The not-so-sweet side of SARS-CoV-2 sugars

The virus responsible for COVID-19 enters cells in part thanks to sugar molecules attached to surface Spike proteins.
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A trio of SARS-CoV-2 spike proteins (pink) attached to the ACE2 receptor (green) found on human cells: without sugar molecules (left), with sugar molecules (middle) and showing the range of movement that these molecules have (right; grey). Image credit: Yang et al. (CC BY 4.0)

COVID-19 is an infectious disease caused by the virus SARS-CoV-2. To access the internal machinery necessary for its replication, the virus needs to latch onto and then enter host cells. Such processes rely on specific ‘glycoproteins’ that carry complex sugar molecules (or glycans), and can be found at the surface of both viruses and host cells. In particular, the viral ‘Spike’ glycoprotein can attach to human proteins called ACE2, which coat the cells that line the inside of the lungs, heart, kidney and brain. Yet the roles played by glycans in these processes remains unclear.

To investigate the role of Spike and ACE-2 glycans, Yang et al. designed a form of SARS-CoV-2 that could be handled safely in the laboratory. How these viruses infect human kidney cells that carry ACE2 was then examined, upon modifying the structures of the sugars on the viral Spike protein as well as the host ACE2 receptor. In particular, the sugar structures displayed by the virus were modified either genetically or chemically, using a small molecule that disrupts the formation of the glycans. Similar methods were also applied to modify the glycans of ACE2. Together, these experiments showed that the sugars present on the Spike protein play a minor role in helping the virus stick to human cells.However, they were critical for the virus to fuse and enter the host cells. These findings highlight the important role of Spike protein sugars in SARS-CoV-2 infection, potentially offering new paths to treat COVID-19 and other coronavirus-related illnesses. In particular, molecules designed to interfere with Spike-proteins and the viral entrance into cells could be less specific to SARS-CoV-2 compared to vaccines, allowing treatments to be efficient even if the virus changes.