Climate variability and aridity modulate the role of leaf shelters for arthropods: A global experiment

Current climate change is disrupting biotic interactions and eroding biodiversity worldwide. However, species sensitive to aridity, high temperatures, and climate variability might find shelter in microclimatic refuges, such as leaf rolls built by arthropods. To explore how the importance of leaf shelters for terrestrial arthropods changes with latitude, elevation, and climate, we conducted a distributed experiment comparing arthropods in leaf rolls versus control leaves across 52 sites along an 11,790 km latitudinal gradient. We then probed the impact of short- versus long-term climatic impacts on roll use, by comparing the relative impact of conditions during the experiment versus average, baseline conditions at the site. Leaf shelters supported larger organisms and higher arthropod biomass and species diversity than non-rolled control leaves. However, the magnitude of the leaf rolls' effect differed between long- and short-term climate conditions, metrics (species richness, biomass, and body size), and trophic groups (predators vs. herbivores). The effect of leaf rolls on predator richness was influenced only by baseline climate, increasing in magnitude in regions experiencing increased long-term aridity, regardless of latitude, elevation, and weather during the experiment. This suggests that shelter use by predators may be innate, and thus, driven by natural selection. In contrast, the effect of leaf rolls on predator biomass and predator body size decreased with increasing temperature, and increased with increasing precipitation, respectively, during the experiment. The magnitude of shelter usage by herbivores increased with the abundance of predators and decreased with increasing temperature during the experiment. Taken together, these results highlight that leaf roll use may have both proximal and ultimate causes. Projected increases in climate variability and aridity are, therefore, likely to increase the importance of biotic refugia in mitigating the effects of climate change on species persistence.

Insights into the Membrane Interacting Properties of the C-Terminal Domain of the Monotopic Glycosyltransferase DGD2 in Arabidopsis thaliana

Glycosyltransferases (GTs) are responsible for regulating the membrane composition of plants. The synthesis of one of the main lipids in the membrane, the galactolipid digalactosyldiacylglycerol, is regulated by the enzyme digalactosyldiacylglycerol synthase 2 (atDGD2) under starving conditions, such as phosphate shortage. The enzyme belongs to the GT-B fold, characterized by two β/α/β Rossmann domains that are connected by a flexible linker. atDGD2 has previously been shown to attach to lipid membranes by the N-terminal domain via interactions with negatively charged lipids. The role of the C-terminal domain in the membrane interaction is, however, not known. Here we have used a combination of in silico prediction methods and biophysical experimental techniques to shed light on the membrane interacting properties of the C-terminal domain. Our results demonstrate that there is an amphipathic sequence, corresponding to residues V240-E258, that interacts with lipids in a charge-dependent way. A second sequence was identified as being potentially important, with a high charge density, but no amphipathic character. The features of the plant atDGD2 observed here are similar in prokaryotic glycosyltransferases. On the basis of our results, and by analogy to other glycosyltransferases, we propose that atDGD2 interacts with the membrane through the N-terminus and with parts of the C-terminus acting as a switch, allowing for a dynamic interaction with the membrane.

Structural and functional characterization of the microtubule interacting and trafficking domains of two oomycete chitin synthases

Chitin synthases (Chs) are responsible for the synthesis of chitin, a key structural cell wall polysaccharide in many organisms. They are essential for growth in certain oomycete species, some of which are pathogenic to diverse higher organisms. Recently, a microtubule interacting and trafficking (MIT) domain, which is not found in any fungal Chs, has been identified in some oomycete Chs proteins. Based on experimental data relating to the binding specificity of other eukaryotic MIT domains, there was speculation that this domain may be involved in the intracellular trafficking of Chs proteins. However, there is currently no evidence for this or any other function for the MIT domain in these enzymes. To attempt to elucidate their function, MIT domains from two Chs enzymes from the oomycete Saprolegnia monoica were cloned, expressed, and characterized. Both were shown to interact strongly with the plasma membrane component, phosphatidic acid, and to have additional putative interactions with proteins thought to be involved in protein transport and localization. Aiding our understanding of these data, the structure of the first MIT domain from a carbohydrate-active enzyme (MIT1) was solved by NMR, and a model structure of a second MIT domain (MIT2) was built by homology modeling. Our results suggest a potential function for these MIT domains in the intracellular transport and/or regulation of Chs enzymes in the oomycetes.

DATABASE

Structural data are available in the Biological Magnetic Resonance Bank (BMRB) database under the accession number 19987 and the PDB database under the accession number 2MPK.

Membrane Interaction of the Glycosyltransferase WaaG

The glycosyltransferase WaaG is involved in the synthesis of lipopolysaccharides that constitute the outer leaflet of the outer membrane in Gram-negative bacteria such as Escherichia coli. WaaG has been identified as a potential antibiotic target, and inhibitor scaffolds have previously been investigated. WaaG is located at the cytosolic side of the inner membrane, where the enzyme catalyzes the transfer of the first outer-core glucose to the inner core of nascent lipopolysaccharides. Here, we characterized the binding of WaaG to membrane models designed to mimic the inner membrane of E. coli. Based on the crystal structure, we identified an exposed and largely α-helical 30-residue sequence, with a net positive charge and several aromatic amino acids, as a putative membrane-interacting region of WaaG (MIR-WaaG). We studied the peptide corresponding to this sequence, along with its bilayer interactions, using circular dichroism, fluorescence quenching, fluorescence anisotropy, and NMR. In the presence of dodecylphosphocholine, MIR-WaaG was observed to adopt a three-dimensional structure remarkably similar to the segment in the crystal structure. We found that the membrane interaction of WaaG is conferred at least in part by MIR-WaaG and that electrostatic interactions play a key role in binding. Moreover, we propose a mechanism of anchoring WaaG to the inner membrane of E. coli, where the central part of MIR-WaaG inserts into one leaflet of the bilayer. In this model, electrostatic interactions as well as surface-exposed Tyr residues bind WaaG to the membrane.

Diffuse binding of Zn(2+) to the denatured ensemble of Cu/Zn superoxide dismutase 1

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Nearly complete backbone assignments for monomeric SOD1 were obtained.

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Monomeric SOD1 is unstructured in 9 M urea.

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Zn²⁺-binding to monomeric SOD1 occurs through diffuse coordination to most His residues.

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The binding geometry of Zn²⁺ is different than in the native, folded SOD1.

The stability and structural properties of the metalloprotein superoxide dismutase 1 (SOD1) are found to depend critically on metal ions. Native SOD1 monomers coordinate one structural Zn²⁺ and one redox-active Cu^2+/1+ to the active site. To do this, the Zn²⁺ ions need to interact with the SOD1 protein on the denatured side of the folding barrier, prior to the formation of the folding nucleus. In this study, we have examined at residue level the nature of this early Zn²⁺ binding by NMR studies on the urea denatured-state of SOD1. Nearly complete backbone chemical shift assignments were obtained in 9 M urea at physiological pH, conditions at which NMR studies are scarce. Our results demonstrate that SOD1 is predominantly unstructured under these conditions. Chemical-shift changes upon Zn²⁺ titration show that denatured SOD1 retains a significant affinity to Zn²⁺ ions, even in 9 M urea. However, the Zn²⁺ interactions are not limited to the native metal-binding ligands in the two binding sites, but are seen for all His residues. Moreover, the native Cu^2+/1+ ligand H46 seems not to bind as well as the other His residues, while the nearby non-native H43 does bind, indicating that the binding geometry is relaxed. The result suggests that the Zn²⁺-binding observed to catalyze folding of SOD1 in physiological buffer is initiated by diffuse, non-specific coordination to the coil, which subsequently funnels by ligand exchange into the native coordination geometry of the folded monomer. Altogether, this diffuse binding is a result with fundamental implications for folding of metalloproteins in general.