Phage infection places strong selective pressure on bacteria to mount a rapid immune response to ensure survival. Although recent studies have uncovered a wide diversity of bacterial immune systems, how these systems function under the tight time constraints of phage infection remains poorly understood. Here we summarize structural, biochemical and genetic insights into bacterial immunity, focusing on steps that link infection sensing to activation and effector function. We highlight how conformational change, protein assembly and spatial organization shape immune activation, and how phages employ counter-defense strategies that disrupt these processes. In this Review, the authors summarize structural, biochemical and genetic insights into bacterial immunity, focusing on steps that link infection sensing to activation and effector function.
Norrin binds Frizzled 4 (FZD4), tetraspanin 12 (TSPAN12) and low-density lipoprotein receptor-related protein 5 (LRP5) to regulate angiogenesis and the integrity of the blood–brain barrier and inner blood–retina barrier (BRB). Aberrant Norrin signal causes retinopathies, BRB defects and deafness. Therapeutic mimetics of Norrin are currently in clinical trials for eye diseases. To characterize the mechanism of Norrin signal biogenesis, we determined the structure of a Norrin–FZD4–TSPAN12–LRP5 signaling complex using a de novo designed FZD4 binder as a ‘protein glue’. Our structure shows that two curved Norrin homodimers bridge two FZD4–TSPAN12 heterodimers. LRP5LA3 simultaneously interacts with TSPAN12 and two Norrin homodimers, positioning the cytosolic FZD4 and LRP5 tails for signal activation. On the basis of our findings, we designed FZD4CRD–LRP5LA3 bispecific binders that act as Norrin surrogates. Our signaling assays showed that oligomerization of the Norrin surrogate enhances signaling potency. Thus, this work reveals how the Norrin signal is generated and may assist in the development of Norrin/Wnt mimetics. Hu, Li et al. uncover how Norrin assembles Frizzled 4, tetraspanin 12 and low-density lipoprotein receptor-related protein 5 into a signaling complex and use this structural insight to design potent Norrin surrogates, providing a framework for developing therapies for retinal vascular diseases.
DNA compaction by human condensin II is crucial for the correct organization of mitotic chromosomes. Mitotic activation of condensin II is a multilayered process, controlled mainly by the opposing activity of the MCPH1 and M18BP1 proteins, acting as an interphase inhibitor and a mitotic activator, respectively. Mitotic phosphorylation favors M18BP1 binding to condensin II, resulting in effective and timely chromosome compaction. However, the molecular mechanisms underlying condensin II activation in driving DNA compaction remain uncharacterized. Here, combining cryo-electron microscopy with single-molecule imaging, we unravel the mechanism of condensin II activation by M18BP1 and elucidate their role in DNA compaction. The specific interaction with phosphorylated M18BP1 induces conformational changes in the condensin II complex, relieving an autoinhibitory conformation and allowing stable DNA binding and DNA compaction. Beyond the activity of isolated condensin II complexes on DNA, we observe the emergence of multimeric species that withstand the forces applied by optical tweezers, suggesting that condensin II multimerization might contribute to the mechanical reinforcement of mitotic chromosomes. Here, the authors show that phosphorylated M18BP1 activates condensin II by relieving an autoinhibitory interface, enabling stable DNA binding and compaction. Additionally, the M18BP1–condensin II complex forms clusters that might mechanically reinforce mitotic chromosomes.
Most eukaryotic membrane proteins are inserted into the membrane at the endoplasmic reticulum (ER). This essential process relies on molecular quality control machineries to prevent mistargeting and incorrect structure formation. Here, we show that the ER membrane protein complex (EMC) forms an evolutionarily conserved supercomplex with the P5A-ATPase Spf1/ATP13A1. Biochemical, structural and cellular analyses demonstrate that the EMC and Spf1 associate in a stoichiometric manner and retain their individual catalytic activities, the transmembrane domain (TMD) insertase function of the EMC and the TMD dislocase activity of Spf1. Our cryo-electron microscopy structures show that the EMC insertase cavity and the Spf1 dislocase cleft are positioned in close proximity within the membrane and reveal that the ATPase cycle of Spf1 remodels the interface between the two machineries and alters cytoplasmic accessibility of both substrate-binding cavities. Together, our study provides structural insights into how membrane protein insertion and extraction machineries can be spatially coordinated at the ER membrane, suggesting that functionally distinct proteostasis factors can assemble into hubs to support membrane protein biogenesis and quality control. Klose et al. provide structural insights into a supercomplex between the EMC and Spf1 that brings together cellular machines for membrane protein insertion and extraction into a single entity, providing a framework for integrated membrane protein quality control.
