Kinetochores are proteinaceous assemblies that mediate the interaction of chromosomes with the mitotic spindle. The 180 kDa Ndc80 complex is a direct point of contact between kinetochores and microtubules. Its four subunits contain coiled coils and form an elongated rod structure with functional globular domains at either end. We crystallized an engineered "bonsai" Ndc80 complex containing a shortened rod domain but retaining the globular domains required for kinetochore localization and microtubule binding. The structure reveals a microtubule-binding interface containing a pair of tightly interacting calponin-homology (CH) domains with a previously unknown arrangement. The interaction with microtubules is cooperative and predominantly electrostatic. It involves positive charges in the CH domains and in the N-terminal tail of the Ndc80 subunit and negative charges in tubulin C-terminal tails and is regulated by the Aurora B kinase. We discuss our results with reference to current models of kinetochore-microtubule attachment and centromere organization.

Microtubules are nucleated in vivo by γ-tubulin complexes. The 300 kDa γ-tubulin small complex (γTuSC), consisting of two molecules of γ-tubulin and one copy each of the accessory proteins Spc97p and Spc98p, is the conserved, essential core of the microtubule nucleating machinery1,2. In metazoa multiple γTuSCs assemble with other proteins into γ-tubulin ring complexes (γTuRCs). The structure of γTuRC suggested that it functions as a microtubule template2–5. Because each γTuSC contains two molecules of γ-tubulin, it was assumed that the γTuRC-specific proteins are required to organize γTuSCs to match thirteen-fold microtubule symmetry. Here, we show that γTuSC forms rings even in the absence of other γTuRC components. The yeast adaptor protein Spc110p stabilizes the rings into extended filaments and is required for oligomer formation under physiological buffer conditions. The 8Å cryo-EM reconstruction of the filament reveals thirteen γ-tubulins per turn, matching microtubule symmetry, with plus ends exposed for interaction with microtubules, implying that one turn of the filament constitutes a microtubule template. The domain structures of Spc97p and Spc98p suggest functions for conserved sequence motifs, with implications for the γTuRC-specific proteins. The γTuSC filaments nucleate microtubules at a low level, and the structure provides a strong hypothesis for how nucleation is regulated, converting this less active form to a potent nucleator.

The world continues to face a life-threatening viral pandemic. The virus underlying the Coronavirus Disease 2019 (COVID-19), Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), has caused over 98 million confirmed cases and 2.2 million deaths since January 2020. Although the most recent respiratory viral pandemic swept the globe only a decade ago, the way science operates and responds to current events has experienced a cultural shift in the interim. The scientific community has responded rapidly to the COVID-19 pandemic, releasing over 125,000 COVID-19–related scientific articles within 10 months of the first confirmed case, of which more than 30,000 were hosted by preprint servers. We focused our analysis on bioRxiv and medRxiv, 2 growing preprint servers for biomedical research, investigating the attributes of COVID-19 preprints, their access and usage rates, as well as characteristics of their propagation on online platforms. Our data provide evidence for increased scientific and public engagement with preprints related to COVID-19 (COVID-19 preprints are accessed more, cited more, and shared more on various online platforms than non-COVID-19 preprints), as well as changes in the use of preprints by journalists and policymakers. We also find evidence for changes in preprinting and publishing behaviour: COVID-19 preprints are shorter and reviewed faster. Our results highlight the unprecedented role of preprints and preprint servers in the dissemination of COVID-19 science and the impact of the pandemic on the scientific communication landscape.

Model organisms are widely used in research as accessible and convenient systems to study a particular area or question in biology. Traditionally only a handful of organisms have been widely studied, but modern research tools are enabling researchers to extend the set of model organisms to include less-studied and more unusual systems. This Forum highlights a range of 'non-model model organisms' as emerging systems for tackling questions across the whole spectrum of biology (and beyond), the opportunities and challenges, and the outlook for the future.

BackgroundCyanobacteria play a significant role in the global carbon cycle. In Synechococcus elongatus , the carbon-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is concentrated into polyhedral, proteinaceous compartments called carboxysomes.Methodology/Principal FindingsUsing live cell fluorescence microscopy, we show that carboxysomes are first detected as small seeds of RuBisCO that colocalize with existing carboxysomes. These seeds contain little or no shell protein, but increase in RuBisCO content over several hours, during which time they are exposed to the solvent. The maturing seed is then enclosed by shell proteins, a rapid process that seals RuBisCO from the cytosol to establish a distinct, solvent-protected microenvironment that is oxidizing relative to the cytosol. These closure events can be spatially and temporally coincident with the appearance of a nascent daughter RuBisCO seed.Conclusions/SignificanceCarboxysomes assemble in a stepwise fashion, inside-to-outside, revealing that cargo is the principle organizer of this compartment’s biogenesis. Our observations of the spatial relationship of seeds to previously formed carboxysomes lead us to propose a model for carboxysome replication via sequential fission, polymerization, and encapsulation of their internal cargo.

Bacterial cytoskeletal proteins participate in a variety of processes, including cell division and DNA segregation. Polymerization of one plasmid-encoded, actin-like protein, ParM, segregates DNA by pushing two plasmids in opposite directions and forms the current paradigm for understanding active plasmid segregation. An essential feature of ParM assembly is its dynamically instability, the stochastic switching between growth and disassembly. It is unclear whether dynamic instability is an essential feature of all actin-like protein-based segregation mechanisms or whether bacterial filaments can segregate plasmids by different mechanisms. We expressed and purified AlfA, a plasmid-segregating actin-like protein from Bacillus subtilis, and found that it forms filaments with a unique structure and biochemistry; AlfA nucleates rapidly, polymerizes in the presence of ATP or GTP, and forms highly twisted, ribbon-like, helical filaments with a left-handed pitch and protomer nucleotide binding pockets rotated away from the filament axis. Intriguingly, AlfA filaments spontaneously associate to form uniformly sized, mixed-polarity bundles. Most surprisingly, our biochemical characterization revealed that AlfA does not display dynamic instability and is relatively stable in the presence of diphosphate nucleotides. These results (i) show that there is remarkable structural diversity among bacterial actin filaments and (ii) indicate that AlfA filaments partition DNA by a novel mechanism.Bacteria contain multiple filament-forming proteins related to eukaryotic actin (6). These actin-like proteins have multiple cellular roles, including determination of cell shape (18), arrangement of organelles (20), and segregation of DNA (36). Little is known about the assembly dynamics of most of these proteins or about the identities and activities of the factors that regulate them. The widely expressed actin-like protein MreB, for example, has been purified and studied in vitro, but its assembly appears to be strongly inhibited by physiological concentrations of monovalent cations, suggesting that its assembly in vivo is facilitated by as-yet-unknown factors (23). At present, the best-understood actin-like protein is ParM, a plasmid-encoded protein that constructs a bipolar spindle capable of pushing plasmids to opposite poles of rod-shaped cells (5,25). In contrast to the eukaryotic actin cytoskeleton, whose assembly and architecture are regulated by a variety of accessory factors, ParM dynamics are regulated by a single factor, a complex composed of multiple copies of the repressor protein ParR bound to a DNA locus, parC (17). The ParR/parC complex binds the ends of ParM filaments and is pushed through the cytoplasm by filament elongation (5, 14, 25). The ability of ParM to function with such minimal regulation appears to be due to its unique assembly dynamics, which are dramatically different from those of eukaryotic actins. One of the most important differences is that ParM filaments are dynamically unstable (13). That is, similar to...

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25The world continues to face an ongoing viral pandemic that presents a serious threat to human 26 health. The virus underlying the COVID-19 disease, SARS-CoV-2, has caused over 3.2 million confirmed 27 cases and 220,000 deaths between January and April 2020. Although the last pandemic of respiratory 28 disease of viral origin swept the globe only a decade ago, the way science operates and responds to 29 current events has experienced a paradigm shift in the interim. The scientific community has 30 responded rapidly to the COVID-19 pandemic, releasing over 16,000 COVID-19 related scientific 31 articles within 4 months of the first confirmed case, of which at least 6,000 were hosted by preprint 32 servers. We focused our analysis on bioRxiv and medRxiv, two growing preprint servers for biomedical 33 research, investigating the attributes of COVID-19 preprints, their access and usage rates, 34 characteristics of their sharing on online platforms, and the relationship between preprints and their 35 published articles. Our data provides evidence for increased scientific and public engagement (COVID-36 19 preprints are accessed and distributed at least 15 times more than non-COVID-19 preprints) and 37 changes in journalistic practice with reference to preprints. We also find evidence for changes in 38 preprinting and publishing behaviour: COVID-19 preprints are shorter, with fewer panels and tables, 39 and reviewed faster. Our results highlight the unprecedented role of preprints and preprint servers in 40 the dissemination of COVID-19 science, and the likely long-term impact of the pandemic on the 41 scientific publishing landscape. 42 43 44 45 46 47 48 49 50 51 52 53 54The first quarter of 2020 has been defined by the COVID-19 outbreak, which has escalated to 55 pandemic status, and caused over 3.2 million cases and 220,000 deaths within 4 months of the first 56 reported case [1,2]. The causative pathogen was rapidly identified as a novel virus within the family 57Coronaviridae and was named severe acute respiratory syndrome coronavirus 2 (or 'SARS-CoV-2') [3]. 58 Although multiple coronaviruses are ubiquitous among humans and cause only mild disease, 59 epidemics of newly emerging coronaviruses were previously observed in SARS coronavirus in 2002 [4] 60 and Middle East respiratory syndrome (MERS) coronavirus in 2012 [5]. The unprecedented extent and 61 rate of spread of COVID-19 has created a critical global health emergency and academic communities 62 have raced to actively respond through research developments. 63 Research developments have traditionally been communicated via published journal articles or 64 conference presentations. Traditional scientific publishing involves the submission of manuscripts to 65 an individual journal, which then organises peer review. Authors often conduct additional experiments 66or analyses to address the reviewers' concerns in one or more revisions. Even after this lengthy 67 process is concluded, almost half of submissions are rejected and require re-submission to a different ...