The REEP5/TRAM1 complex binds SARS-CoV-2 NSP3 and promotes virus replication.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), like other coronaviruses, replicates their genome in virus-induced cytosolic membrane-bound replication organelles (ROs). SARS-CoV-2 promotes the biogenesis of ROs by inducing the rearrangement of endoplasmic reticulum (ER) membranes. NSP3, NSP4, and NSP6 are transmembrane viral non-structural proteins (NSPs) and essential players in the formation of ROs. To understand how these three NSPs work synergistically with host-binding proteins, we performed affinity purifications followed by mass spectrometry analyses to study the host-viral protein-protein interactome of NSP3, NSP4, and NSP6 expressed individually and in combination. Through this analysis, we identified two host transmembrane proteins, REEP5 and TRAM1, as critical interacting partners of NSP3 that localize at the membrane of the RO. REEP5 interacts with TRAM1 endogenously and binds NSP3 during SARS-CoV-2 infection. REEP5 knockout reduces ER membrane rearrangements and inhibits SARS-CoV-2 replication. Collectively, our study shows that the host REEP5/TRAM1 complex binds NSP3, promoting RO biogenesis and viral replication. IMPORTANCE Generation of virus-host protein-protein interactions (PPIs) maps may provide clues to uncover SARS-CoV-2-hijacked cellular processes. However, these PPIs maps were created by expressing each viral protein singularly, which does not reflect the life situation in which certain viral proteins synergistically interact with host proteins. Our results reveal the host-viral protein-protein interactome of SARS-CoV-2 NSP3, NSP4, and NSP6 expressed individually or in combination. Furthermore, REEP5/TRAM1 complex interacts with NSP3 at ROs and promotes viral replication. The significance of our research is identifying virus-host interactions that may be targeted for therapeutic intervention.