Fengbin Wang

Long-range extracellular electron transfer enables respiring microbes to use minerals, organisms, or electrodes as electron acceptors by transporting electrons microns away from the cell surface. This process is primarily studied in Geobacter sulfurreducens , which produces at least three distinct, micrometer-long, multiheme cytochrome nanowires capable of electron transfer. However, the distribution and higher-order structure of such nanowires remains largely unknown. Here, we employed cryo-electron microscopy to determine the atomic structure of a unique cytochrome nanowire from Desulfuromonas soudanensis WTL, a halophilic, iron- and electrode-reducing bacterium from deep subsurface brine. These filaments are based on an OmcE tetraheme cytochrome homolog that assembles into highly ordered bundles of anti-parallel filaments. Electronic measurements of purified nanowire bundle films reveal that these bundles are conductive, with conductance increasing in a concentration-dependent manner. The apparent midpoint potential of these OmcE bundles was −145 mV vs SHE. They exhibit redox activity over a ~370 mV window attributable to five electrostatically coupled hemes, suggesting heme–heme interaction beyond the boundaries of individual OmcE repeat units. Observations of these bundles under diverse buffer conditions suggest that they represent stable supramolecular assemblies, although direct in vivo confirmation within intact biofilms remains a future goal. Furthermore, a similar cytochrome bundle structure was observed in Geobacter metallireducens , showing that this quaternary structure may be a common feature among nanowires secreted by electroactive microbes. Our findings demonstrate that cytochrome nanowires are widespread and can form specialized bundle interfaces. This novel state could facilitate conductive biofilm formation and form the basis for direct microbial electron exchange. IMPORTANCE: All available structures of filaments containing a central core of closely spaced multiheme c-type cytochromes are derived from only a few organisms and are based on individual fibers easily imaged by cryo-electron microscopy. While imaging enriched preparations of extracellular appendages from a bacterium isolated from deep subsurface brine, we observed ordered bundles of anti-parallel cytochrome filaments. This quaternary arrangement contained specific contacts between adjacent nanowires, and a similar structure could be found in a relative from a related genus. This first report of bundling by nanowires suggests that such conductive structures may be missed in traditional surveys searching for novel filaments and provides an explanation for how nanowires could directly link adjacent cells in biofilms and granular communities.

Enzyme-responsive peptide nanomaterials can induce non-apoptotic cancer cell death, yet the supramolecular states that mediate activity in cellular environments are usually inferred rather than directly resolved. Here, we integrate multiscale in situ imaging with biochemical assays to map distinct assembly states of the phosphatase-responsive peptide NBD-L4pY across cell-free and cellular environments. Cell-free cryo-electron microscopy (cryo-EM) reveals concentration- and history-dependent cross-β polymorphs. An equilibrated NBD-L4pY solution shows no detectable filamentous assemblies, whereas rapid dilution from a higher concentration produces a mixed filament population containing a ∼7 nm C13 nanotube. At the cell interface, this accessible C13 packing architecture becomes predominant and is resolved in situ at 3.4 Å. Cryo-electron tomography (cryo-ET) places these nanotubes to membrane contact and invagination sites associated with localized membrane perturbation and early permeability changes. At later times, cryo-ET of lamellae prepared by cryo-focused ion beam (cryo-FIB) milling reveals morphologically distinct intracellular assemblies associated with filamentous actin and membranous organelles. Fractionation shows profilin-1 (PFN1) redistribution into peptide-rich pellets, whereas reconstitution demonstrates dose-dependent impairment of actin polymerization. Together, the data support a two-stage model in which cellular environments first favor a membrane-active nanotube and subsequently promote supramolecular remodeling into morphologically distinct intracellular assemblies associated with altered actin regulation. Thus, context-dependent structure selection and remodeling, rather than enzyme activation alone, emerge as key determinants of bioactive supramolecular materials.

Programmed lytic cell death, including pyroptosis and necroptosis, involves intracellular enzymes that form membrane-rupturing pores. Tumor-associated ectoenzymes such as alkaline phosphatase (ALP), however, offer the potential to initiate lytic death extrinsically. Here, we design a phospho-biphenyl-capped peptide precursor that is selectively dephosphorylated by ALP on cancer cell surfaces, triggering enzyme-instructed peptide self-assembly (EISA) into in situ peptide filaments. These supramolecular filaments physically breach the plasma membrane, overwhelm ESCRT-dependent membrane repair, and induce catastrophic calcium influx, cytoskeletal collapse, and organelle dysfunction. While cryo-EM uncovers 2.5-2.9 Å resolution details of ordered dimeric packing that underlies their mechanical rigidity and membrane-rupturing capability, cryo-electron tomography (cryo-ET) reveals the filament penetration of the plasma membrane in live cells. By reprogramming ALP from an immune checkpoint ectoenzyme into a pro-death catalyst, this work establishes a molecular mechanism linking enzymatic catalysis to supramolecular order and membrane failure. More broadly, it outlines a supramolecular chemical-biology framework in which enzyme-triggered assemblies function as programmable executors of cell death.

Bacterial biofilms represent a survival strategy that enables microbial communities to withstand environmental stress. Stenotrophomonas maltophilia is an emerging, antibiotic-resistant, Gram-negative, opportunistic pathogen that frequently colonizes the lungs of individuals with cystic fibrosis. Its chaperone-usher pathway (CUP) pilus, SMF-1, is present in nearly all clinical isolates and is essential for biofilm development; however, its molecular architecture has remained unknown. Here, we present a 4.0 Å cryo-EM structure revealing that SMF-1 is an archaic, rather than a classic, CUP pilus. SMF-1 forms thin, zigzag filaments that assemble into defined antiparallel “pili-couples,” which further aggregate into thick bundles. These bundles act as robust intercellular tethers, facilitating the rapid cell-to-cell aggregation required for biofilm initiation. Despite high sequence and structural similarity to classic CUP systems, SMF-1 lacks the interfaces required to form a rod-like architecture, suggesting it may represent an evolutionary intermediate between the CUP classes. Finally, we demonstrate that SMF-1 producing bacteria initiate biofilm formation within 24 hours and that flagella can further accelerate this process. Together, these findings uncover a conserved bundling mechanism that promotes bacterial colonization and may contribute to pathogenicity.

Microbial nanowires are appendages that Bacteria and Archaea use to transfer electrons to external surfaces, such as minerals, electrodes, or other microbes. While initial studies suggested that nanowires were modified pili, recent advancements in cryo-EM revealed that microbial nanowires are composed of multi-heme c-type cytochromes. In this review, we discuss the discovery of microbial nanowires, advancements that allowed elucidation of their near-atomic resolution cryo-EM structures, and the impact of heme arrangement on electron transfer. We also discuss how new structural information can be used to identify filaments in images from published literature. The structural insights gained from these studies provide a deeper understanding of the mechanisms underlying long-range electron transport in microbial nanowires and their potential applications in bioelectronics and energy-generating microbial fuel cells.

Helicobacter pylori relies on flagellar motility to colonize the gastric niche. While H. pylori flagella serve multiple functions, including swimming motility, it has recently been appreciated that flagellar basal body properties also contribute to biofilm initiation. The H. pylori flagellar filament contains two spatially segregated flagellins, FlaA and FlaB, but their roles in the biofilm initiation process remained undefined. Here, we confirmed that both FlaA and FlaB are required for optimal motility, but found that they exert opposite effects on biofilm initiation: cells without FlaA display low biofilm initiation, while cells without FlaB display elevated initiation. To understand the molecular basis for this divergence, cryo-electron microscopy (cryo-EM) analysis of native flagellar filaments revealed that FlaA and FlaB have distinct supercoiled waveforms. We confirmed these intrinsic waveform curvatures by analyzing filaments from ΔflaA and ΔflaB mutants, providing a physical basis for their functional specialization. Genetic epistasis experiments further demonstrated that the enhanced early biofilm formation of the ΔflaB mutant depends on PilO, a stator-associated component of the flagellar motor recently shown to drive a reciprocal biofilm and motility response. Our findings establish that H. pylori has developed functionally specialized flagellins that work with motor-dependent signaling to dynamically balance surface colonization and motility.

Long-range extracellular electron transfer enables respiring microbes to use minerals, other organisms, or electrodes as electron acceptors by transporting electrons microns away from the cell surface. This process is primarily studied in Geobacter sulfurreducens , which produces at least three different micrometer-long, multi-heme cytochrome nanowires capable of electron transfer. However, the distribution and higher-order structure of these types of cytochrome nanowires remains largely unknown. Here, we employed cryo-electron microscopy to determine the atomic structure of a unique cytochrome nanowire from Desulfuromonas soudanensis WTL, a halophilic, iron- and electrode-reducing bacterium found in deep subsurface brine. These filaments are based on a homolog of the OmcE tetraheme cytochrome, which then assemble into highly ordered bundles of antiparallel filaments. This arrangement likely arises from the association of nanowires extending from adjacent cells. Furthermore, a similar cytochrome bundle structure was observed in Geobacter metallireducens , suggesting that this quaternary structure may be a common feature among nanowires secreted by electroactive microbes. Our findings demonstrate that cytochrome nanowires in diverse taxa can form specialized bundle interfaces, potentially facilitating conductive biofilm formation and representing a novel strategy for microbial electron exchange. More broadly, this work expands understanding of electron transfer mechanisms and demonstrates the production of multi-heme filaments across distinct lineages. These insights could guide future research into cytochrome nanowire secretion and conductive biofilm engineering, with potential applications in electrochemical technologies.

Cannulae are structurally rigid tubular protein filaments that accumulate on the extracellular surface of archaea within the family Pyrodictiaceae during cell growth. These obligate anaerobes propagate under hyperthermophilic conditions in which cannulae form a biomatrix that interconnects and sustains cells. The persistence of cannulae in this environment suggests that these filaments display significant thermostability, which has attracted technological interest in their development as synthetic protein-based biomaterials. Here, we report cryoEM structural analyses of ex vivo and in vitro assembled recombinant cannulae. We demonstrate that the interactions between protomers in native and recombinant cannulae is based on donor strand complementation (DSC), a form of non-covalent polymerization previously observed for bacterial chaperone-usher pili. Unexpectedly, calcium ion coordination at the subunit interfaces reinforces the network of donor strand interactions in the cannulae. This study provides insight into the mechanism of assembly of cannulae and the structural origin of their high stability and rigidity.

Cyanobacteria are vital photosynthetic prokaryotes, but some form harmful algal blooms (cyanoHABs) that disrupt ecosystems and produce toxins. The mechanisms by which these blooms form have yet to be fully understood, particularly the role of extracellular components. Here, we present a 2.4 Å cryo-EM structure of a pilus, termed the cyanobacterial tubular (CT) pilus, found in the cyanoHAB-forming Microcystis aeruginosa. The pilin exhibits a unique protein fold, forming a tubular pilus structure with tight, double-layer anti-parallel β-sheet interactions. We show that CT pili are essential for buoyancy by facilitating the formation of micro-colonies, which increases drag force and prevents sinking. The CT pilus surface is heavily glycosylated with ten monosaccharide modifications per pilin. Furthermore, CT pili can enrich microcystin, potentially enhancing cellular resilience, and co-localize with iron-enriched extracellular matrix components. Thus, we propose that this pilus plays an important role in the proliferation of cyanoHABs. This just discovered pilus family appears to be widely distributed across several cyanobacterial orders. Our structural and functional characterization of CT pili provide insights into cyanobacterial cell morphology, physiology, and toxin interactions, and identify potential targets for disrupting bloom formation.