Structural investigation of the HIV-1 envelope glycoprotein gp160 cleavage site

Viral envelope glycoprotein processing by proprotein convertases

The proprotein convertases (PCs) are a family of nine mammalian enzymes that play key roles in the maintenance of cell homeostasis by activating or inactivating proteins via limited proteolysis under temporal and spatial control. A wide range of pathogens, including major human pathogenic viruses can hijack cellular PCs for their own purposes. In particular, productive infection with many enveloped viruses critically depends on the processing of their fusion-active viral envelope glycoproteins by cellular PCs. Based on their crucial role in virus-host interaction, PCs can be important determinants for viral pathogenesis and represent promising targets of therapeutic antiviral intervention. In the present review we will cover basic aspects and recent developments of PC-mediated maturation of viral envelope glycoproteins of selected medically important viruses. The molecular mechanisms underlying the recognition of PCs by viral glycoproteins will be described, including recent findings demonstrating differential PC-recognition of viral and cellular substrates. We will further discuss a possible scenario how viruses during co-evolution with their hosts adapted their glycoproteins to modulate the activity of cellular PCs for their own benefit and discuss the consequences for virus-host interaction and pathogenesis. Particular attention will be given to past and current efforts to evaluate cellular PCs as targets for antiviral therapeutic intervention, with emphasis on emerging highly pathogenic viruses for which no efficacious drugs or vaccines are currently available.

Domain flexibility modulates the heterogeneous assembly mechanism of anthrax toxin protective antigen

The three protein components of anthrax toxin are nontoxic individually, but they form active holotoxin complexes upon assembly. The role of the protective antigen (PA) component of the toxin is to deliver two other enzyme components, lethal factor and edema factor, across the plasma membrane and into the cytoplasm of target cells. PA is produced as a proprotein, which must be proteolytically activated; generally, cell surface activation is mediated by a furin family protease. Activated PA can then assemble into one of two noninterconverting oligomers, a homoheptamer and a homooctamer, which have unique properties. Herein we describe molecular determinants that influence the stoichiometry of PA in toxin complexes. By tethering PA domain 4 (D4) to domain 2 with two different-length cross-links, we can control the relative proportions of PA heptamers and octamers. The longer cross-link favors octamer formation, whereas the shorter one favors formation of the heptamer. X-ray crystal structures of PA (up to 1.45 Å resolution), including these cross-linked PA constructs, reveal that a hinge-like movement of D4 correlates with the relative preference for each oligomeric architecture. Furthermore, we report the conformation of the flexible loop containing the furin cleavage site and show that, for efficient processing, the furin site cannot be moved ~5 or 6 residues within the loop. We propose that there are different orientations of D4 relative to the main body of PA that favor the formation of either the heptamer or the octamer.

Secreted antiviral entry inhibitory (SAVE) peptides for gene therapy of HIV infection

Gene therapeutic strategies for human immunodeficiency virus type 1 (HIV-1) infection could potentially overcome the limitations of standard antiretroviral drug therapy (ART). However, in none of the clinical gene therapy trials published to date, therapeutic levels of genetic protection have been achieved in the target cell population for HIV-1. To improve systemic antiviral efficacy, C peptides, which are efficient inhibitors of HIV-1 entry, were engineered for high-level secretion by genetically modified cells. The size restrictions for efficient peptide export through the secretory pathway were overcome by expressing the C peptides as concatemers, which were processed into monomers by furin protease cleavage. These secreted antiviral entry inhibitory (SAVE) peptides mediated a substantial protective bystander effect on neighboring nonmodified cells, thus suppressing virus replication even if only a small fraction of cells was genetically modified. Accordingly, these SAVE peptides may provide a strong benefit to AIDS patients in future, and, if applied by direct in vivo gene delivery, could present an effective alternative to antiretroviral drug regimen.

Heparin enhances the furin cleavage of HIV-1 gp160 peptides

Infectious HIV-1 requires gp160 cleavage by furin at the REKR511 downward arrow motif (site1) into the gp120/gp41 complex, whereas the KAKR503 (site2) sequence remains uncleaved. We synthesized 41mer and 51mer peptides, comprising site1 and site2, to study their conformation and in vitro furin processing. We found that, while the previously reported 19mer and 13mer analogues represent excellent in vitro furin substrates, the present extended sequences require heparin for optimal processing. Our data support the hypothesis of a direct binding of heparin with site1 and site2, allowing selective exposure/accessibility of the REKR sequence, which is only then optimally cleaved by furin.

Structural investigation of the HIV-1 envelope glycoprotein gp160 cleavage site 3: role of site-specific mutations

Proteolytic processing of HIV gp160 to produce gp120 and gp41 is performed by PC enzymes. This process is a prerequisite for the virus infectivity, since both gp120 and gp41 participate in the virus HIV-1 entry mechanism. The structure of the gp120/gp41 junction remains to be elucidated, and the structural features required for molecular recognition between HIV-1 gp160 and proteolytic enzymes have not been clarified. Furin is the best PC candidate for the gp160 proteolytic processing known to date. In previous studies on model peptides, we have shown the relevance of an N-terminal helix for the proper recognition of the gp160 processing site by furin. Here we analyze the effect of point mutations in peptides lacking a regular N-terminal helix. To this end, we present the structure-activity characterization of three peptide analogues of the HIV gp160 processing site that all present mutations in proline at positions P3 and/or P2', while sharing the same N-terminal sequence, containing helix-breaking D-amino acids. Conformational analysis of the peptides was carried out in solution by NMR techniques, and furin's efficiency in cleaving them was measured. Structural findings are presented and discussed in relation to the different exhibited activity.

Alterations of pr-M cleavage and virus export in pr-M junction chimeric dengue viruses

During the export of flavivirus particles through the secretory pathway, a viral envelope glycoprotein, prM, is cleaved by the proprotein convertase furin; this cleavage is required for the subsequent rearrangement of receptor-binding E glycoprotein and for virus infectivity. Similar to many furin substrates, prM in vector-borne flaviviruses contains basic residues at positions P1, P2, and P4 proximal to the cleavage site; in addition, a number of charged residues are found at position P3 and between positions P5 and P13 that are conserved for each flavivirus antigenic complex. The influence of additional charged residues on pr-M cleavage and virus replication was investigated by replacing the 13-amino-acid, cleavage-proximal region of a dengue virus (strain 16681) with those of tick-borne encephalitis virus (TBEV), yellow fever virus (YFV), and Japanese encephalitis virus (JEV) and by comparing the resultant chimeric viruses generated from RNA-transfected mosquito cells. Among the three chimeric viruses, cleavage of prM was enhanced to a larger extent in JEVpr/16681 than in YFVpr/16681 but was slightly reduced in TBEVpr/16681. Unexpectedly, JEVpr/16681 exhibited decreased focus size, reduced peak titer, and depressed replication in C6/36, PS, and Vero cell lines. The reduction of JEVpr/16681 multiplication correlated with delayed export of infectious virions out of infected cells but not with changes in specific infectivity. Binding of JEVpr/16681 to immobilized heparin and the heparin-inhibitable infection of cells were not altered. Thus, diverse pr-M junction-proximal sequences of flaviviruses differentially influence pr-M cleavage when tested in a dengue virus prM background. More importantly, greatly enhanced prM cleavability adversely affects dengue virus export while exerting a minimal effect on infectivity. Because extensive changes of charged residues at the pr-M junction, as in JEVpr/16681, were not observed among a large number of dengue virus isolates, these results provide a possible mechanism by which the sequence conservation of the pr-M junction of dengue virus is maintained in nature.