SARS-CoV-2 is an enveloped, positive-sense, single-stranded RNA virus. Which of the following structural features distinguishes it from other respiratory viruses like influenza?

Explanation

## SARS-CoV-2 Structural Features **Key Point:** The crown-like spike (S) protein with its receptor-binding domain (RBD) is the defining structural feature of coronaviruses, including SARS-CoV-2. This protein is responsible for binding to the human ACE2 receptor and is the primary target of neutralizing antibodies. ### Spike Protein Architecture The SARS-CoV-2 spike protein: 1. Forms homotrimers that project from the viral surface 2. Contains two functional subunits: - **S1 subunit:** Contains the RBD that binds ACE2 - **S2 subunit:** Mediates membrane fusion 3. Undergoes conformational changes during viral entry 4. Is heavily glycosylated, creating a "crown" appearance under electron microscopy **High-Yield:** The RBD of the spike protein is the primary epitope for neutralizing antibodies produced during infection or vaccination. Mutations in the RBD (e.g., in Omicron, Delta variants) can reduce antibody recognition and vaccine efficacy. ### SARS-CoV-2 Genome Organization Unlike influenza, SARS-CoV-2 has: - **Non-segmented, positive-sense RNA genome** (~29.9 kb) - **10 open reading frames (ORFs)** encoding: - 4 structural proteins (S, E, M, N) - 16 non-structural proteins (NSPs) with enzymatic functions **Mnemonic:** **SEMEN** = Spike, Envelope, Membrane, N-protein (the 4 structural proteins of coronavirus) ### Comparison: SARS-CoV-2 vs. Influenza | Feature | SARS-CoV-2 | Influenza | |---------|-----------|----------| | **Genome** | Non-segmented, positive-sense ssRNA (~30 kb) | Segmented, negative-sense ssRNA (8 segments, ~13.6 kb total) | | **Surface proteins** | Spike (S), Envelope (E), Membrane (M) | Hemagglutinin (HA), Neuraminidase (NA) | | **Receptor** | ACE2 | Sialic acid | | **Envelope origin** | ER/Golgi | Plasma membrane | | **Replication site** | Cytoplasm (ER/Golgi) | Nucleus | **Clinical Pearl:** The spike protein's RBD is used in mRNA vaccines (Pfizer, Moderna) and protein subunit vaccines (Novavax) to elicit protective immunity without exposing the patient to infectious virus. [cite:Robbins 10e Ch 8]