An ongoing outbreak of SARS-CoV-2 has raised global concerns in 2020 with millions of confirmed cases, which is identified as another clade within the Betacoronavirus genus, Coronaviridae family. This article will help to figure out how this kind of virus infect human body by introducing the interaction between the four stages of the life cycle of coronavirus and the cytoskeleton.
The first stage: virus invasion
The first stage of coronavirus infection is Spike(S) protein-mediated attachment to the cell surface via S protein to the ceramide acid portion (acidic carbohydrate with 9 carbon atoms) or heparan sulfate. This infection strategy is very effective because there are many types of receptor molecules on the surface of all mammalian cells, thus creating abundant conditions for plasma membrane attachment. After binding, the virus particles actively rearrange the cytoskeleton by regulating the FAK/Cofilin/Rac/Cdc42 pathway.
The second stage: transport
In the second phase, some studies revealed that actin and tubulin are complementary cytoskeletal components of intracellular transport. Researchers explored the function of F-Actin in intracellular localization. Jasplakinolide (a cell-permeable F-actin stabilizing compound) inhibits the plasma membrane binding of virus particles during virus invasion, while cytochalsin D (a F-actin depolymerization compound) does not inhibit the invasion of the virus, but disrupts the normal positioning of the virion from the surrounding nucleus to the cytoplasmic region. The C-terminal peptide (S protein) of the coronavirus spike protein binds to several β-tubulin subtypes in a coronavirus-specific manner, and the chaotic peptides show that the binding is not due to random ionic charge interactions. Therefore, the transport of virions within cells utilizes a variety of cytoskeletal structural proteins to navigate and locate specific areas within the cell.
The third stage: assembly and maturity
After the virions are transported to the perinuclear area, the coronavirus RNA leaks from the vesicles and virions, and enters the nucleus for reverse transcription and replication. The DNA replicon is then transcribed as RNA and enters the Golgi/ER/microtubule organization center from the nucleus. Initially, the nucleocapsid (N) protein binds to the RNA copy and to the vesicle membrane, and then the N and E proteins mature further, which is required for the assembly of basic virus-like particles (VLPs). If the S protein is co-expressed, it will be incorporated into the virus particles. Under the synergistic effect of a variety of cytoskeleton and membrane regulatory proteins (such as HDAC6, ubiquitin and Rab GTPases), the assembly is assisted by concentrating packaging components.
The fourth stage: release
The genetic fusion of the SARS nucleocapsid or SARS S protein with GFP makes it possible to track non-infectious virus particles by fluorescence microscopy. Scientists used this technique to monitor the release of SARS-CoV and found vesicles fused into a multi-particle mass. This transport is sensitive to Brefeldin A, indicating that the secretory pathway is being used. Other studies have found that nocodazole can effectively inhibit the transport of virions to the plasma membrane, indicating that microtubules are an important part of the virus’s export outlet. Rab11 involves KHC binding for microtubule transport and then binding to myosin to help traverse the periocular actin matrix and release from the cells.
In general, there are many ways for coronavirus to enter the cell through the attachment and invagination of the plasma membrane. Subsequently, actin, tubulin, dynein and myosin cytoskeletal components need to be transported to the correct location for replication. After reverse transcription and transcription, the positive-strand RNA is packaged on the scaffold of the Golgi/ER/microtubule complex. Virus particles wrapped in vesicles move along the microtubules, then fuse with the plasma membrane and escape from the cells.