Single Molecule Microbiology

Abstract: Bacterial cell division requires recruitment of peptidoglycan (PG) synthases to the division site by the tubulin homologue, FtsZ. Septal PG synthases promote septum growth. FtsZ treadmilling is proposed to drive the processive movement of septal PG synthases and septal constriction in some bacteria; however, the precise mechanisms spatio-temporally regulating PG synthase movement and activity and FtsZ treadmilling are poorly understood. Here using single-molecule imaging of division proteins in the Gram-positive pathogen Staphylococcus aureus, we showed that the septal PG synthase complex FtsW/PBP1 and its putative activator protein, DivIB, move with similar velocity around the division site. Impairing FtsZ treadmilling did not affect FtsW or DivIB velocities or septum constriction rates. Contrarily, PG synthesis inhibition decelerated or stopped directional movement of FtsW and DivIB, and septum constriction. Our findings suggest that a single population of processively moving FtsW/PBP1 associated with DivIB drives cell constriction independently of FtsZ treadmilling in S. aureus.

Abstract: Latent infection by Mycobacterium tuberculosis (Mtb) impedes effective tuberculosis therapy and eradication. The protein PerM is essential for chronic Mtb infections in mice and acts via the divisome protein FtsB to modulate cell division. Using transgenic co-expression in Escherichia coli, we studied the Mtb PerM-FtsB interaction in isolation from other Mtb proteins, engineering PerM to enhance expression in the E. coli membrane. We confirmed the reported instability of Mtb FtsB, and we linked FtsB instability to a segment of FtsB predicted to bind cell-division proteins FtsL and FtsQ. Though narrowly conserved, the PerM-FtsB interaction emerges as a potential target for therapy targeting persistent infections by disrupting regulation of cell division. Using fluorescence microscopy, we found that stability of both FtsB and PerM hinges on their interaction via a C-terminal helix in FtsB. Molecular dynamics results supported the observation that FtsB stabilized PerM, and suggested that interactions at the PerM-FtsB interface differ from our initial structure prediction in a way that is consistent with PerM sequence conservation. Integrating protein structure prediction, molecular dynamics and single-molecule microscopy, our approach is primed to screen potential inhibitors of the PerM-FtsB interaction and can be straightforwardly adapted to explore other putative interactions.

Abstract: Accurate estimation of the effects of mutations on SARS-CoV-2 viral fitness can inform public-health responses such as vaccine development and predicting the impact of a new variant; it can also illuminate biological mechanisms including those underlying the emergence of variants of concern. Recently, Lan et al reported a high-quality model of SARS-CoV-2 secondary structure and its underlying dimethyl sulfate (DMS) reactivity data. I investigated whether secondary structure can explain some variability in the frequency of observing different nucleotide substitutions across millions of patient sequences in the SARS-CoV-2 phylogenetic tree. Nucleotide basepairing was compared to the estimated “mutational fitness” of substitutions, a measurement of the difference between a substitution’s observed and expected frequency that is correlated with other estimates of viral fitness. This comparison revealed that secondary structure is often predictive of substitution frequency, with significant decreases in substitution frequencies at basepaired positions. Focusing on the mutational fitness of C→T, the most common type of substitution, I describe C→T substitutions at basepaired positions that characterize major SARS-CoV-2 variants; such mutations may have a greater impact on fitness than appreciated when considering substitution frequency alone.

Abstract: SARS-CoV-2 genomes collected at the onset of the Covid-19 pandemic are valuable because they could help understand how the virus entered the human population. In 2021, Jesse Bloom reported on the recovery of a dataset of raw sequencing reads that had been removed from the NCBI SRA database at the request of the data generators, a scientific team at Wuhan University (Wang et al., 2020b). Bloom suggested that the data may have been removed in order to obfuscate the origin of SARS-CoV-2, and he questioned the generating authors’ statements that the samples had been collected on and after January 30, 2020. Here, we show that sample collection dates were published in 2020 by Wang et al. together with the sequencing reads, and match the dates given by the authors in 2021. We examine mutations in these sequences and confirm that they are entirely consistent with the previously known genetic diversity of SARS-CoV-2 of late January 2020. Finally, we explain how an apparent phylogenetic rooting paradox described by Bloom was resolved by subsequent analysis. Our reanalysis demonstrates that allegations of cover-up or of metadata manipulation were unwarranted.

Abstract: Coronavirus replication is associated with the remodeling of cellular membranes, resulting in the formation of double-membrane vesicles (DMVs). A DMV-spanning pore was identified as a putative portal for viral RNA. However, the exact components and the structure of the SARS-CoV-2 DMV pore remain to be determined. Here, we investigate the structure of the DMV pore by in situ cryo-electron tomography combined with subtomogram averaging. We identify non-structural protein (nsp) 3 and 4 as minimal components required for the formation of a DMV-spanning pore, which is dependent on nsp3-4 proteolytic cleavage. In addition, we show that Mac2-Mac3-DPUP-Ubl2 domains are critical for nsp3 oligomerization and crown integrity which influences membrane curvature required for biogenesis of DMVs. Altogether, SARS-CoV-2 nsp3-4 have a dual role by driving the biogenesis of replication organelles and assembly of DMV-spanning pores which we propose here to term replicopores.

Abstract: Zoonotic spillovers of viruses have occurred through the animal trade worldwide. The start of the COVID-19 pandemic was traced epidemiologically to the Huanan Wholesale Seafood Market, the site with the most reported wildlife vendors in the city of Wuhan, China. Here, we analyze publicly available qPCR and sequencing data from environmental samples collected in the Huanan market in early 2020. We demonstrate that the SARS-CoV-2 genetic diversity linked to this market is consistent with market emergence, and find increased SARS-CoV-2 positivity near and within a particular wildlife stall. We identify wildlife DNA in all SARS-CoV-2 positive samples from this stall. This includes species such as civets, bamboo rats, porcupines, hedgehogs, and one species, raccoon dogs, known to be capable of SARS-CoV-2 transmission. We also detect other animal viruses that infect raccoon dogs, civets, and bamboo rats. Combining metagenomic and phylogenetic approaches, we recover genotypes of market animals and compare them to those from other markets. This analysis provides the genetic basis for a short list of potential intermediate hosts of SARS-CoV-2 to prioritize for retrospective serological testing and viral sampling.

Abstract: The bacterial divisome is a macromolecular machine composed of more than 30 proteins that controls cell wall constriction during division. Here, we present a model of the structure and dynamics of the core complex of the E. coli divisome, supported by a combination of structure prediction, molecular dynamics simulation, single-molecule imaging, and mutagenesis. We focus on the septal cell wall synthase complex formed by FtsW and FtsI, and its regulators FtsQ, FtsL, FtsB, and FtsN. The results indicate extensive interactions in four regions in the periplasmic domains of the complex. FtsQ, FtsL, and FtsB support FtsI in an extended conformation, with the FtsI transpeptidase domain lifted away from the membrane through interactions among the C-terminal domains. FtsN binds between FtsI and FtsL in a region rich in residues with superfission (activating) and dominant negative (inhibitory) mutations. Mutagenesis experiments and simulations suggest that the essential domain of FtsN links FtsI and FtsL together, potentially modulating interactions between the anchor-loop of FtsI and the putative catalytic cavity of FtsW, thus suggesting a mechanism of how FtsN activates the cell wall synthesis activities of FtsW and FtsI.

Abstract: Double membrane vesicles (DMVs) in coronavirus-infected cells feature pores that span both membranes. DMV pores were observed to have six-fold symmetry and include the nsp3 protein. Co-expression of SARS-CoV nsp3 and nsp4 induces DMV formation, and elements of nsp3 and nsp4 have been identified that are essential for membrane disruption. I describe a predicted luminal binding interface between nsp3 and nsp4 that is membrane-associated, conserved in SARS-CoV-2 during the COVID-19 pandemic and in diverse coronaviruses, and stable in molecular dynamics simulation. Combined with structure predictions for the full-length nsp4 monomer and cryo-EM data, this suggests a DMV pore model in which nsp4 spans both membranes with nsp3 and nsp4 inserted into the same bilayer. This approach may be able to identify additional protein-protein interactions between coronavirus proteins.