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Cyclic di-AMP is the only known essential second messenger in bacteria and archaea, regulating different proteins indispensable for numerous physiological processes. In particular, it controls various potassium and osmolyte transporters involved in osmoregulation. InBacillus subtilis, the K+/H+symporter KimA of the KUP family is inactivated by c-di-AMP. KimA sustains survival at potassium limitation at low external pH by mediating K+ions uptake. However, at elevated intracellular K+concentrations, further K+accumulation would be toxic. In this study, we reveal the molecular basis of how c-di-AMP binding inhibits KimA. We report cryo-EM structures of KimA with bound c-di-AMP in detergent solution and reconstituted in amphipols. By combining structural data with functional assays and molecular dynamics simulations we reveal how c-di-AMP modulates transport. We show that an intracellular loop in the transmembrane domain interacts with c-di-AMP bound to the adjacent cytosolic domain. This reduces the mobility of transmembrane helices at the cytosolic side of the K+binding site and therefore traps KimA in an inward-occluded conformation.

Potassium homeostasis is vital for all organisms, but is challenging in single-celled organisms like bacteria and yeast and immobile organisms like plants that constantly need to adapt to changing external conditions. KUP transporters facilitate potassium uptake by the co-transport of protons. Here, we uncover the molecular basis for transport in this widely distributed family. We identify the potassium importer KimA from Bacillus subtilis as a member of the KUP family, demonstrate that it functions as a K+/H+ symporter and report a 3.7 Å cryo-EM structure of the KimA homodimer in an inward-occluded, trans-inhibited conformation. By introducing point mutations, we identify key residues for potassium and proton binding, which are conserved among other KUP proteins., KUP transporters facilitate potassium uptake by the co-transport of protons and are key players in potassium homeostasis. Here authors identify the potassium importer KimA from Bacillus subtilis as a new member of the KUP transporter family and show the cryo-EM structure of KimA in an inward-occluded, trans-inhibited conformation.

Envelope glycoproteins from genetically-divergent virus families comprise fusion peptides (FPs) that have been posited to insert and perturb the membranes of target cells upon activation of the virus-cell fusion reaction. Conserved sequences rich in aromatic residues juxtaposed to the external leaflet of the virion-wrapping membranes are also frequently found in viral fusion glycoproteins. These membrane-proximal external regions (MPERs) have been implicated in the promotion of the viral membrane restructuring event required for fusion to proceed, hence, proposed to comprise supplementary FPs. However, it remains unknown whether the structure-function relationships governing canonical FPs also operate in the mirroring MPER sequences. Here, we combine infrared spectroscopy-based approaches with cryo-electron microscopy to analyze the alternating conformations adopted, and perturbations generated in membranes by CpreTM, a peptide derived from the MPER of the HIV-1 Env glycoprotein. Altogether, our structural and morphological data support a cholesterol-dependent conformational plasticity for this HIV-1 sequence, which could assist cell-virus fusion by destabilizing the viral membrane at the initial stages of the process., This study was supported by the Spanish MCIU (Grants RTI2018-095624-B-C21; MCIU/AEI/FEDER, UE to JLN and BA; and PID2019-111096GA-I00; MCIU/AEI/FEDER, UE to AC) and Basque Government (Grant: IT1196-19).

AMPylation, the post-translational modification with adenosine monophosphate (AMP), is catalyzed by effector proteins from a variety of pathogens. Legionella pneumophila is thus far the only known pathogen that, in addition to encoding an AMPylase (SidM/DrrA), also encodes a deAMPylase, called SidD, that reverses SidM-mediated AMPylation of the vesicle transport GTPase Rab1. DeAMPylation is catalyzed by the N-terminal phosphatase-like domain of SidD. Here, we determined the crystal structure of full length SidD including the uncharacterized C-terminal domain (CTD). A flexible loop rich in aromatic residues within the CTD was required to target SidD to model membranes in vitro and to the Golgi apparatus within mammalian cells. Deletion of the loop (Δloop) or substitution of its aromatic phenylalanine residues rendered SidD cytosolic, showing that the hydrophobic loop is the primary membrane-targeting determinant of SidD. Notably, deletion of the two terminal alpha helices resulted in a CTD variant incapable of discriminating between membranes of different composition. Moreover, a L. pneumophila strain producing SidDΔloop phenocopied a L. pneumophila ΔsidD strain during growth in mouse macrophages and displayed prolonged co-localization of AMPylated Rab1 with LCVs, thus revealing that membrane targeting of SidD via its CTD is a critical prerequisite for its ability to catalyze Rab1 deAMPylation during L. pneumophila infection., Author summary The frequency of Legionnaires' disease outbreaks, a serious pneumonia caused by the intracellular pathogen Legionella pneumophila, has increased more than 4-fold between the years 2000 and 2015 [CDC, Summary of Notifiable Diseases, United States, 2015]. L. pneumophila secretes hundreds of bacterial effectors to manipulate host cell processes. Rab GTPases, which control intracellular vesicle trafficking in eukaryotes, are a known target of Legionella effectors. SidM and SidD both target host cell Rab1, but with opposing activities. While SidM catalyzes the attachment of adenosine monophosphate (AMP) to Rab1, SidD catalyzes AMP removal from Rab1, a process known as deAMPylation. Here we present the crystal structure of full length SidD, including the previously uncharacterized C-terminal domain (CTD). We discovered that the CTD assumed a unique fold that was critical for membrane localization of SidD within host cells, and that this localization was dependent on two structural elements: a stretch of hydrophobic amino acid residues that represents a general membrane targeting determinant, and a two-helix bundle that functions as the specificity determinant. Deletion of either of these elements interfered with the correct localization of SidD within mammalian cells, confirming that positioning of bacterial effectors is critically important for their biological function.

The signaling nucleotide cyclic di-AMP (c-di-AMP) is the only known essential second messenger in bacteria. Recently, c-di-AMP has been identified as being essential for controlling potassium uptake in the model organism Bacillus subtilis and several other bacteria. A B. subtilis strain lacking c-di-AMP is not viable at high potassium concentrations, unless the bacteria acquire suppressor mutations. In this study, we isolated such suppressor mutants and found mutations that reduced the activities of the potassium transporters KtrCD and KimA. Although c-di-AMP–mediated control of KtrCD has previously been demonstrated, it is unknown how c-di-AMP affects KimA activity. Using the DRaCALA screening assay, we tested for any interactions of KimA and other potential target proteins in B. subtilis with c-di-AMP. This assay identified KimA, as well as the K(+)/H(+) antiporter KhtT, the potassium exporter CpaA (YjbQ), the osmoprotectant transporter subunit OpuCA, the primary Mg(2+) importer MgtE, and DarB (YkuL), a protein of unknown function, as bona fide c-di-AMP–binding proteins. Further, binding of c-di-AMP to KimA inhibited potassium uptake. Our results indicate that c-di-AMP controls KimA-mediated potassium transport at both kimA gene expression and KimA activity levels. Moreover, the discovery that potassium exporters are c-di-AMP targets indicates that this second messenger controls potassium homeostasis in B. subtilis at a global level by binding to riboswitches and to different classes of transport proteins involved in potassium uptake and export.

Intervening protein sequences (inteins) from extremely halophilic haloarchaea can be inactive under low salinity but could be activated by increasing the salt content to a specific concentration for each intein. The halo-obligatory inteins confer high solubility under both low and high salinity conditions. We showed the broad utility of salt-dependent protein splicing in cis and trans by demonstrating backbone cyclization, self-cleavage for purification, and scarless protein ligation for segmental isotopic labeling. Artificially split MCM2 intein derived from Halorhabdus utahensis remained highly soluble and was capable of protein trans-splicing with excellent ligation kinetics by reassembly under high salinity conditions. Importantly, the MCM2 intein has the active site residue of Ser at the +1 position, which remains in the ligated product, instead of Cys as found in many other efficient split inteins. Since Ser is more abundant than Cys in proteins, the novel split intein could widen the applications of segmental labeling in protein NMR spectroscopy and traceless protein ligation by exploiting a Ser residue in the native sequences as the +1 position of the MCM2 intein. The split halo-obligatory intein was successfully used to demonstrate the utility in NMR investigation of intact proteins by producing segmentally isotope-labeled intact TonB protein from Helicobacter pylori.

P-type ATPases ubiquitously pump cations across biological membranes to maintain vital ion gradients. Among those, the chimeric K+ uptake system KdpFABC is unique. While ATP hydrolysis is accomplished by the P-type ATPase subunit KdpB, K+ has been assumed to be transported by the channel-like subunit KdpA. A first crystal structure uncovered its overall topology, suggesting such a spatial separation of energizing and transporting units. Here, we report two cryo-EM structures of the 157 kDa, asymmetric KdpFABC complex at 3.7 Å and 4.0 Å resolution in an E1 and an E2 state, respectively. Unexpectedly, the structures suggest a translocation pathway through two half-channels along KdpA and KdpB, uniting the alternating-access mechanism of actively pumping P-type ATPases with the high affinity and selectivity of K+ channels. This way, KdpFABC would function as a true chimeric complex, synergizing the best features of otherwise separately evolved transport mechanisms.

Other than more widely used methods, the use of styrene maleic acid copolymers allows the direct extraction of membrane proteins from the lipid bilayer into SMALPs keeping it in its native lipid surrounding., Other than more widely used methods, the use of styrene maleic acid allows the direct extraction of membrane proteins from the lipid bilayer into SMALPs keeping it in its native lipid surrounding. Here we present the combined use of SMALPs and LILBID-MS, allowing determination of oligomeric states of membrane proteins of different functionality directly from the native nanodiscs.

Asparaginyl endopeptidases (AEPs) catalyze head-to-tail backbone cyclization of naturally occurring cyclic peptides such as cyclotides, and have become an important peptide-engineering tool for macrocyclization and peptide ligation. Here, we report efficient protein ligation in trans by mimicking efficient backbone cyclization by an AEP without any excess of reactants. We demonstrate a practical application of segmental isotopic labeling for NMR studies of a single-domain globular protein without any refolding step using the recombinant AEP prepared from Escherichia coli. This simple protein ligation approach using an AEP could be applied for incorporation of various biophysical probes into proteins as well as post-translational production of full-length proteins.

The multisubunit Golgi-associated retrograde protein (GARP) complex is required for tethering and fusion of endosome-derived transport vesicles to the trans -Golgi network. Mutation of leucine-967 to glutamine in the Vps54 subunit of GARP is responsible for spinal muscular atrophy in the wobbler mouse, an animal model of amyotrophic lateral sclerosis. The crystal structure at 1.7 Å resolution of the mouse Vps54 C-terminal fragment harboring leucine-967, in conjunction with comparative sequence analysis, reveals that Vps54 has a continuous α-helical bundle organization similar to that of other multisubunit tethering complexes. The structure shows that leucine-967 is buried within the α-helical bundle through predominantly hydrophobic interactions that are critical for domain stability and folding in vitro. Mutation of this residue to glutamine does not prevent integration of Vps54 into the GARP complex but greatly reduces the half-life and levels of the protein in vivo. Severely reduced levels of mutant Vps54 and, consequently, of the whole GARP complex underlie the phenotype of the wobbler mouse.

Two errors were made in this article. First, a reference was omitted from the second sentence of the fourth paragraph in the Introduction section. The sentence should read: SidD lacks any obvious sequence homology with the AT-N or other known proteins, although fold recognition analysis of the N-terminal portion of SidD predicted limited resemblance with members of the metal-dependent protein phosphatase (PPM) family (Rigden 2011). The reference is: Rigden DJ. (2011) Identification and modelling of a PPM protein phosphatase fold in the Legionella pneumophila deAMPylase SidD. FEBS Lett. 585:2749-54. doi: 10.1016/j.febslet.2011.08.006. Second, there were some omissions from the acknowledgements section, which should instead read as follows: We thank K. B. Decker, Y. Lin, and A.H. Gaspar for comments on the manuscript, and A. Vidaurrazaga for assistance with biochemical experiments. This study made use of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) under the Block Allocation Group (BAG) MX1420, the Diamond Light Source (Oxfordshire, UK) under the rapid access mode MX7512 and BAG MX8302, the beamline PROXIMA1 at the SOLEIL synchrotron (Saint-Aubin, France), the X-ray crystallography platform and proteomics platform (member of CIBERehd and ProteoRed-ISCIII) at CIC bioGUNE (Derio, Spain), and the SGIker analytical facility at UPV/EHU (Leioa, Spain). We thank all the staff from these facilities for technical and human support. Access to synchrotron facilities was part funded by the the BioStruct-X project (proposal number 2460).