclosporine resistance in HIV-1. These results suggest that HIV-2 and SIV Nef are able to compensate for the need for cyclophilin A for full infectivity and that amino acids present at the C termini of these proteins are important for this function.

Regulation of peptide bond cis/trans isomerization by enzyme catalysis and its implication in physiological processes

In some cases, the slow rotational movement underlying peptide bond cis/trans isomerizations is found to control the biological activity of proteins. Peptide bond cis/trans isomerases as cyclophilins, Fk506-binding proteins, parvulins, and bacterial hsp70 generally assist in the interconversion of the polypeptide substrate cis/trans isomers, and rate acceleration is the dominating mechanism of action in cells. We present evidence disputing the hypothesis that some of the molecular properties of these proteins play an auxiliary role in enzyme function.

Cyclophilin D, a component of the permeability transition-pore, is an apoptosis repressor

The permeability transition (PT)-pore is an important proapoptotic protein complex in mitochondria. Although it is activated by many signals for apoptosis induction, the role of its various subunits in cell death induction has remained largely unknown. We found that of its components, only the voltage-dependent anion channel in the outer mitochondrial membrane and the adenine nucleotide translocator-1 (ANT-1), a PT-pore subunit of the inner membrane, are apoptosis inducers. We also report that ANT-1's direct interactor, cyclophilin D, can specifically repress ANT-1-induced apoptosis. In addition, cotransfection experiments revealed that for a diverse range of apoptosis inducers, cyclophilin D shows the same repression profile as the compound bongkrekic acid, a specific inhibitor of the PT-pore. This activity seems to be independent of its chaperone activity, the only known function of cyclophilin D to date. Importantly, cyclophilin D is specifically up-regulated in human tumors of the breast, ovary, and uterus, suggesting that inhibition of the PT-pore via up-regulation of cyclophilin D plays a role in tumorigenesis.

Intracellular trafficking of HIV-1 cores: journey to the center of the cell

After entry into the cytoplasm, many diverse viruses, including both RNA and DNA viruses, require import into the nucleus and access to the cellular nuclear machinery for productive replication to proceed. Because diffusion through the crowded cytoplasmic environment is greatly restricted, most (if not all) of these viruses must first be actively transported from the site of cytoplasmic entry to the nuclear periphery (Luby-Phelps 2000; Lukacs et al. 2000; Sodeik 2000). Having reached the nucleus, viruses have evolved assorted methods to overcome the formidable physical barrier that is presented by the nuclear envelope. This review examines how these issues relate to human immunodeficiency virus type-1 (HIV-1) infection. Specifically, HIV-1 uncoating, cytoplasmic transport, and nuclear entry are addressed.

Cyclophilin A interacts with HIV-1 Vpr and is required for its functional expression

Viral protein R (Vpr) of human immunodeficiency virus, type 1 (HIV-1) is the major virion-associated accessory protein that affects a number of biological functions in the retroviral life cycle, including promotion of the transport of the preintegration complex into the nucleus and the induction of G2 host cell cycle arrest. Our recent investigation of the conformational heterogeneity of the proline residues in the N terminus of Vpr suggested a functional interaction between Vpr and a host peptidylprolyl cis/trans isomerase (PPIase) that might regulate the cis/trans interconversion of the imidic bond within the conserved proline residues of Vpr in vivo. Using surface plasmon resonance spectroscopy, Far Western blot, and pulldown experiments a physical interaction of Vpr with the major host PPIase cyclophilin A (CypA) is now demonstrated. The interaction domain involves the N-terminal region of Vpr including an essential role for proline in position 35. The CypA inhibitor cyclosporin A and non-immunosuppressive PPIase inhibitors such as NIM811 and sanglifehrin A block expression of Vpr without affecting pre- or post-translational events such as transcription, intracellular transport, or virus incorporation of Vpr. Similarly to CypA inhibition, Vpr expression is also reduced in HIV-1 infected CypA-/- knock-out T cells. This study thus shows that in addition to the interaction between CypA and HIV-1 capsid occurring during early steps in virus replication, CypA is also important for the de novo synthesis of Vpr and that in the absence of CypA activity, the Vpr-mediated cell cycle arrest is completely lost in HIV-1-infected T cells.

Role of HIV-1 Gag domains in viral assembly

After entry of the human immunodeficiency virus type 1 (HIV-1) into T cells and the subsequent synthesis of viral products, viral proteins and RNA must somehow find each other in the host cells and assemble on the plasma membrane to form the budding viral particle. In this general review of HIV-1 assembly, we present a brief overview of the HIV life cycle and then discuss assembly of the HIV Gag polyprotein on RNA and membrane substrates from a biochemical perspective. The role of the domains of Gag in targeting to the plasma membrane and the role of the cellular host protein cyclophilin are also reviewed.

Structural insights into the catalytic mechanism of cyclophilin A

Cyclophilins constitute a ubiquitous protein family whose functions include protein folding, transport and signaling. They possess both sequence-specific binding and proline cis-trans isomerase activities, as exemplified by the interaction between cyclophilin A (CypA) and the HIV-1 CA protein. Here, we report crystal structures of CypA in complex with HIV-1 CA protein variants that bind preferentially with the substrate proline residue in either the cis or the trans conformation. Cis- and trans-Pro substrates are accommodated within the enzyme active site by rearrangement of their N-terminal residues and with minimal distortions in the path of the main chain. CypA Arg55 guanidinium group probably facilitates catalysis by anchoring the substrate proline oxygen and stabilizing sp3 hybridization of the proline nitrogen in the transition state.

Treatment of human immunodeficiency virus type 1 virions depleted of cyclophilin A by natural endogenous reverse transcription restores infectivity

We have previously shown that virions with nef deleted can be restored to wild-type infectivity by treatment to induce natural endogenous reverse transcription (NERT). Since Nef and cyclophilin A (CyPA) appear to act in similar ways on postentry events, we determined whether NERT treatment would restore infectivity to virions depleted of CyPA. Our results show that the infectivity of virions depleted of CyPA by treatment with cyclosporine A could be restored by NERT treatment, while mutants in the CyPA binding loop of capsid could only be partially restored. These results suggest that CyPA is involved in some aspect of the uncoating process.

Potential roles of cellular proteins in HIV-1

Human immunodeficiency virus type-1 particles contain host proteins, both on their surface and interior. This review summarises the cellular proteins found in these virions and covers some of their potential roles in the viral life cycle and pathogenesis. For most proteins studied, their role and function are either unknown or in the hypothesis stage. This reflects the relatively recent emphasis given to these proteins by the HIV-1 field as well as the incomplete understanding of their function in the cell. The study of cellular proteins in HIV-1 promises to help us better understand the interaction of this virus with the cell, the immune system, and the whole human host as well as to shed light on the nature of AIDS and suggest more targets for therapeutic intervention. Finally, many of the cell systems themselves are still poorly understood. The extensive study of HIV-1 has already brought increased attention to the fields of immunology and vaccine science and, in the same way, might assist our understanding of the cellular pathways themselves.

Cyclophilin a plays distinct roles in human immunodeficiency virus type 1 entry and postentry events, as revealed by spinoculation

Cyclophilin A (CypA) is necessary for effective human immunodeficiency virus type 1 (HIV-1) replication. However, the functions of CypA and the precise steps at which CypA acts in the HIV-1 life cycle remain to be determined. By using a methodology that bypasses the need for attachment factors-spinoculation-we present evidence that CypA participates in both entry and postentry events.