The SARS-CoV-2 envelope and membrane proteins modulate maturation and retention of the spike protein, allowing assembly of virus-like particles

Abstract

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a β-coronavirus, is the causative agent of the COVID-19 pandemic. Like for other coronaviruses, its particles are composed of four structural proteins, namely Spike S, Envelope E, Membrane M and Nucleoprotein N proteins. The involvement of each of these proteins and their interplays during the assembly process of this new virus are poorly-defined and are likely β-coronavirus-type different. Therefore, we sought to investigate how SARS-CoV-2 behaves for its assembly by expression assays of S, in combination with E, M and/or N. By combining biochemical and imaging assays, we showed that E and M regulate intracellular trafficking of S and hence its furin-mediated processing. Indeed, our imaging data revealed that S remains at ERGIC or Golgi compartments upon expression of E or M, like for SARS-CoV-2 infected cells. By studying a mutant of S, we showed that its cytoplasmic tail, and more specifically, its C-terminal retrieval motif, is required for the M-mediated retention in the ERGIC, whereas E induces S retention by modulating the cell secretory pathway. We also highlighted that E and M induce a specific maturation of S N-glycosylation, which is observed on particles and lysates from infected cells independently of its mechanisms of intracellular retention. Finally, we showed that both M, E and N are required for optimal production of virus-like-proteins. Altogether, our results indicated that E and M proteins influence the properties of S proteins to promote assembly of viral particles. Our results therefore highlight both similarities and dissimilarities in these events, as compared to other β-coronaviruses. Author Summary The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the COVID-19 pandemic. Its viral particles are composed of four structural proteins, namely Spike S, Envelope E, Membrane M and Nucleoprotein N proteins, though their involvement in the virion assembly remain unknown for this particular coronavirus. Here we showed that presence of E and M influence the localization and maturation of S protein, in term of cleavage and N-glycosylation maturation. Indeed, E protein is able to slow down the cell secretory pathway whereas M-induced retention of S requires the retrieval motif in S C-terminus. We also highlighted that E and M might regulate the N glycosylation maturation of S independently of its intracellular retention mechanism. Finally, we showed that the four structural proteins are required for optimal formation of virus-like particles, highlighting the involvement of N, E and M in assembly of infectious particles. Altogether, our results highlight both similarities and dissimilarities in these events, as compared to other β-coronaviruses.

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