Natural transformation is a dominant force in bacterial evolution by promoting horizontal gene transfer. This process may have devastating consequences, such as the spread of antibiotic resistance or the emergence of highly virulent clones. However, uptake and recombination of foreign DNA are most often deleterious to competent species. Therefore, model naturally transformable Gram-negative bacteria, including the human pathogen Neisseria meningitidis , have evolved means to preferentially take up homotypic DNA containing short and genus-specific sequence motifs. Despite decades of intense investigations, the DNA uptake sequence receptor in Neisseria species has remained elusive. We show here, using a multidisciplinary approach combining biochemistry, molecular genetics, and structural biology, that meningococcal type IV pili bind DNA through the minor pilin ComP via an electropositive stripe that is predicted to be exposed on the filaments surface and that ComP displays an exquisite binding preference for DNA uptake sequence. Our findings illuminate the earliest step in natural transformation, reveal an unconventional mechanism for DNA binding, and suggest that selective DNA uptake is more widespread than previously thought.

Biofilms are communities of bacteria that grow encased in an extracellular matrix that often contains proteins. The spatial organization and the molecular interactions between matrix scaffold proteins remain in most cases largely unknown. Here, we report that Bap protein of Staphylococcus aureus self-assembles into functional amyloid aggregates to build the biofilm matrix in response to environmental conditions. Specifically, Bap is processed and fragments containing at least the N-terminus of the protein become aggregation-prone and self-assemble into amyloid-like structures under acidic pHs and low concentrations of calcium. The molten globule-like state of Bap fragments is stabilized upon binding of the cation, hindering its self-assembly into amyloid fibers. These findings define a dual function for Bap, first as a sensor and then as a scaffold protein to promote biofilm development under specific environmental conditions. Since the pH-driven multicellular behavior mediated by Bap occurs in coagulase-negative staphylococci and many other bacteria exploit Bap-like proteins to build a biofilm matrix, the mechanism of amyloid-like aggregation described here may be widespread among pathogenic bacteria. Major components of the biofilm matrix scaffold are proteins that assemble to create a unified structure that maintain bacteria attached to each other and to surfaces. We provide evidence that a surface protein present in several staphylococcal species forms functional amyloid aggregates to build the biofilm matrix in response to specific environmental conditions. Under low Ca²⁺ concentrations and acidic pH, Bap is processed and forms insoluble aggregates with amyloidogenic properties. When the Ca²⁺ concentration increases, metal-coordinated Bap adopts a structurally more stable conformation and as a consequence, the N-terminal region is unable to assemble into amyloid aggregates. The control of Bap cleavage and assembly helps to regulate biofilm matrix development as a function of environmental changes.

Enteropathogenic Escherichia coli (EPEC) induce characteristic attaching and effacing (A/E) lesions on epithelial cells. This event is mediated, in part, by binding of the bacterial outer membrane protein, intimin, to a second EPEC protein, Tir (translocated intimin receptor), which is exported by the bacteria and integrated into the host cell plasma membrane. In this study, we have localized the intimin‐binding domain of Tir to a central 107‐amino‐acid region, designated Tir‐M. We provide evidence that both the amino‐ and carboxy‐termini of Tir are located within the host cell. In addition, using immunogold labelling electron microscopy, we have confirmed that intimin can bind independently to host cells even in the absence of Tir. This Tir‐independent interaction and the ability of EPEC to induce A/E lesions requires an intact lectin‐like module residing at the carboxy‐terminus of the intimin polypeptide. Using the yeast two‐hybrid system and gel overlays, we show that intimin can bind both Tir and Tir‐M even when the lectin‐like domain is disrupted. These data provide strong evidence that intimin interacts not only with Tir but also in a lectin‐like manner with a host cell intimin receptor.

Innovating to be nondisruptive: Insights into the architecture and stoichiometry of membrane complexes have grown with advances in cryo–electron microscopy and native mass spectroscopy. However, most of these studies are not in the context of native membrane. Chorev et al. released intact membrane complexes directly from native lipid membrane vesicles into a mass spectrometer. They analyzed components of the Escherichia coli inner and outer membranes and the bovine mitochondrial inner membrane. For several identified complexes, they found a stoichiometry that differs from published results and, in some cases, confirmed interactions that could not be characterized structurally. Science , this issue p. 829