Identification of a membrane-associated element (MAE) in the C-terminal region of SARS-CoV-2 nsp6 that is essential for viral replication.

ABSTRACT

The coronavirus disease 2019 (COVID-19) pandemic, caused by the novel coronavirus severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), has rapidly spread worldwide since its emergence in late 2019. Its ongoing evolution poses challenges for antiviral drug development. Coronavirus nsp6, a multiple-spanning transmembrane protein, participates in the biogenesis of the viral replication complex, which accommodates the viral replication-transcription complex. The roles of its structural domains in viral replication are not well studied. Herein, we predicted the structure of the SARS-CoV-2 nsp6 protein using AlphaFold2 and identified a highly folded C-terminal region (nsp6C) downstream of the transmembrane helices. The enhanced green fluorescent protein (EGFP)-fused nsp6C was found to cluster in the cytoplasm and associate with membranes. Functional mapping identified a minimal membrane-associated element (MAE) as the region from amino acids 237 to 276 (LGV-KLL), which is mainly composed of the α-helix H1 and the α-helix H2; the latter exhibits characteristics of an amphipathic helix (AH). Mutagenesis studies and membrane flotation experiments demonstrate that AH-like H2 is required for MAE-mediated membrane association. This MAE was functionally conserved across MERS-CoV, HCoV-OC43, HCoV-229E, HCoV-HKU1, and HCoV-NL63, all capable of mediating membrane association. In a SARS-CoV-2 replicon system, mutagenesis studies of H2 and replacements of H1 and H2 with their homologous counterparts demonstrated requirements of residues on both sides of the H2 and properly paired H1-H2 for MAE-mediated membrane association and viral replication. Notably, mutations I266A and K274A significantly attenuated viral replication without dramatically affecting membrane association, suggesting a dual role of the MAE in viral replication mediating membrane association as well as participating in protein-protein interactions.IMPORTANCESevere acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) assembles a double-membrane vesicle (DMV) by the viral non-structural proteins for viral replication. Understanding the mechanisms of the DMV assembly is of paramount importance for antiviral development. Nsp6, a multiple-spanning transmembrane protein, plays an important role in the DMV biogenesis. Herein, we predicted the nsp6 structure of SARS-CoV-2 and other human coronaviruses using AlphaFold2 and identified a putative membrane-associated element (MAE) in the highly conserved C-terminal regions of nsp6. Experimentally, we verified a functionally conserved minimal MAE composed of two α-helices, the H1, and the amphipathic helix-like H2. Mutagenesis studies confirmed the requirement of H2 for MAE-mediated membrane association and viral replication and demonstrated a dual role of the MAE in viral replication, by mediating membrane association and participating in residue-specific interactions. This functionally conserved MAE may serve as a novel anti-viral target.