Biology subjects

Saffarian, S.

Publications and source records attributed to Saffarian, S..

12 recordsLinked to original sources

bioRxiv2026

A mechanism-aware stoichiometry platform resolves functional viral thresholds and induces antiviral hypersensitivity

Antiviral discovery is frequently hindered by a stoichiometric blind spot--a lack of resolution regarding the functional density of viral enzymes required for replication. Traditional screening pipelines rely on target-based or phenotypic assays that cannot distinguish between simple molecular binding and the crossing of a functional stoichiometric cliff. Here, we present a mechanism-aware platform that resolves these enzymatic requirements with single-virion precision. By integrating quantitative cryo-electron microscopy and genomic validation with Monte Carlo modeling, we map the stoichiometric landscapes of HIV-1 Protease (PR) and Reverse Transcriptase (RT). We uncover a striking disparity in enzymatic demand: a high-redundancy buffering capacity for PR ([~]40 monomers) contrasted with a high-threshold requirement for RT ([~]95 subunits). We demonstrate that by systematically de-buffering the virion, our platform induces a state of antiviral hypersensitivity, enabling the detection of therapeutic activity in novel and clinical inhibitors that remain invisible to traditional workflows. Furthermore, this multiplexed profiling enables de novo target identification, as inhibitors trigger failure exclusively on their respective stoichiometric arms. This platform provides a deterministic roadmap for de-risking drug discovery and identifying viral sub-stoichiometric vulnerabilities.

biophysics↗

bioRxiv2025

Cryo-ET Reveals Distinct Gag Lattice Architectures in Virus-like Particles and Immature HIV-1

HIV-1 is released from infected cells as immature virions whose membranes are supported by a Gag lattice. During maturation, this lattice is cleaved by the viral protease to release capsid proteins that assemble into the mature core. The architecture of the Gag lattice is central to this process, and the Gag lattice is targeted by maturation inhibitors that block cleavage. Using cryo-electron tomography, we compared Gag-only virus-like particles (VLPs) with immature HIV-1 virions and found that VLPs assemble denser and more complete lattices, exhibiting a strong correlation between lattice curvature and Gag copy number. In contrast, immature virions incorporate fewer Gag molecules and display weaker coupling between curvature and Gag stoichiometry. These findings show that while Gag alone can form the canonical immature lattice, additional viral components fine-tune lattice organization and curvature, potentially regulating protease accessibility, virion release, and the onset of HIV-1 maturation.

biophysics↗

bioRxiv2024

Enhanced Yield and Gentle Purification of HIV for Cryo-Electron Tomography Analysis of Virion Maturation.

HIV is a lentivirus characterized by the formation of its mature core. Visualization and structural examination of HIV requires purification of virions to high concentrations. The yield and integrity of these virions are crucial for ensuring a uniform representation of all viral particles in subsequent analyses. In this study, we present a method for purification of HIV virions which minimizes forces applied to virions while maximizing the efficiency of collection. This method allows us to capture between 1,000 and 5,000 HIV virions released from individual HEK293 cells after transfection with the NL4.3 HIV backbone, a 10 fold advantage over other methods. We utilized this approach to investigate HIV core formation from several constructs: pNL4-3(RT:D185A&D186A) with an inactive reverse transcriptase, NL4.3(IN: V165A&R166A) with a type-II integrase mutation, and NL4.3(: {Delta}(105-278)&{Delta}(301-332)) featuring an edited packaging signal. Notably, virions from NL4.3(: {Delta}(105-278)&{Delta}(301-332)) displayed a mixed population, comprising immature virions, empty cores, and cores with detectable internal density. Conversely, virions derived from NL4.3(IN: V165A&R166A) exhibited a type II integrase mutant phenotype characterized by empty cores and RNP density localized around the cores, consistent with previous studies. In contrast, virions released from pNL4-3(RT:D185A&D186A) displayed mature cores containing detectable RNP density. We suggest that the purification methods developed in this study can significantly facilitate the characterization of enveloped viruses.

microbiology↗

bioRxiv2024

Kinetic landscape of single virus-like particles highlights the efficacy of SARS-Cov-2 internalization

The efficiency of virus internalization into target cells is a major determinant of infectivity. SARS-CoV-2 internalization occurs via S-protein-mediated cell binding followed either by direct fusion with the plasma membrane or endocytosis and subsequent fusion with the endosomal membrane. Despite the crucial role of virus internalization, the precise kinetics of the processes involved remains elusive. We developed a pipeline, which combines live-cell microscopy and advanced image analysis, for measuring the rates of multiple internalization-associated molecular events of single SARS-CoV-2-virus-like particles (VLPs), including endosome ingression, pH change, and nucleocapsid release. Our live-cell imaging experiments demonstrate that only a few minutes after binding to the plasma membrane, VLPs ingress into Rab5-negative endosomes via Dynamin-dependent scission. Less than two minutes later, the pH of VLPs drops below 5 followed by an increase in VLP speed, yet these two events are not interrelated. Nucleocapsid release from the VLPs occurs with similar kinetics to the pH drop, suggesting that VLP fusion occurs during endosome acidification. Neither Omicron mutations nor abrogation of the S protein polybasic cleavage site altered the rate of VLP internalization events, indicating that they do not affect these processes. Finally, we observe that VLP internalization occurs two to three times faster in VeroE6 than in A549 cells, which may contribute to the greater susceptibility of the former cell line to SARS-CoV-2 infection. Taken together, our precise measurements of the kinetics of VLP internalization-associated processes shed light on their contribution to the effectiveness of SARS-CoV-2 propagation in cells. Time-lapse videos of the studied internalization events can be accessed in the dedicated COVIDynamics database.

cell biology↗

bioRxiv2024

Competitive assembly resolves the stoichiometry of essential proteins in infectious HIV-1 virions.

During assembly on the plasma membrane, HIV-1 virions incorporate Gag-Pol as well as gp120/gp41 trimers. The Pol region consists of protease, reverse transcriptase and integrase precursors which are essential enzymes required for maturation, reverse transcription, and integration of the viral genome in the next host. gp120/gp41 trimers catalyze the fusion of the virion with its next host. Only a fraction of released virions are infectious. The stoichiometry of gp120/gp41 and Gag-Pol proteins in HIV virions was previously measured using cryotomography and ratiometric protein analysis, but what is the stoichiometry of these proteins in infectious virions remained to be determined. Here we developed a method based on competition between infectious HIV backbones with noninfectious mutants and measured 100 {+/-} 10 Gag-Pol and 15 {+/-} 3 gp120/gp41 proteins incorporated in infectious virions assembled in HEK293 cells from NL4.3 HIV-1 backbone. Our measurements are in broad agreement with cryotomography and ratiometric protein analysis and therefore stoichiometry of gp120/gp41 and Gag-Pol in infectious virions is the same as all released virions. With the development of appropriate mutants and infectivity assays, our method is applicable to other infectious viruses. Statement of significanceThere are 30 million people who have succumbed to the AIDS pandemic with 600,000 additional deaths per year. HIV has an accelerated rate of mutational accumulation with the virus mutating out of neutralizing antibodies within the same patient making development of vaccines challenging. Like most enveloped viruses, only a fraction of released virions are infectious and the question of what selects these virions has remained a mystery. The method developed in this article will allow stoichiometric measurements on infectious virions and therefore allows further studies of causes of infectivity.

biophysics↗

bioRxiv2022

Defects in the HIV immature lattice support essential lattice remodeling within budded virions

For HIV virions to become infectious, the immature lattice of Gag polyproteins attached to the virion membrane must be cleaved. Cleavage cannot initiate without the protease formed by the homo-dimerization of domains linked to Gag. However, only 5% of the Gag polyproteins, termed Gag-Pol, carry this protease domain, and they are embedded within the structured lattice. The mechanism of Gag-Pol dimerization is unknown. Here, we use reaction-diffusion simulations of the immature Gag lattice as derived from experimental structures, showing that dynamics of the lattice on the membrane is unavoidable due to the missing 1/3 of the spherical protein coat. These dynamics allow for Gag-Pol molecules carrying the protease domains to detach and reattach at new places within the lattice. Surprisingly, dimerization timescales of minutes or less are achievable for realistic binding energies and rates despite retaining most of the large-scale lattice structure. We derive a formula allowing extrapolation of timescales as a function of interaction free energy and binding rate, thus predicting how additional stabilization of the lattice would impact dimerization times. We further show that during assembly, dimerization of Gag-Pol occurs stochastically and therefore must be actively suppressed to prevent early activation. By direct comparison to recent biochemical measurements within budded virions, we find that only moderately stable hexamer contacts (-12kBT<{Delta}G<-8kBT) retain both the dynamics and lattice structures that are consistent with experiment. These dynamics are likely essential for proper maturation, and our models quantify and predict lattice dynamics and protease dimerization timescales that define a key step in understanding formation of infectious viruses. Statement of SignificanceFor retroviruses such as HIV-1, the Gag polyprotein assembles an immature lattice that ensures successful budding from the cell plasma membrane. The first step in the subsequent maturation requires a pair of protease domains embedded within the lattice to form a homodimer. We show here that this homo-dimerization can proceed within minutes despite involving a small subset of Gag monomers, due to the incompleteness of the immature lattice. Using reaction-diffusion simulations, we quantify timescales of first dimerization events between the protease domains and define a formula to extrapolate across a range of energies and rates. Our models illustrate how protein contacts can be weakened to disrupt lattice assembly or stabilized to slow the remodeling essential for viral infectivity.

biophysics↗

bioRxiv2022

Competitive HIV budding suggests that a self-packaging gRNA:Gag-Pol complex directs HIV assembly and enforces infectivity.

To resolve the assembly mechanism of infectious HIV virions, we tested the ability of HIV to assemble infectious virions in the presence of a titrated mix of infectious/ non-infectious proviral genomes. The analysis of our assembly competitions shows that during translation, 15 {+/-} 5 Gag-Pols bind back to their parental gRNA creating a gRNA1: Gag-Pol15 complex. This complex initiates the infectious virion assembly through interactions mediated by cis packaged Gag/Gag-pols and the plasma membrane. Our analysis also shows the number of Gag-Pol and Env proteins packaged in an infectious HIV virion and the minimum functional units of these proteins required for viral infectivity. We suggest that aside from orchestrating the infectious virion assembly the gRNA1: Gag-Pol15 complex plays a major role in HIV evolution and likely hampers effectiveness of antiviral therapies. One-Sentence SummaryCompetitive HIV budding reveals gRNA1: Gag-Pol15 complex orchestrating viral assembly.

microbiology↗

bioRxiv2022

Optimized production and fluorescent labelling of SARS-CoV-2 Virus-Like-Particles to study virus assembly and entry.

SARS-CoV-2 is an RNA enveloped virus responsible for the COVID-19 pandemia that conducted in 6 million deaths worldwide so far. SARS-CoV-2 particles are mainly composed of the 4 main structural proteins M, N, E and S to form 100nm diameter viral particles. Based on productive assays, we propose an optimal transfected plasmid ratio mimicking the virus RNA ratio allowing SARS-CoV-2 Virus-Like Particle (VLPs) formation composed of the viral structural proteins M, N, E and S. Furthermore, monochrome, dual-color fluorescent or photoconvertible VLPs were produced. Thanks to live fluorescence and super-resolution microscopy, we quantified VLPs size and concentration. It shows a diameter of 110 and 140 nm respectively for MNE-VLPs and MNES-VLPs with a minimum concentration of 10e12 VLP/ml. SARS-CoV-2 VLPs could tolerate the integration of fluorescent N and M tagged proteins without impairing particle assembly. In this condition, we were able to establish incorporation of the mature Spike in fluorescent VLPs. The Spike functionality was then shown by monitoring fluorescent MNES-VLPs docking and endocytosis in human pulmonary cells expressing the receptor hACE2. This work provides new insights on the use of non-fluorescent and fluorescent VLPs to study and visualize the SARS-CoV-2 viral life cycle in a safe environment (BSL-2 instead of BSL-3). Moreover, optimized SARS-CoV-2 VLP production can be further adapted to vaccine design strategies.

microbiology↗

bioRxiv2020

Structural stability of SARS-CoV-2 degrades with temperature

SARS-CoV-2 is a novel coronavirus which has caused the COVID-19 pandemic. Other known coronaviruses show a strong pattern of seasonality, with the infection cases in humans being more prominent in winter. Although several plausible origins of such seasonal variability have been proposed, its mechanism is unclear. SARS-CoV-2 is transmitted via airborne droplets ejected from the upper respiratory tract of the infected individuals. It has been reported that SARS-CoV-2 can remain infectious for hours on surfaces. As such, the stability of viral particles both in liquid droplets as well as dried on surfaces is essential for infectivity. Here we have used atomic force microscopy to examine the structural stability of individual SARS-CoV-2 virus like particles at different temperatures. We demonstrate that even a mild temperature increase, commensurate with what is common for summer warming, leads to dramatic disruption of viral structural stability, especially when the heat is applied in the dry state. This is consistent with other existing non-mechanistic studies of viral infectivity, provides a single particle perspective on viral seasonality, and strengthens the case for a resurgence of COVID-19 in winter. Statement of Scientific SignificanceThe economic and public health impact of the COVID-19 pandemic are very significant. However scientific information needed to underpin policy decisions are limited partly due to novelty of the SARS-CoV-2 pathogen. There is therefore an urgent need for mechanistic studies of both COVID-19 disease and the SARS-CoV-2 virus. We show that individual virus particles suffer structural destabilization at relatively mild but elevated temperatures. Our nanoscale results are consistent with recent observations at larger scales. Our work strengthens the case for COVID-19 resurgence in winter.

biophysics↗

bioRxiv2020

Minimal system for assembly of SARS-CoV-2 virus like particles

SARS-CoV-2 virus is the causative agent of COVID-19. Here we demonstrate that non-infectious SARS-CoV-2 virus like particles (VLPs) can be assembled by co-expressing the viral proteins S, M and E in mammalian cells. The assembled SARS-CoV-2 VLPs possess S protein spikes on particle exterior, making them ideal for vaccine development. The particles range in shape from spherical to elongated with a characteristic size of 129 {+/-} 32 nm. We further show that SARS-CoV-2 VLPs dried in ambient conditions can retain their structural integrity upon repeated scans with Atomic Force Microscopy up to a peak force of 1 nN.

biophysics↗

bioRxiv2019

Interferometric Fluorescent Cross Correlation Spectroscopy

We present a method utilizing single photon interference and fluorescence correlation spectroscopy (FCS) to simultaneously measure transport of fluorescent molecules within aqueous samples. Our method, within seconds, measures transport in thousands of homogenous voxels (100 nm)3 and eliminates photo-physical artifacts associated with blinking of fluorescent molecules. A comprehensive theoretical framework is presented and validated by measuring transport of quantum dots, associated with VSV-G receptor along cellular membranes as well as within viscous gels.

biophysics↗

bioRxiv2019

Abrogating ALIX interactions results in stuttering of the ESCRT machinery

ESCRTs are cellular proteins that catalyze the fission of membranes and play an important role in biology of disease including cancer and infectious virus release 1. ESCRT associated protein ALIX plays an essential role in HIV budding 2-4, exosome release 5,6, down regulation of G-protein coupled receptors 7, cytokinesis 8,9 and multi vesicular budding 1. The consensus view is that ALIX plays a role by binding to the viral late domains 2-4/Syntenin late domain 5/Cepp55 8,9 and helps recruit downstream protein CHMP4 2,8,10,11 which along with VPS4 catalyzes the fission of the membrane 12,13. Using live imaging we have visualized the recruitment of ALIX, CHMP4 and VPS4 during budding of HIV with abrogated Gag-ALIX interactions. Based on the canonical view, we were expecting to find reduced recruitment of ALIX under these conditions. Instead we report observing multiple rounds of transient recruitment of ALIX, CHMP4 and VPS4 prior to virion release. We further show that during each, transient recruitment, stoichiometry of all ESCRT components remained the same regardless of mutations abrogating ALIX and Gag interaction. In addition, mutations abrogating interactions between Gag and TSG101 result in recruitment of ESCRTs with a substantial delay while maintaining similar stoichiometry. Our results demonstrate that recruitment of ESCRTs is driven by a robust network of interactions resulting in an \"On/Off\" switch behavior and ALIXs interactions with late domains of HIV Gag play a crucial role during final catalysis of membrane fission after assembly of the full ESCRT machinery.

cell biology↗