SPHIRE (SPARX for High-Resolution Electron Microscopy) is a novel open-source, user-friendly software suite for the semi-automated processing of single particle electron cryo-microscopy (cryo-EM) data. The protocol presented here describes in detail how to obtain a near-atomic resolution structure starting from cryo-EM micrograph movies by guiding users through all steps of the single particle structure determination pipeline. These steps are controlled from the new SPHIRE graphical user interface and require minimum user intervention. Using this protocol, a 3.5 Å structure of TcdA1, a Tc toxin complex from Photorhabdus luminescens, was derived from only 9500 single particles. This streamlined approach will help novice users without extensive processing experience and a priori structural information, to obtain noise-free and unbiased atomic models of their purified macromolecular complexes in their native state.

The thylakoid membrane of chloroplasts and cyanobacteria is a unique internal membrane system harbouring the complexes of the photosynthetic electron transfer chain. Despite their apparent importance, little is known about the biogenesis and maintenance of thylakoid membranes. Although membrane fusion events are essential for the formation of thylakoid membranes, proteins involved in membrane fusion have yet to be identified in photosynthetic cells or organelles. Here we show that IM30, a conserved chloroplast and cyanobacterial protein of approximately 30 kDa binds as an oligomeric ring in a well-defined geometry specifically to membranes containing anionic lipids. Triggered by Mg 2 þ , membrane binding causes destabilization and eventually results in membrane fusion. We propose that IM30 establishes contacts between internal membrane sites and promotes fusion to enable regulated exchange of proteins and/or lipids in cyanobacteria and chloroplasts.

The Rod–Zw10–Zwilch (RZZ) complex assembles as a fibrous corona on kinetochores before microtubule attachment during mitotic spindle formation. Mosalaganti et al. provide new structural insight into the Spindly–RZZ complex that suggests that it resembles a dynein adaptor–cargo pair in the kinetochore corona.

Biogenesis and dynamics of thylakoid membranes likely involves membrane fusion events. Membrane attachment of the inner membrane-associated protein of 30 kDa (IM30) affects the structure of the lipid bilayer, finally resulting in membrane fusion. Yet, how IM30 triggers membrane fusion is largely unclear. IM30 monomers pre-assemble into stable tetrameric building blocks, which further align to form oligomeric ring structures, and differently sized IM30 rings bind to membranes. Based on a 3D reconstruction of IM30 rings, we locate the IM30 loop 2 region at the bottom of the ring and show intact membrane binding but missing fusogenic activity of loop 2 mutants. However, helix 7, which has recently been shown to mediate membrane binding, was located at the oppossite, top side of IM30 rings. We propose that a two-sided IM30 ring complex connects two opposing membranes, finally resulting in membrane fusion. Thus, IM30-mediated membrane fusion requires a Janus-faced IM30 ring.

The IM30 (inner membrane-associated protein of 30 kDa), also known as the Vipp1 (vesicle-inducing protein in plastids 1), has a crucial role in thylakoid membrane biogenesis and maintenance. Recent results suggest that the protein binds peripherally to membranes containing negatively charged lipids. However, although IM30 monomers interact and assemble into large oligomeric ring complexes with different numbers of monomers, it is still an open question whether ring formation is crucial for membrane interaction. Here we show that binding of IM30 rings to negatively charged phosphatidylglycerol membrane surfaces results in a higher ordered membrane state, both in the head group and in the inner core region of the lipid bilayer. Furthermore, by using gold nanorods covered with phosphatidylglycerol layers and single particle spectroscopy, we show that not only IM30 rings but also lower oligomeric IM30 structures interact with membranes, although with higher affinity. Thus, ring formation is not crucial for, and even counteracts, membrane interaction of IM30.Engulfment of an ancient cyanobacterium by a primordial cell has resulted in the development of modern day chloroplasts, the organelles where photosynthesis takes place. Consequently, the fine structures of cyanobacterial cells and chloroplasts share many similarities. Notably, chloroplasts and cyanobacteria both contain an extra internal membrane system, the thylakoid membrane (TM) 2 network. Furthermore, both perform oxygenic photosynthesis, and the TMs harbor all protein complexes and pigments involved in the photosynthetic light reaction. However, although assembly and maintenance of TMs is vital for photosynthesis, the details of TM biogenesis and maintenance are still largely unsolved mysteries (1).In 1994 the IM30 (inner membrane-associated protein of 30 kDa) was discovered in chloroplasts of Pisum sativum (2), and a homologous protein appears to be expressed in almost every organism that is able to conduct oxygenic photosynthesis (2-5). Several in vivo studies have shown that the protein plays an essential role in TM formation and maintenance (reviewed in Ref. 6), e.g. the depletion of the protein in Arabidopsis thaliana or the cyanobacterium Synechocystis sp. PCC 6803 has resulted in a decreased amount of TMs (3, 7-9). Under cold stress conditions, depletion of IM30 resulted in the formation of vesicular structures at the chloroplast inner envelope membrane in Arabidopsis, which led to the name Vipp1 (vesicle inducing protein in plastids 1) (3). Nevertheless, although diverse physiological functions were proposed for IM30 in the past two decades, the exact protein function remained elusive for a long time. Only recently, IM30 has been identified to be able to reorganize lipid bilayer structures and to mediate membrane fusion in chloroplasts and cyanobacteria (10). Similar to its bacterial homolog, the PspA (phage shock protein A), which is involved in a bacterial stress response in situations provoking membrane stress (11), IM30 specifically interacts wi...