Hartooni, N. | Search bioRxiv

bioRxiv2021

Single-molecule analysis of specificity and multivalency in binding of short linear substrate motifs to the APC/C

Robust regulatory signals in the cell often depend on interactions between short linear motifs (SLiMs) and globular proteins. Many of these interactions are poorly characterized because the binding proteins cannot be produced in the amounts needed for traditional methods. To address this problem, we developed a single-molecule off-rate (SMOR) assay based on microscopy of fluorescent ligand binding to immobilized protein partners. We used it to characterize substrate binding to the Anaphase-Promoting Complex/Cyclosome (APC/C), a ubiquitin ligase that triggers chromosome segregation. We find that SLiMs in APC/C substrates (the D box and KEN box) display distinct affinities and specificities for the substrate-binding subunits of the APC/C, and we show that multiple SLiMs in a substrate generate a high-affinity multivalent interaction. The remarkably adaptable substrate-binding mechanisms of the APC/C have the potential to govern the order of substrate destruction in mitosis.

biophysics↗

bioRxiv2020

Phosphorylation modulates liquid-liquid phase separation of the SARS-CoV-2 N protein

The nucleocapsid (N) protein of coronaviruses serves two major functions: compaction of the RNA genome in the virion and regulation of viral gene transcription in the infected cell1–3. The N protein contains two globular RNA-binding domains surrounded by regions of intrinsic disorder4. Phosphorylation of the central disordered region is required for normal viral genome transcription5,6, which occurs in a cytoplasmic structure called the replication transcription complex (RTC)7–11. It is not known how phosphorylation controls N protein function. Here we show that the N protein of SARS-CoV-2, together with viral RNA, forms biomolecular condensates12–15. Unmodified N protein forms partially ordered gel-like structures that depend on multivalent RNA-protein and protein-protein interactions. Phosphorylation reduces a subset of these interactions, generating a more liquid-like droplet. We speculate that distinct oligomeric states support the two functions of the N protein: unmodified protein forms a structured oligomer that is suited for nucleocapsid assembly, and phosphorylated protein forms a liquid-like compartment for viral genome processing. Inhibitors of N protein phosphorylation could therefore serve as antiviral therapy.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biochemistry↗