Structure of the endosomal CORVET tethering complex.

Cells depend on their endolysosomal system for nutrient uptake and downregulation of plasma membrane proteins. These processes rely on endosomal maturation, which requires multiple membrane fusion steps. Early endosome fusion is promoted by the Rab5 GTPase and its effector, the hexameric CORVET tethering complex, which is homologous to the lysosomal HOPS. How these related complexes recognize their specific target membranes remains entirely elusive. Here, we solve the structure of CORVET by cryo-electron microscopy and revealed its minimal requirements for membrane tethering. As expected, the core of CORVET and HOPS resembles each other. However, the function-defining subunits show marked structural differences. Notably, we discover that unlike HOPS, CORVET depends not only on Rab5 but also on phosphatidylinositol-3-phosphate (PI3P) and membrane lipid packing defects for tethering, implying that an organelle-specific membrane code enables fusion. Our data suggest that both shape and membrane interactions of CORVET and HOPS are conserved in metazoans, thus providing a paradigm how tethering complexes function.

Conformational variability of cyanobacterial ChlI, the AAA+ motor of magnesium chelatase involved in chlorophyll biosynthesis.

IMPORTANCE: Photosynthesis is an essential life process that relies on chlorophyll. In photosynthetic organisms, chlorophyll synthesis involves multiple steps and depends on magnesium chelatase. This enzyme complex is responsible for inserting magnesium into the chlorophyll precursor, but the molecular mechanism of this process is not fully understood. By using cryogenic electron microscopy and conducting functional analyses, we have discovered that the motor subunit ChlI of magnesium chelatase undergoes conformational changes in the presence of ATP. Our findings offer new insights into how energy is transferred from ChlI to the other components of magnesium chelatase. This information significantly contributes to our understanding of the initial step in chlorophyll biosynthesis and lays the foundation for future studies on the entire process of chlorophyll production.

Structure of the HOPS tethering complex, a lysosomal membrane fusion machinery.

Lysosomes are essential for cellular recycling, nutrient signaling, autophagy, and pathogenic bacteria and viruses invasion. Lysosomal fusion is fundamental to cell survival and requires HOPS, a conserved heterohexameric tethering complex. On the membranes to be fused, HOPS binds small membrane-associated GTPases and assembles SNAREs for fusion, but how the complex fulfills its function remained speculative. Here, we used cryo-electron microscopy to reveal the structure of HOPS. Unlike previously reported, significant flexibility of HOPS is confined to its extremities, where GTPase binding occurs. The SNARE-binding module is firmly attached to the core, therefore, ideally positioned between the membranes to catalyze fusion. Our data suggest a model for how HOPS fulfills its dual functionality of tethering and fusion and indicate why it is an essential part of the membrane fusion machinery.

Our cells break down the nutrients that they receive from the body to create the building blocks needed to keep us alive. This is done by compartments called lysosomes that are filled with a cocktail of proteins called enzymes, which speed up the breakdown process. Lysosomes are surrounded by a membrane, a barrier of fatty molecules that protects the rest of the cell from being digested. When new nutrients reach the cell, they travel to the lysosome packaged in vesicles, which have their own fatty membrane. To allow the nutrients to enter the lysosome without creating a leak, the membranes of the vesicles and the lysosome must fuse. The mechanism through which these membranes fuse is not fully clear. It is known that both fusing membranes must contain proteins called SNAREs, which wind around each other when they interact. However, this alone is not enough. Other proteins are also required to tether the membranes together before they fuse. To understand how these tethers play a role, Shvarev, Schoppe, König et al. studied the structure of the HOPS complex from yeast. This assembly of six proteins is vital for lysosomal fusion and, has a composition similar to the equivalent complex in humans. Using cryo-electron microscopy, a technique that relies on freezing purified proteins to image them with an electron microscope and reveal their structure, allowed Shvarev, Schoppe, König et al. to provide a model for how HOPS interacts with SNAREs and membranes. In addition to HOPS acting as a tether to bring the membranes together, it can also bind directly to SNAREs. This creates a bridge that allows the proteins to wrap around each other, driving the membranes to fuse. HOPS is a crucial component in the cellular machinery, and mutations in the complex can cause devastating neurological defects. The complex is also targeted by viruses ­ such as SARS-CoV-2 ­ that manipulate HOPS to reduce its activity. Shvarev, Schoppe, König et al.'s findings could help researchers to develop drugs to maintain or recover the activity of HOPS. However, this will require additional information about its structure and how the complex acts in the biological environment of the cell.

Frozen motion: how cryo-EM changes the way we look at ABC transporters.

ATP-binding cassette (ABC) transporters are widely present molecular machines that transfer substrates across the cell membrane. ABC transporters are involved in numerous physiological processes and are often clinical targets. Structural biology is fundamental to obtain the molecular details underlying ABC transporter function and suggest approaches to modulate it. Until recently, X-ray crystallography has been the only method capable of providing high-resolution structures of ABC transporters. However, modern cryo-electron microscopy (cryo-EM) opens entirely new ways of studying these dynamic membrane proteins. Cryo-EM enables analyses of targets that resist X-ray crystallography, challenging multicomponent complexes, and the exploration of conformational dynamics. These unique capacities have turned cryo-EM into the dominant technique for structural studies of membrane proteins, including ABC transporters.

The HlyD-like membrane fusion protein All5304 is essential for acid stress survival of the filamentous cyanobacterium Anabaena sp. PCC 7120.

Acid stress is an environmental problem for plants and fresh water cyanobacteria like the filamentous, heterocyst forming species Anabaena sp. PCC 7120 (hereafter Anabaena sp.). Heterocyst differentiation, cell-cell communication and nitrogen fixation has been deeply studied in this model organism, but little is known about the cellular response of Anabaena sp. to decreased pH values, causing acid stress. ATP-binding cassette (ABC) transporters are involved in acid stress response in other bacteria, by exporting proteins responsible for survival under acidification. The genome of Anabaena sp. encodes numerous ABC transporter components, whose function is not known yet. Here, we describe the function of the gene all5304 encoding a protein with homology to membrane fusion proteins of tripartite efflux pumps driven by ABC transporters like HlyBD-TolC of Escherichia coli. The all5304 mutant shows less resistance against low pH, even though the expression of the gene is independent from the pH of the medium. We compared the exoproteome of the wild type and mutant cultures and identified three proteins-candidate substrates of the putative transporter. Including the in silico analysis of All5304, our results suggest that All5304 functions as part of an efflux pump, secreting of a protein necessary for acid tolerance in Anabaena sp.

Two DevBCA-like ABC transporters are involved in the multidrug resistance of the cyanobacterium Anabaena sp. PCC 7120.

The filamentous heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 is an important model organism for studying cell differentiation, nitrogen fixation, and photosynthesis. This cyanobacterium possesses a high number of membrane transporters. Not much is known about the roles of the membrane transporters, especially the ATP-binding cassette (ABC) transporters, in the multidrug resistance of this cyanobacterium. In the present work, we performed a mutational analysis of the genes alr4280/alr4281/alr4282 and alr3647/alr3648/alr3649 that code for the components of putative ABC exporters and are homologous to the DevBCA heterocyst-specific glycolipid exporter. We show that these genes are essential for resistance to different drugs and are not essential for heterocyst development.

Roles of DevBCA-like ABC transporters in the physiology of Anabaena sp. PCC 7120.

The filamentous, photosynthetic cyanobacterium Anabaena sp. PCC 7120 can be considered as a true multicellular bacterium. Along the filament of cells, nitrogen fixation is spatially separated from the incompatible process of oxygenic photosynthesis by the formation of specialized heterocysts in a semiregular pattern. Heterocyst development involves many proteins, including a group of DevBCA-HgdD-like tripartite efflux pumps driven by ATP-binding cassette (ABC) transporters and that share similarity with MacAB or LolCDE transporters. In this minireview, we summarize the results from our studies of this group of transporters in Anabaena sp. PCC 7120 and discuss what remains to be elucidated.

Glycolipid composition of the heterocyst envelope of Anabaena sp. PCC 7120 is crucial for diazotrophic growth and relies on the UDP-galactose 4-epimerase HgdA.

The nitrogenase complex in the heterocysts of the filamentous freshwater cyanobacterium Anabaenasp. PCC 7120 fixes atmospheric nitrogen to allow diazotrophic growth. The heterocyst cell envelope protects the nitrogenase from oxygen and consists of a polysaccharide and a glycolipid layer that are formed by a complex process involving the recruitment of different proteins. Here, we studied the function of the putative nucleoside-diphosphate-sugar epimerase HgdA, which along with HgdB and HgdC is essential for deposition of the glycolipid layer and growth without a combined nitrogen source. Using site-directed mutagenesis and single homologous recombination approach, we performed a thoroughly functional characterization of HgdA and confirmed that the glycolipid layer of the hgdAmutant heterocyst is aberrant as shown by transmission electron microscopy and chemical analysis. The hgdA gene was expressed during late stages of the heterocyst differentiation. GFP-tagged HgdA protein localized inside the heterocysts. The purified HgdA protein had UDP-galactose 4-epimerase activity in vitro. This enzyme could be responsible for synthesis of heterocyst-specific glycolipid precursors, which could be transported over the cell wall by the ABC transporter components HgdB/HgdC.

The ABC Transporter Components HgdB and HgdC are Important for Glycolipid Layer Composition and Function of Heterocysts in Anabaena sp. PCC 7120.

Anabaena sp. PCC 7120 is a filamentous cyanobacterium able to fix atmospheric nitrogen in semi-regularly spaced heterocysts. For correct heterocyst function, a special cell envelope consisting of a glycolipid layer and a polysaccharide layer is essential. We investigated the role of the genes hgdB and hgdC, encoding domains of a putative ABC transporter, in heterocyst maturation. We investigated the subcellular localization of the fusion protein HgdC-GFP and followed the differential expression of the hgdB and hgdC genes during heterocyst maturation. Using a single recombination approach, we created a mutant in hgdB gene and studied its phenotype by microscopy and analytical chromatography. Although heterocysts are formed in the mutant, the structure of the glycolipid layer is aberrant and also contains an atypical ratio of the two major glycolipids. As shown by a pull-down assay, HgdB interacts with the outer membrane protein TolC, which indicates a function as a type 1 secretion system. We show that the hgdB-hgdC genes are essential for the creation of micro-oxic conditions by influencing the correct composition of the glycolipid layer for heterocyst function. Our observations confirm the significance of the hgdB-hgdC gene cluster and shed light on a novel mode of regulation of heterocyst envelope formation.

ATP-binding cassette transporters of the multicellular cyanobacterium Anabaena sp. PCC 7120: a wide variety for a complex lifestyle.

Two hundred genes or 3% of the known or putative protein-coding genes of the filamentous freshwater cyanobacterium Anabaena sp. PCC 7120 encode domains of ATP-binding cassette (ABC) transporters. Detailed characterization of some of these transporters (14-15 importers and 5 exporters) has revealed their crucial roles in the complex lifestyle of this multicellular photoautotroph, which is able to differentiate specialized cells for nitrogen fixation. This review summarizes the characteristics of the ABC transporters of Anabaena sp. PCC 7120 known to date.