Dynamic ensembles of SARS-CoV-2 N-protein reveal head-to-head coiled-coil-driven oligomerization and phase separation.

The SARS-CoV-2 nucleocapsid (N) protein is essential for the viral life cycle, facilitating RNA packaging, replication, and host-cell interactions. Its ability to self-assemble and undergo phase separation is critical for these functions but remains poorly understood. Using an integrated approach combining small-angle X-ray scattering (SAXS), nuclear magnetic resonance spectroscopy, computational modeling, and biophysical assays, we uncover key mechanisms underpinning N-protein's dynamic self-assembly. We show that the N-protein's interdomain linker (IDL) contains a conserved coiled-coil (CC) motif that drives transient interactions between protein subunits, enabling the formation of progressively larger complexes at higher concentrations. SAXS analysis and ensemble modeling reveal that the IDL exists in a concentration-dependent equilibrium between monomeric, dimeric, and trimeric states. The CC motif facilitates parallel, head-to-head oligomerization of N-protein dimers, transitioning between compact (closed) and extended (open) configurations depending on the interaction network within the IDL. This linker-driven assembly modulates phase separation, impacting the size, stability, and dynamics of biomolecular condensates. Here, we present the most comprehensive conformational landscape analysis of the N-protein to date, providing a detailed model of its self-assembly and phase separation. Our findings highlight how the structural plasticity of the IDL and CC-mediated interactions are pivotal to its roles in the SARS-CoV-2 life cycle.