Endosome

Lipid bilayer fusion

Biophysics

Pathology

Cell

Paleontology

Virology

Cleavage (geology)

Enzyme

Chemistry

Virus

Furin

Coronavirus

TMPRSS2

Cell membrane

Disease

Coronavirus disease 2019 (COVID-19)

Fracture (geology)

Infectious disease (medical specialty)

Genetics

Biochemistry

Medicine

Cell culture

Biology

Cell fusion

Cell biology

Proceedings of the National Academy of Sciences

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cell entry starts with membrane attachment and ends with spike (S) protein–catalyzed membrane fusion depending on two cleavage steps, namely, one usually by furin in producing cells and the second by TMPRSS2 on target cells. Endosomal cathepsins can carry out both. Using real-time three-dimensional single-virion tracking, we show that fusion and genome penetration require virion exposure to an acidic milieu of pH 6.2 to 6.8, even when furin and TMPRSS2 cleavages have occurred. We detect the sequential steps of S1-fragment dissociation, fusion, and content release from the cell surface in TMPRRS2-overexpressing cells only when exposed to acidic pH. We define a key role of an acidic environment for successful infection, found in endosomal compartments and at the surface of TMPRSS2-expressing cells in the acidic milieu of the nasal cavity.

SARS-CoV-2 requires acidic pH to infect cells