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.

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.

2-Aminopurine (2AP) is a fluorescent analog of guanosine and adenosine and has been used to probe nucleic acid structure and dynamics. Its spectral features in nucleic acids have been interpreted phenomenologically, in the absence of a rigorous electronic description of the context-dependence of 2AP fluorescence. Now, by using time-dependent density functional theory, we describe the excited-state properties of 2AP in a B-form dinucleotide stacked with guanosine, adenosine, cytosine, or thymine. Calculations predict that 2AP fluorescence is quenched statically when stacked with purines, because of mixing of the molecular orbitals in the ground state. In contrast, quenching is predicted to be dynamic when 2AP is stacked with pyrimidines, because of formation of a low-lying dark excited state. The different quenching mechanisms will result in different experimentally measured fluorescence lifetimes and quantum yields. The nucleotide 2-aminopurine (2AP) has been used as a site-specific probe of nucleic acid structure and dynamics (1-11) because it base pairs with cytosine in a wobble configuration (4,5,12) or with thymine in a Watson-Crick geometry (7,11). Thermodynamic measurements of DNAs containing 2AP:C or 2AP:T show that the former pairing is more destabilizing (11,12). Incorporating 2AP into DNA quenches its fluorescence (2, 8, 11), reducing its quantum yield from that of the free nucleoside (0.65 in aqueous solution). This reduction is attributed to stacking interactions with nearest neighbor nucleobases, and, therefore, fluorescence properties of 2AP have been used to probe the equilibrium stacking properties of DNA duplexes containing these mismatched pairs (2,8,10). Although the fluorescence decay of 2AP in solution is single exponential, with a lifetime of Ϸ10 ns, in the context of a DNA molecule, four decay components, from 50 ps to 8 ns, are typically needed to describe its lifetime (2). The short decay time observed for 2AP in a duplex is attributed to the fully stacked state, whereas the longest lifetime comes from unstacked 2AP (2, 7). The distribution of stacked states can be altered by temperature, solvent, flanking bases, and bound protein, and so all of these conditions can be probed by using 2AP fluorescence. Dynamics of stacked bases adjacent to a mutagenic mismatch in DNA (such as 2AP:C and 2AP:T) may be part of the recognition mechanism by replication͞repair enzymes (3, 13), so interpretation of 2AP fluorescence data is critical for describing these environments.To understand the effect of base stacking on fluorescent properties of 2AP, we have used time-dependent density functional theory (TDDFT) (14, 15) to calculate the excited-state properties of 2AP alone and in stacked B-and A-form dinucleotides (dimers). Analysis of monomer calculations indicates the accuracy with which TDDFT describes spectral properties: excited-state transition energies, transition dipole directions, and oscillator strengths for 2AP, thymine, cytosine, adenine, and guanine (see figures) are in excellent agreement w...

The isolated 102 amino acid N-terminal RNA binding domain (RBD) of the human U1A protein specifically interacts with a short RNA hairpin containing the U1 snRNA stem/loop II sequence. This recognition is nucleotide-specific, for substitutions of critical nucleotides in the RNA loop decrease binding affinity up to 10(6)-fold, as measured by nitrocellulose filter binding experiments. The magnitude of the loss of binding free energy with single-nucleotide substitution in the conserved GCA sequence suggests that the interaction between the RBD and RNA occurs through a number of interdependent specific contacts in the complex. 13C and 15N NMR experiments, using isotopically-labeled RNA together with unlabeled protein, show that the chemical shifts of many protons from the bound RNA are substantially different from those of the free RNA, especially in the loop region of the hairpin. All these data suggest that there is a conformational change in the RNA upon formation of the RBD-RNA complex.

We have used NMR to study the structure of the yeast tRNA(Phe) sequence which was synthesized by using T7 RNA polymerase. Many resonances in the imino 1H- spectrum of the transcript have been assigned, including those of several tertiary interactions. When the Mg2+ concentration is high, the transcript appears to fold normally, and the spectral features of the transcript resemble those of tRNA(Phe). The transcript has been shown to be aminoacylated with kinetics similar to the modified tRNA(Phe) [Sampson, J. R., & Uhlenbeck, O. C. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 1033-1037], suggesting that the structure of the two molecules must be similar. In the absence of Mg2+ or at [tRNA]:[Mg2+] ratios less than 0.2, the transcript does not adopt the native structure, as shown by both chemical shifts and NOE patterns. In these low Mg2+ conditions, a second GU base pair is found, suggesting a structural rearrangement of the transcript. NMR data indicate that the structure of a mutant having G20 changed to U20 is nearly identical with that of the normal sequence, suggesting that the low aminoacylation activity of this variant is not due to a substantially different conformation.