In the current model of mitochondrial trafficking, Miro1 and Miro2 Rho‐GTPases regulate mitochondrial transport along microtubules by linking mitochondria to kinesin and dynein motors. By generating Miro1/2 double‐knockout mouse embryos and single‐ and double‐knockout embryonic fibroblasts, we demonstrate the essential and non‐redundant roles of Miro proteins for embryonic development and subcellular mitochondrial distribution. Unexpectedly, the TRAK1 and TRAK2 motor protein adaptors can still localise to the outer mitochondrial membrane to drive anterograde mitochondrial motility in Miro1/2 double‐knockout cells. In contrast, we show that TRAK2‐mediated retrograde mitochondrial transport is Miro1‐dependent. Interestingly, we find that Miro is critical for recruiting and stabilising the mitochondrial myosin Myo19 on the mitochondria for coupling mitochondria to the actin cytoskeleton. Moreover, Miro depletion during PINK1/Parkin‐dependent mitophagy can also drive a loss of mitochondrial Myo19 upon mitochondrial damage. Finally, aberrant positioning of mitochondria in Miro1/2 double‐knockout cells leads to disruption of correct mitochondrial segregation during mitosis. Thus, Miro proteins can fine‐tune actin‐ and tubulin‐dependent mitochondrial motility and positioning, to regulate key cellular functions such as cell proliferation.

Undesired complement activation is a major cause of tissue injury in various pathological conditions and contributes to several immune complex diseases. Compstatin, a 13-residue peptide, is an effective inhibitor of the activation of complement component C3 and thus blocks a central and crucial step in the complement cascade. The precise binding site on C3, the structure in the bound form, and the exact mode of action of compstatin are unknown. Here we present the crystal structure of compstatin in complex with C3c, a major proteolytic fragment of C3. The structure reveals that the compstatin-binding site is formed by the macroglobulin (MG) domains 4 and 5. This binding site is part of the structurally stable MG-ring formed by domains MG 1-6 and is far away from any other known binding site on C3. Compstatin does not alter the conformation of C3c, whereas compstatin itself undergoes a large conformational change upon binding. We propose a model in which compstatin sterically hinders the access of the substrate C3 to the convertase complexes, thus blocking complement activation and amplification. These insights are instrumental for further development of compstatin as a potential therapeutic.The complement system is a key part of the innate and adaptive immune system and plays a major role in homeostasis by clearing altered host cells and invading pathogens (1, 2). Inappropriate activation of the complement system leads to tissue injury, causing or aggravating various pathological conditions, such as autoimmune diseases, burn injuries, Alzheimer disease, stroke, and heart attack (reviewed in Ref.3). Several complement inhibitors are under development, targeting various steps in the complement activation pathways. None of these compounds have been approved for clinical use yet (3-5). We studied a 13-residue cyclic peptide, called compstatin, which inhibits complement response by preventing the proteolytic activation of C3 3 (6). Activation of C3 by the C3 convertases is a central amplification step in complement activation. All three recognition and initiation pathways, the classical, lectin, and alternative pathways, converge in the activation of C3. Proteolytic activation of C3 yields C3b, which covalently binds to pathogenic or self surfaces, providing a strong signal for clearance of the tagged particles. Because compstatin blocks this critical step of complement activation and because it is a small non-immunogenic peptide, compstatin has the potential to be developed into a therapeutic agent.Compstatin (ICVVQDWGHHRCT-NH 2 , circularized by disulfide bond Cys-2-Cys-12) was discovered by a phage-display, random peptide library search (6). Compstatin has been shown to be an effective inhibitor of complement activation in several clinically relevant models (reviewed in Ref. 5). For example, compstatin was shown to prolong the survival of kidneys in an ex vivo xenograft model (7), inhibited complement activation during the contact of whole blood with biomaterial in a model of extracorporeal circulation (8), and in...

Mitochondrial Rho (Miro) GTPases localize to the outer mitochondrial membrane and are essential machinery for the regulated trafficking of mitochondria to defined subcellular locations. However, their sub-mitochondrial localization and relationship with other critical mitochondrial complexes remains poorly understood. Here, using super-resolution fluorescence microscopy, we report that Miro proteins form nanometer-sized clusters along the mitochondrial outer membrane in association with the Mitochondrial Contact Site and Cristae Organizing System (MICOS). Using knockout mouse embryonic fibroblasts we show that Miro1 and Miro2 are required for normal mitochondrial cristae architecture and Endoplasmic Reticulum-Mitochondria Contacts Sites (ERMCS). Further, we show that Miro couples MICOS to TRAK motor protein adaptors to ensure the concerted transport of the two mitochondrial membranes and the correct distribution of cristae on the mitochondrial membrane. The Miro nanoscale organization, association with MICOS complex and regulation of ERMCS reveal new levels of control of the Miro GTPases on mitochondrial functionality.

Background: NAIP5 and NLRC4 induce an innate immune response to intracellular flagellin. Results: Flagellin fragments were identified that induce signaling-competent NAIP5-NLRC4 inflammasomes with 11-and 12-fold symmetry. Conclusion: Conserved flagellin terminal regions induce an inflammasome in which NAIP5 and NLRC4 appear to occupy equivalent positions. Significance: We provide fundamental insights into the formation and structure of hetero-oligomeric inflammasomes.