Initiation of Infection
The first step of reproduction is the entry of the virion into the host cell. This process occurs via receptor-mediated endocytosis—the virus binds to specific cellular receptors, triggering membrane invagination and internalization of the viral particle.
Once inside the cell, the virus undergoes uncoating. The protein shell breaks down, releasing the viral genome—the positive-sense RNA strand—directly into the cytoplasm of the infected cell. All subsequent events of the viral cycle unfold within the cytoplasm.
Early Translation: Immediate Protein Synthesis
The key feature of this pathogen group lies in the nature of their nucleic acid. The released genomic plus-strand RNA is structurally ready to function as mRNA, binding directly to host cell ribosomes.
Early translation proceeds as follows:
- Ribosomes read the information from the viral RNA and synthesize a single giant polyprotein.
- This polyprotein is non-functional in its initial form and undergoes enzymatic cleavage.
- This proteolytic processing yields four non-structural proteins (NSP 1–4).
The most critical of these non-structural proteins is RNA-dependent RNA polymerase—an enzyme essential for the next stage, as the host cell itself cannot synthesize RNA from an RNA template.
Viral Genome Replication
Equipped with its own polymerase, the virus initiates large-scale replication of its genetic material. The viral polymerase uses the original genomic plus-strand RNA as a template for transcription.
First, a complementary negative-sense RNA strand is synthesized. It does not encode proteins and serves exclusively as a template. Using this negative strand as a mold, the enzyme actively produces two types of RNA molecules:
- Full-length genomic plus-strand RNA copies (49S). These are full-sized genetic molecules that will be packaged into new progeny virions.
- Incomplete (subgenomic) 26S mRNA molecules. These shortened strands are required exclusively to drive the massive synthesis of the structural elements of the future virus.
Structural Protein Synthesis and Modification
The assembly of a complete virion requires various types of proteins, so their production is strictly compartmentalized within the cell:
- Capsid (C) protein is synthesized on free ribosomes floating in the cytoplasm. It rapidly accumulates in preparation for assembly with new genomic copies.
- Envelope glycoproteins (E1–3) are synthesized on ribosomes tightly bound to the membranes of the endoplasmic reticulum (ER).
The synthesized glycoproteins immediately embed into the ER membrane. They then undergo intracellular transport to the Golgi apparatus, where they undergo further glycosylation (addition of carbohydrate residues). The final step involves the translocation and insertion of fully processed viral glycoproteins into the host plasma membrane.
Nucleocapsid Assembly and Virion Release
The final stage involves the formation of new viral particles. The capsid C protein locates the newly synthesized genomic RNA in the cytoplasm and binds tightly to it, forming the nucleocapsid.
Next, the nucleocapsid migrates to the inner surface of the host plasma membrane. It specifically targets areas that have been modified and now contain embedded viral glycoproteins.
The process concludes with budding. The nucleocapsid evaginates the cell membrane outward, gradually wrapping itself in it. Ultimately, a mature virion enveloped in a lipid bilayer studded with viral spikes (glycoproteins) pinches off from the cell. The cell releases these new viruses to infect neighboring tissues via a specialized form of exocytosis.