The SARS-CoV-2 nucleocapsid (N) protein is an abundant RNA-binding protein critical for viral genome packaging, yet the molecular details that underlie this process are poorly understood. Here we combine single-molecule spectroscopy with all-atom simulations to uncover the molecular details that contribute to N protein function. N protein contains three dynamic disordered regions that house putative transiently-helical binding motifs. The two folded domains interact minimally such that full-length N protein is a flexible and multivalent RNA-binding protein. N protein also undergoes liquid-liquid phase separation when mixed with RNA, and polymer theory predicts that the same multivalent interactions that drive phase separation also engender RNA compaction. We offer a simple symmetry-breaking model that provides a plausible route through which single-genome condensation preferentially occurs over phase separation, suggesting that phase separation offers a convenient macroscopic readout of a key nanoscopic interaction.

SARS-CoV-2 has intricate mechanisms for initiating infection, immune evasion/suppression and replication that depend on the structure and dynamics of its constituent proteins. Many protein structures have been solved, but far less is known about their relevant conformational changes. To address this challenge, over a million citizen scientists banded together through the Folding@home distributed computing project to create the first exascale computer and simulate 0.1 seconds of the viral proteome. Our adaptive sampling simulations predict dramatic opening of the apo spike complex, far beyond that seen experimentally, explaining and predicting the existence of 'cryptic' epitopes. Different spike variants modulate the probabilities of open versus closed structures, balancing receptor binding and immune evasion. We also discover dramatic conformational changes across the proteome, which reveal over 50 'cryptic' pockets that expand targeting options for the design of antivirals. All data and models are freely available online, providing a quantitative structural atlas.

The SARS-CoV-2 nucleocapsid (N) protein is an abundant RNA binding protein that plays a variety of roles in the viral life cycle including replication, transcription, and genome packaging. Despite its critical and multifunctional nature, the molecular details that underlie how N protein mediates these functions are poorly understood. Here we combine single-molecule spectroscopy with all-atom simulations to uncover the molecular details that contribute to the function of SARS-CoV-2 N protein. N protein contains three intrinsically disordered regions and two folded domains. All three disordered regions are highly dynamic and contain regions of transient helicity that appear to act as local binding interfaces for protein-protein or protein-RNA interactions. The two folded domains do not significantly interact with one another, such that full-length N protein is a flexible and multivalent RNA binding protein. As observed for other proteins with similar molecular features, we found that N protein undergoes liquid-liquid phase separation when mixed with RNA. Polymer models predict that the same multivalent interactions that drive phase separation also engender RNA compaction. We propose a simple model in which symmetry breaking through specific binding sites promotes the formation of metastable single-RNA condensate, as opposed to large multi-RNA phase separated droplets. We speculate that RNA compaction to form dynamic single-genome condensates may underlie the early stages of genome packaging. As such, assays that measure how compounds modulate phase separation could provide a convenient tool for identifying drugs that disrupt viral packaging.