Coronaviruses use ACE2 monomers as entry receptors

The angiotensin-converting enzyme 2 (ACE2) has been identified as entry receptor on cells enabling binding and infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via trimeric spike (S) proteins protruding from the viral surface. It has been suggested that trimeric S proteins preferably bind to plasma membrane areas with high concentrations of preferably multimeric ACE2 receptors to achieve a higher binding and infection efficiency. However, our current knowledge about the influence of ACE2 expression and organization in the plasma membrane on SARS-CoV-2 infection efficiency remains elusive. Here we used direct stochastic optical reconstruction microscopy (dSTORM) in combination with different labeling approaches to visualize the distribution and quantify the expression of ACE2 on different cells. Our results reveal that endogenous ACE2 receptors are present as monomers in the plasma membrane with densities of only 1-2 receptors um-2. In addition, binding of trimeric S proteins does not induce clustering of ACE2 receptors in the plasma membrane. Supported by infection studies using vesicular stomatitis virus (VSV) particles bearing S proteins our data demonstrate that a single S protein interaction per virus particle with a monomeric ACE2 receptor is sufficient for infection which attests SARS-CoV-2 a high infectivity. ### Competing Interest Statement The authors have declared no competing interest.