Functional Landscape of Motifs within the Sarbecovirus Spike Cytoplasmic Tail

Abstract Sarbecoviruses exhibit extensive diversity in host range and zoonotic potential. Although the ectodomain of the viral spike protein has been well characterized, the functional landscape of the cytoplasmic tail (CT) remains poorly defined. To address this gap, we systematically generated CT truncation variants of representative spike proteins from all major ACE2-utilizing clades to define the roles of conserved host factor–interacting motifs associated with COPI, COPII, FERM, and SNX27. Using vesicular stomatitis virus (VSV)- and lentivirus-based pseudotyping systems, we evaluated viral entry in cells expressing varying levels of ACE2 and TMPRSS2. Our results demonstrate that CT-associated motifs differentially regulate viral infectivity. Specifically, truncation of the COPI- or SNX27-binding motifs markedly reduces entry efficiency, whereas disruption of the COPII-binding motif produces the opposite outcome. By contrast, removal of the FERM-binding motif consistently enhances infectivity across lineages. Mechanistically, truncation of this motif increases spike expression, cell surface localization, incorporation into virions, and particle stability. Importantly, despite these pronounced effects on viral infectivity, deletion of the FERM-binding motif does not affect antigenicity, receptor dependence, or sensitivity to protease inhibitors, as demonstrated by neutralization and inhibition assays. In addition, this approach substantially increases spike protein density on virus-like particles (VLPs). Collectively, by extending the analysis beyond SARS-CoV-1 and SARS-CoV-2, our study reveals a generalizable mechanism in which cytoskeletal anchoring mediated by the FERM-binding motif acts as a limiting determinant of viral assembly. These findings provide a practical framework for optimizing pseudovirus platforms and guiding vaccine development against emerging viral threats. Competing Interest Statement The authors have declared no competing interest.