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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 and pH change. Our live-cell imaging experiments demonstrate that only a few minutes after binding to the plasma membrane, VLPs ingress into RAP5-negative endosomes via dynamin-dependent scission. Less than two minutes later, VLP speed increases in parallel with a pH drop below 5, yet these two events are not interrelated. By co-imaging fluorescently labeled nucleocapsid proteins, we show that nucleocapsid release occurs with similar kinetics to VLP acidification. Neither Omicron mutations nor abrogation of the S protein polybasic cleavage site affected the rate of VLP internalization, indicating that they do not confer any significant advantages or disadvantages during this process. 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. 

Abstract 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. 

ABSTRACT Alphaherpesviruses such as herpes simplex virus and pseudorabies virus (PRV) are neuroinvasive double-stranded DNA (dsDNA) viruses that establish lifelong latency in peripheral nervous system (PNS) neurons of their native hosts. Following reactivation, infection can spread back to the initial mucosal site of infection or, in rare cases, to the central nervous system, with usually serious outcomes. During entry and egress, viral capsids depend on microtubule-based molecular motors for efficient and fast transport. In axons of PNS neurons, cytoplasmic dynein provides force for retrograde movements toward the soma, and kinesins move cargo in the opposite, anterograde direction. The dynamic properties of virus particles in cells can be imaged by fluorescent protein fusions to the small capsid protein VP26, which are incorporated into capsids. However, single-color fluorescent protein tags fail to distinguish the virus inoculum from progeny. Therefore, we established a dual-color system by growing a recombinant PRV expressing a red fluorescent VP26 fusion (PRV180) on a stable cell line expressing a green VP26 fusion (PK15-mNG-VP26). The resulting dual-color virus preparation (PRV180G) contains capsids tagged with both red and green fluorescent proteins, and 97% of particles contain detectable levels of mNeonGreen (mNG)-tagged VP26. After replication in neuronal cells, all PRV180G progeny exclusively contain monomeric red fluorescent protein (mRFP)-VP26-tagged capsids. We used PRV180G for an analysis of axonal capsid transport dynamics in PNS neurons. Fast dual-color total internal reflection fluorescence (TIRF) microscopy, single-particle tracking, and motility analyses reveal robust, bidirectional capsid motility mediated by cytoplasmic dynein and kinesin during entry, whereas egressing progeny particles are transported exclusively by kinesins. IMPORTANCE Alphaherpesviruses are neuroinvasive viruses that infect the peripheral nervous system (PNS) of infected hosts as an integral part of their life cycle. Establishment of a quiescent or latent infection in PNS neurons is a hallmark of most alphaherpesviruses. Spread of infection to the central nervous system is surprisingly rare in natural hosts but can be fatal. Pseudorabies virus (PRV) is a broad-host-range swine alphaherpesvirus that enters neuronal cells and utilizes intracellular transport processes to establish infection and to spread between cells. By using a virus preparation with fluorescent viral capsids that change color depending on the stage of the infectious cycle, we find that during entry, axons of PNS neurons support robust, bidirectional capsid motility, similar to cellular cargo, toward the cell body. In contrast, progeny particles appear to be transported unidirectionally by kinesin motors toward distal egress sites.