For over three years, SARS-CoV-2, the pathogen that causes the respiratory disease known as COVID-19, has been keeping us on our toes due to its high infectiousness and frequently severe, potentially deadly, clinical outcomes. How does the infection mechanism work on a molecular level? Gerti Beliu, Markus Sauer, University of Würzburg, Germany, and colleagues have provided new insights on this.
The surfaces of SARS-CoV-2 viruses are covered in 24 to 40 protruding spikes consisting of trimers of three identical proteins. After over two years of research, it is undisputed that the critical first step of an infection is the binding of these spikes to the angiotensin-converting enzyme 2 (ACE2). ACE2 is present almost everywhere in the body and is involved in diverse physiological functions, such as the regulation of blood pressure and circulation. ACE2 acts as an entry receptor for the viruses. After binding, the virus particle invades the cell.
Some questions have, thus far, remained unanswered: Do the spikes, which consist of three subunits, simultaneously bind to multiple ACE2 receptors? If they do, is ACE2 already present as a dimer or oligomer in the membrane? Or do several ACE2 molecules aggregate as a result of binding to the spikes? The team says that neither is the case.
Distribution of ACE2 on Cells
The researchers labeled the ACE2 receptors of various cell lines that are used as models for COVID infection with various techniques involving fluorescence dyes and studied them with dSTORM (direct stochastic optical reconstruction microscopy). This fluorescence imaging method has an extremely high resolution, beyond the diffraction limit of classic methods.
Using this approach, the team was able to determine the number and distribution of the ACE2 receptors in the plasma membrane of various cell lines. They also performed dSTORM experiments with reference receptors that form monomers, dimers, and heterodimers and found that the method can distinguish between monomeric and dimeric receptors.
It was shown that the ACE2 receptors are evenly distributed—as monomers—with a density of about 1–2 ACE2 molecules per square micrometer, which is low in comparison to most other membrane receptors. Experiments after addition of trimeric viral spikes showed unambiguously that binding of these proteins does not induce any formation of ACE2 dimers or oligomers.
Single Protein, Monomeric Receptor
Infection studies using a modified type of virus (vesicular stomatitis virus, VSV) bearing spike proteins supported the conclusion that an interaction between one single spike protein per virus particle with a single monomeric ACE2 receptor is enough to cause infection—likely one reason for the high infectiousness of SARS-CoV-2.
This new quantitative molecular information about the interactions between spike proteins and ACE2 on the cell membrane could offer new perspectives for the development of improved drugs for treating COVID infections.
- Coronaviruses Use ACE2 Monomers as Entry‐Receptors,
Patrick Eiring, Teresa Klein, Simone Backes, Marcel Streit, Marvin Jungblut, Sören Doose, Gerti Beliu, Markus Sauer,
Angew. Chem. Int. Ed. 2023.