## Structural Comparison: SARS-CoV-2 vs SARS-CoV-1 **Key Point:** The furin cleavage site (FCS) in the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein is the most significant structural difference between the two viruses and is absent in SARS-CoV-1. ### Furin Cleavage Site Significance The FCS in SARS-CoV-2: - Located at the S1/S2 boundary of the spike protein - Allows proteolytic cleavage by host furin protease - Enhances viral entry efficiency and cell tropism - Contributes to increased transmissibility compared to SARS-CoV-1 - Enables infection of a broader range of cell types **High-Yield:** The furin cleavage site is a key structural feature that explains SARS-CoV-2's higher transmissibility and broader tissue tropism compared to SARS-CoV-1. ### Shared Features (Not Discriminating) | Feature | SARS-CoV-1 | SARS-CoV-2 | |---------|-----------|----------| | Genome type | Single-stranded RNA (+ve sense) | Single-stranded RNA (+ve sense) | | Envelope | Lipid bilayer from host | Lipid bilayer from host | | Spike protein | Present | Present | | ACE2 receptor binding | Yes | Yes | **Clinical Pearl:** The furin cleavage site may explain why SARS-CoV-2 causes more asymptomatic infections and has higher transmissibility — the virus can infect respiratory epithelium more efficiently than SARS-CoV-1, which had more severe but less frequent infections. [cite:Harrison 21e Ch 197]
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