Herpesvirus entry into cells is mediated by the viral fusogen gB, which is thought to refold from the prefusion to the postfusion form in a series of large conformational changes that energetically couple refolding to membrane fusion. In contrast to most viral fusogens, gB requires a conserved heterodimer, gH/gL, as well as other nonconserved proteins. In a further mechanistic twist, gB-mediated cell-cell fusion appears restricted by its intraviral or cytoplasmic domain (cytodomain) because mutations within it result in a hyperfusogenic phenotype. Here, we characterized a panel of hyperfusogenic HSV-1 gB cytodomain mutants and show that they are fully functional in cell-cell fusion at shorter coincubation times and at lower temperatures than those for wild-type (WT) gB, which suggests that these mutations reduce the kinetic energy barrier to fusion. Despite this, the mutants require both gH/gL and gD. We confirm previous observations that the gH cytotail is an essential component of the cell-cell fusion mechanism and show that the N-terminal portion of the gH cytotail is critical for this process. Moreover, the fusion levels achieved by all gB constructs, WT and mutant, were proportionate to the length of the gH cytotail. Putting these results together, we propose that the gH cytotail, in addition to the gH/gL ectodomain, plays an essential role in gB activation, potentially acting as a "wedge" to release the gB cytodomain "clamp" and enable gB activation. IMPORTANCEHerpesviruses infect their hosts for life and cause a substantial disease burden. Herpes simplex viruses cause oral and genital sores as well as rare yet severe encephalitis and a panoply of ocular ailments. Infection initiates when the viral envelope fuses with the host cell membrane in a process orchestrated by the viral fusogen gB, assisted by the viral glycoproteins gH, gL, and gD and a cellular gD receptor. This process is more complicated than that of most other viruses and is subject to multiple regulatory inputs. Antiviral and vaccine development would benefit from a detailed mechanistic knowledge of this process and how it is regulated. Herpesviruses, large, enveloped, double-stranded DNA (dsDNA) viruses, enter cells by the merger of the viral envelope and a host cell membrane, catalyzed by the conserved viral glycoprotein gB. As for other viral fusogens, gB is thought to refold from the prefusion to the postfusion form in a series of large conformational changes that provides the energy necessary to overcome the kinetic barrier associated with membrane fusion (1). However, unlike most viral fusogens, gB does not mediate fusion on its own and requires a conserved heterodimer, gH/gL (2), as well as other nonconserved proteins. For example, herpes simplex virus 1 (HSV-1) and HSV-2, members of the alphaherpesvirus subfamily, require the receptor-binding glycoprotein gD and a cellular gD receptor such as nectin-1 in addition to gB and gH/gL (3). These five proteins also mediate the fusion of transfected cells in the absence of any other ...

The double-stranded DNA polyomavirus Merkel cell polyomavirus (MCV) causes Merkel cell carcinoma, an aggressive but rare human skin cancer that most often affects the immunosuppressed and elderly. As in other polyomaviruses, the large T-antigen of MCV recognizes the viral origin of replication by binding repeating G(A/G)GGC pentamers. The spacing, number, orientation, and necessity of repeats for viral replication differ, however, from other family members such as SV40 and murine polyomavirus. We report here the 2.9 Å crystal structure of the MCV large T-antigen origin binding domain in complex with a DNA fragment from the MCV origin of replication. Consistent with replication data showing that three of the G(A/G)GGC-like binding sites near the center of the origin are required for replication, the crystal structure contains three copies of the origin-binding domain. This stoichiometry was verified using isothermal titration calorimetry. The affinity for G(A/G)GGC-containing dsDNA was found to be ~740 nM, approximately 8-fold weaker than the equivalent domain in SV40 for the analogous region of the SV40 origin. The difference in affinity is partially attributable to DNA-binding residue Lys 331 (Arg 154 in SV40). In contrast to SV40, a small protein-protein interface is observed between MCV origin-binding domains when bound to the central region of the origin. This protein-protein interface is reminiscent of that seen in bovine papilloma virus E1 protein. Mutational analysis indicates, however, that this interface contributes little to the DNA binding energy.

Herpes simplex viruses (HSVs) are unusual in that unlike most enveloped viruses, they require at least four entry glycoproteins, gB, gD, gH, and gL, for entry into target cells in addition to a cellular receptor for gD. The dissection of the herpes simplex virus 1 (HSV-1) entry mechanism is complicated by the presence of more than a dozen proteins on the viral envelope. To investigate HSV-1 entry requirements in a simplified system, we generated vesicular stomatitis virus (VSV) virions pseudotyped with HSV-1 essential entry glycoproteins gB, gD, gH, and gL but lacking the native VSV fusogen G. These virions, referred to here as VSV⌬G-BHLD virions, infected a cell line expressing a gD receptor, demonstrating for the first time that the four essential entry glycoproteins of HSV-1 are not only required but also sufficient for cell entry. To our knowledge, this is the first time the VSV pseudotyping system has been successfully extended beyond two proteins. Entry of pseudotyped virions required a gD receptor and was inhibited by HSV-1 specific anti-gB or anti-gH/gL neutralizing antibodies, which suggests that membrane fusion during the entry of the pseudotyped virions shares common requirements with the membrane fusion involved in HSV-1 entry and HSV-1-mediated syncytium formation. The HSV pseudotyping system established in this study presents a novel tool for systematic exploration of the HSV entry and membrane fusion mechanisms. IMPORTANCEHerpes simplex viruses (HSVs) are human pathogens that can cause cold sores, genital herpes, and blindness. No vaccines or preventatives are available. HSV entry into cells-a prerequisite for a successful infection-is a complex process that involves multiple viral and host proteins and occurs by different routes. Detailed mechanistic knowledge of the HSV entry is important for understanding its pathogenesis and would benefit antiviral and vaccine development, yet the presence of more than a dozen proteins on the viral envelope complicates the dissection of the HSV entry mechanisms. In this study, we generated heterologous virions displaying the four essential entry proteins of HSV-1 and showed that they are capable of cell entry and, like HSV-1, require all four entry glycoproteins along with a gD receptor. This HSV pseudotyping system pioneered in this work opens doors for future systematic exploration of the herpesvirus entry mechanisms.T o enter living cells to replicate, viruses must overcome the barrier of the cellular membrane. Enveloped viruses accomplish this task by facilitating the merger of their envelope with a target cell membrane, during which capsids are delivered into the cytosol and infection ensues. Entry is initiated by binding of a virus to an appropriate receptor on the surface of the host cell and is catalyzed by a virus-encoded membrane fusogen. In most enveloped viruses, the receptor binding and the fusogenic functions are executed by a single protein (1).Herpesviruses are double-stranded DNA, enveloped viruses with intricate envelopes that contain...

Pharmacological inhibitors of cysteine proteases have provided useful insights into the regulation of calpain activity in erythrocytes. However, the precise biological function of calpain activity in erythrocytes remains poorly understood. Erythrocytes express calpain-1, an isoform regulated by calpastatin, the endogenous inhibitor of calpains. In the present study, we investigated the function of calpain-1 in mature erythrocytes using our calpain-1-null [KO (knockout)] mouse model. The calpain-1 gene deletion results in improved erythrocyte deformability without any measurable effect on erythrocyte lifespan in vivo. The calcium-induced sphero-echinocyte shape transition is compromised in the KO erythrocytes. Erythrocyte membrane proteins ankyrin, band 3, protein 4.1R, adducin and dematin are degraded in the calcium-loaded normal erythrocytes but not in the KO erythrocytes. In contrast, the integrity of spectrin and its state of phosphorylation are not affected in the calcium-loaded erythrocytes of either genotype. To assess the functional consequences of attenuated cytoskeletal remodelling in the KO erythrocytes, the activity of major membrane transporters was measured. The activity of the K+–Cl− co-transporter and the Gardos channel was significantly reduced in the KO erythrocytes. Similarly, the basal activity of the calcium pump was reduced in the absence of calmodulin in the KO erythrocyte membrane. Interestingly, the calmodulin-stimulated calcium pump activity was significantly elevated in the KO erythrocytes, implying a wider range of pump regulation by calcium and calmodulin. Taken together, and with the atomic force microscopy of the skeletal network, the results of the present study provide the first evidence for the physiological function of calpain-1 in erythrocytes with therapeutic implications for calcium imbalance pathologies such as sickle cell disease.

BackgroundCDI is a 2-hit process requiring C. difficile spores and antibiotic-mediated dysbiosis, a low diversity state of the gut microbiome. Recurrent CDI (rCDI) is common and may be related to inadequate antibiotic concentrations (e.g., metronidazole; MET) or persistent dysbiosis (e.g., vancomycin; VAN). SER-262 is an oral investigational microbiome drug rationally designed to reduce rCDI by restoring colonization resistance.MethodsSERES-262-001 was a Phase 1b randomized placebo (PBO)-controlled single and multidose study. Subjects with primary CDI (n = 96) were enrolled in 8 cohorts (SER-262: PBO, 5:1). Subjects were dosed after MET (n = 57) or VAN (n = 39) per investigator discretion. Engraftment of SER-262 strains was evaluated using strain-specific molecular probes in fecal samples; microbial diversity was measured via whole metagenomic shotgun sequencing. Endpoints included safety and rCDI rates up to 8 weeks posttreatment and strain engraftment at 1, 4, 8, 12, and 24 weeks.ResultsSER-262 safety was comparable to PBO. Although overall rCDI rates were similar in SER-262 (n = 80) and PBO (n = 16) subjects (18.8% vs. 12.5%, respectively), in a post-hoc analysis we observed reduced rates of rCDI in the VAN+SER-262 arm compared with MET+SER-262 (6.3 vs. 27.1%, respectively, P = 0.02, Figure 1). Overall, 8 of 12 SER-262 strains showed significant engraftment relative to PBO. However, greater SER-262 strain engraftment was observed in VAN-treated subjects compared with MET-treated subjects (P < 0.001, Figure 2). To better understand the impact of dysbiosis on engraftment, we evaluated baseline microbial diversity by prior antibiotic received and observed that the diversity of Bacteroidetes and Firmicute species was lower in VAN-treated subjects compared with MET-treated subjects (P < 0.001, Figure 3).ConclusionIn this first phase 1b study of a fermented microbiome drug in subjects with primary CDI, SER-262 was safe and well-tolerated. The higher efficacy rates of SER-262 in reducing rCDI among VAN-treated subjects may be due to low baseline microbial diversity, which creates an ecologic niche for greater engraftment of dose species. Treatment of C. difficile with VAN, followed by restoration of colonization resistance with SER-262, is a promising 2-pronged therapeutic paradigm to reduce rCDI. Disclosures All authors: No reported disclosures.