Sequences in the cytoplasmic tail of SARS-CoV-2 Spike facilitate expression at the cell surface and syncytia formation

Abstract

The Spike (S) protein of SARS-CoV-2 binds ACE2 to direct fusion with host cells. S comprises a large external domain, a transmembrane domain, and a short cytoplasmic tail. Understanding the intracellular trafficking of S is relevant to SARS-CoV-2 infection, and to vaccines expressing full-length S from mRNA or adenovirus vectors. Here we report a proteomic screen for cellular factors that interact with the cytoplasmic tail of S. We confirm interactions with the COPI and COPII vesicle coats, ERM family actin regulators, and the WIPI3 autophagy component. The COPII binding site promotes exit from the endoplasmic reticulum, and although binding to COPI should retain S in the early Golgi where viral budding occurs, there is a suboptimal histidine residue in the recognition motif. As a result, S leaks to the surface where it accumulates and can direct the formation of multinucleate syncytia. Thus, the trafficking signals in the tail of S indicate that syncytia play a role in the SARS-CoV-2 lifecycle.

An extended motif in the SARS-CoV-2 spike modulates binding and release of host coatomer in retrograde trafficking

“…2e, f ). Cellular studies suggest enhanced interactions of this mutant spike sequence with COPI subunits 15 , 30 . It is likely that this mutation affects the local conformation of the full-length spike protein leading to modulation of COPI interactions in a cellular environment.…”

“…Recent investigations of SARS-CoV-2 spike and previously of SARS-CoV spike with COPI have employed cellular lysates 10 , 15 , 30 . Hence, we asked if there is direct interaction between the purified components.…”

“…Cellular and biochemical investigations in recent years and during the ongoing COVID-19 pandemic have suggested a role of COPI interactions in sarbecovirus spike trafficking, maturation, glycan processing, and syncytia formation during infection 10 , 15 , 30 . These studies have established a platform to investigate the underlying chemistry of spike-COPI interaction.…”

SARS‐CoV‐2 Alpha, Beta, and Delta variants display enhanced Spike‐mediated syncytia formation

“…Merging of viral and cellular membranes allows for viral contents to be deposited into the cell to begin the viral life cycle. Within the cell, newly synthesized S protein, envelope, and membrane proteins are inserted into the endoplasmic reticulum (ER) and trafficked and processed through the ER‐Golgi network (Nal et␣al , 2005 ; Duan et␣al , 2020 ; Cattin‐Ortolá et␣al , 2021 ). Virion are formed by budding into ER‐Golgi membranes and are then transported to the surface in order to be released from the cell (Klein et␣al , 2020 ).…”

“…Virion are formed by budding into ER‐Golgi membranes and are then transported to the surface in order to be released from the cell (Klein et␣al , 2020 ). While the majority of the S protein is sequestered within the ER, motifs within its cytoplasmic tail allow for leakage from the Golgi apparatus and localization at the plasma membrane (Cattin‐Ortolá et␣al , 2021 ). The S protein at the surface of an infected cell interacts with receptors on adjacent cells, fusing the plasma membranes together and merging the cytoplasmic contents.…”