Biology subjects

Morgan, D. O.

Publications and source records attributed to Morgan, D. O..

9 recordsLinked to original sources

bioRxiv2026

Intramolecular loops control SARS-CoV-2 nucleocapsid protein self-association and nucleic acid binding dependent on phosphorylation

The nucleocapsid protein of SARS-CoV-2 scaffolds genomic RNA into ribonucleoprotein complexes (RNP) for assembly in the virion, but also fulfills critical intracellular functions in replication and the suppression of host defense. It is comprised of a folded nucleic acid binding domain (NTD) and a dimerization domain, connected by a disordered linker containing a serine/arginine-rich (SR) region and a leucine-rich sequence (LRS). The switch between intracellular and assembly functions of N-protein is controlled by phosphorylation of the SR region, but the molecular details are unclear. Here we describe a model in which two mutually exclusive intramolecular loops bind the NTD and dynamically control self-association and nucleic acid binding properties dependent on the SR linker phosphorylation state. The model is supported by biophysical properties and interactions of full-length protein, point mutants, and peptide fragments. We find SR linker phosphorylation compacts the protein and inhibits nucleic acid binding and RNP formation, while enhancing self-association through promotion of transient coiled-coils in the LRS of the linker. These changes shift the nucleocapsid protein to a configuration poised for multi-valent interactions that support intracellular functions.

biophysics↗

bioRxiv2025

Substrate recognition by human separase

The protein complex cohesin encircles the sister chromatids in early mitosis1. At anaphase onset, sister separation is triggered by cleavage of the cohesin subunit SCC1/RAD21 by the cysteine protease separase2-5. SCC1 contains two cleavage sites, where cleavage is stimulated by SCC1 phosphorylation5,6. The molecular mechanisms of substrate recognition and cleavage are only partly understood7. Here, we determined a series of cryoEM structures of human separase in apo-or substrate-bound forms that, together with biochemical analysis, provide novel insights into the regulation of separase cleavage activity. We verify the first SCC1 cleavage site and reassign the second site. We show that multiple substrates, including separase autocleavage sites8,9 and the two SCC1 cleavage sites, interact with several docking sites in separase, including four phosphate-binding sites. We also describe the structural basis of the interaction between the cohesin subunit SA1/A2 and separase, which promotes cleavage at the second site in SCC1. Finally, using cross-linking mass spectrometry and cryoEM, we propose a model of how cohesin is targeted by human separase. Our work provides an extensive functional and structural framework that explains one of the most fundamental events in cell division.

biochemistry↗

bioRxiv2024

Phosphate-binding pocket on cyclin B governs CDK substrate phosphorylation and mitotic timing

Cell cycle progression is governed by complexes of the cyclin-dependent kinases (CDKs) and their regulatory subunits cyclin and Cks1. CDKs phosphorylate hundreds of substrates, often at multiple sites. Multisite phosphorylation depends on Cks1, which binds initial priming phosphorylation sites to promote secondary phosphorylation at other sites. Here, we describe a similar role for a recently discovered phosphate-binding pocket (PP) on B-type cyclins. Mutation of the PP in Clb2, the major mitotic cyclin of budding yeast, alters bud morphology and delays the onset of anaphase. Mutation of the PP reduces multi-site phosphorylation of CDK substrates in vitro, including the Cdc16 and Cdc27 subunits of the anaphase-promoting complex/cyclosome and the Bud6 and Spa2 subunits of the polarisome. We conclude that the cyclin PP, like Cks1, controls the pattern of multisite phosphorylation on CDK substrates, thereby helping to establish the robust timing of cell-cycle events.

biochemistry↗

bioRxiv2023

Assembly of SARS-CoV-2 ribonucleosomes by truncated N* variant of the nucleocapsid protein

The Nucleocapsid (N) protein of SARS-CoV-2 compacts the RNA genome into viral ribonucleoprotein (vRNP) complexes within virions. Assembly of vRNPs is inhibited by phosphorylation of the N protein SR region. Several SARS-CoV-2 variants of concern carry N protein mutations that reduce phosphorylation and enhance the efficiency of viral packaging. Variants of the dominant B.1.1 viral lineage also encode a truncated N protein, termed N* or {Delta}(1-209), that mediates genome packaging despite lacking the N-terminal RNA-binding domain and SR region. Here, we show that {Delta}(1-209) and viral RNA assemble into vRNPs that are remarkably similar in size and shape to those formed with full-length N protein. We show that assembly of {Delta}(1-209) vRNPs requires the leucine-rich helix (LH) of the central disordered region, and that the LH promotes N protein oligomerization. We also find that fusion of a phosphomimetic SR region to {Delta}(1-209) inhibits RNA binding and vRNP assembly. Our results provide new insights into the mechanisms by which RNA binding promotes N protein self-association and vRNP assembly, and how this process is modulated by SR phosphorylation.

microbiology↗

bioRxiv2022

Reconstitution of the SARS-CoV-2 ribonucleosome provides insights into genomic RNA packaging and regulation by phosphorylation

The nucleocapsid (N) protein of coronaviruses is responsible for compaction of the [~]30-kb RNA genome in the [~]100-nm virion. Cryo-electron tomography suggests that each virion contains 35-40 viral ribonucleoprotein (vRNP) complexes, or ribonucleosomes, arrayed along the genome. There is, however, little mechanistic understanding of the vRNP complex. Here, we show that N protein, when combined with viral RNA fragments in vitro, forms cylindrical 15-nm particles similar to the vRNP structures observed within coronavirus virions. These vRNPs form in the presence of stem-loop-containing RNA and depend on regions of N protein that promote protein-RNA and protein-protein interactions. Phosphorylation of N protein in its disordered serine/arginine (SR) region weakens these interactions and disrupts vRNP assembly. We propose that unmodified N binds stem-loop-rich regions in genomic RNA to form compact vRNP complexes within the nucleocapsid, while phosphorylated N maintains uncompacted viral RNA to promote the proteins transcriptional function.

biochemistry↗

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↗

bioRxiv2021

Multisite phosphorylation by Cdk1 initiates delayed negative feedback to control mitotic transcription

Cell-cycle progression is driven by the phosphorylation of cyclin-dependent kinase (Cdk) substrates1-3. The order of substrate phosphorylation depends in part on the general rise in Cdk activity during the cell cycle4-7, together with variations in substrate docking to sites on associated cyclin and Cks subunits3, 6, 8-10. Many substrates are modified at multiple sites to provide more complex regulation9, 11-14. Here, we describe an elegant regulatory circuit based on multisite phosphorylation of Ndd1, a transcriptional co-activator of genes required for mitotic progression15, 16. As cells enter mitosis, Ndd1 phosphorylation by Cdk1 is known to promote mitotic cyclin (CLB2) gene transcription, resulting in positive feedback17-20. Consistent with these findings, we show that low Cdk1 activity promotes CLB2 expression at mitotic entry. We also find, however, that CLB2 expression is inhibited by high levels of Cdk1 activity in a mitotic arrest. Inhibition is accompanied by Ndd1 degradation, and we present evidence that high mitotic Cdk1-Clb2 activity generates phosphodegrons on Ndd1, leading to its degradation. Complete Ndd1 phosphorylation by the Clb2-Cdk1-Cks1 complex requires the phosphothreonine-binding site of Cks1, as well as a novel phosphate-binding pocket on the cyclin Clb221. We therefore propose that initial phosphorylation by Cdk1 primes the protein for secondary phosphorylation at phosphodegrons, resulting in degradation only at high Cdk1 activity. Together, our results suggest that rising levels of mitotic Cdk1 activity act at multiple phosphorylation sites on Ndd1, first triggering rapid positive feedback and then promoting delayed negative feedback, resulting in a pulse of mitotic gene expression.

cell biology↗

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↗

bioRxiv2019

Polyanions provide selective control of APC/C interactions with the activator subunit

Transient interactions between the Anaphase-Promoting Complex/Cyclosome (APC/C) and its activator subunit Cdc20 or Cdh1 generate oscillations in ubiquitination activity necessary to maintain the order of cell cycle events. Activator binds the APC/C with high affinity and exhibits negligible dissociation kinetics in vitro, and it is not clear how the rapid turnover of APC/C-activator complexes is achieved in vivo. Here, we describe a mechanism that controls APC/C-activator interactions based on the availability of substrates. We find that APC/C-activator dissociation is stimulated by abundant cellular polyanions such as nucleic acids and polyphosphate. Polyanions also interfere with the ubiquitination of low-affinity substrates. However, engagement with high-affinity substrates blocks the inhibitory effects of polyanions on activator binding and APC/C activity. This mechanism amplifies the effects of substrate affinity on APC/C function, stimulating processive ubiquitination of high-affinity substrates and suppressing ubiquitination of low-affinity substrates.

biochemistry↗