Sequences in the cytoplasmic tail of SARS-CoV-2 spike facilitate syncytia formation (preprint)

The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binds the cell surface protein ACE2 to mediate fusion of the viral membrane with target cells. S comprises a large external domain, a transmembrane domain (TMD) and a short cytoplasmic tail. To elucidate the intracellular trafficking of S protein in host cells we applied proteomics to identify cellular factors that interact with its cytoplasmic tail. We confirm interactions with components of the COPI, COPII and SNX27/retromer vesicle coats, and with FERM domain actin regulators and the WIPI3 autophagy component. The interaction with COPII promotes efficient exit from the endoplasmic reticulum (ER), and although COPI-binding should retain S in the early Golgi system where viral budding occurs, the binding is weakened by a suboptimal histidine residue in the recognition motif. As a result, S leaks to the surface where it accumulates as it lacks an endocytosis motif of the type found in many other coronaviruses. It is known that when at the surface S can direct cell:cell fusion leading to the formation of multinucleate syncytia. Thus, the trafficking signals in the cytoplasmic tail of S protein indicate that syncytia formation is not an inadvertent by-product of infection but rather a key aspect of the replicative cycle of SARS-CoV-2 and potential cause of pathological symptoms.

Furin cleavage of SARS-CoV-2 Spike promotes but is not essential for infection and cell-cell fusion (preprint)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects cells by binding to the host cell receptor Ace2 and undergoing virus-host membrane fusion. Fusion is triggered by the protease TMPRSS2, which processes the viral Spike (S) protein to reveal the fusion peptide. SARS-CoV-2 has evolved a multibasic site at the S1-S2 boundary, which is thought to be cleaved by furin in order to prime S protein for TMPRSS2 processing. Here we show that CRISPR-Cas9 knockout of furin reduces, but does not prevent, the production of infectious SARS-CoV-2 virus. Comparing S processing in furin knockout cells to multibasic site mutants reveals that while loss of furin substantially reduces S1-S2 cleavage it does not prevent it. SARS-CoV-2 S protein also mediates cell-cell fusion, potentially allowing virus to spread virion-independently. We show that loss of furin in either donor or acceptor cells reduces, but does not prevent, TMPRSS2-dependent cell-cell fusion, unlike mutation of the multibasic site that completely prevents syncytia formation. Our results show that while furin promotes both SARS-CoV-2 infectivity and cell-cell spread it is not essential, suggesting furin inhibitors will not prevent viral spread.