Structural basis of NLR activation and innate immune signalling in plants

Animals and plants have NLRs (nucleotide-binding leucine-rich repeat receptors) that recognize the presence of pathogens and initiate innate immune responses. In plants, there are three types of NLRs distinguished by their N-terminal domain: the CC (coiled-coil) domain NLRs, the TIR (Toll/interleukin-1 receptor) domain NLRs and the RPW8 (resistance to powdery mildew 8)-like coiled-coil domain NLRs. CC-NLRs (CNLs) and TIR-NLRs (TNLs) generally act as sensors of effectors secreted by pathogens, while RPW8-NLRs (RNLs) signal downstream of many sensor NLRs and are called helper NLRs. Recent studies have revealed three dimensional structures of a CNL (ZAR1) including its inactive, intermediate and active oligomeric state, as well as TNLs (RPP1 and ROQ1) in their active oligomeric states. Furthermore, accumulating evidence suggests that members of the family of lipase-like EDS1 (enhanced disease susceptibility 1) proteins, which are uniquely found in seed plants, play a key role in providing a link between sensor NLRs and helper NLRs during innate immune responses. Here, we summarize the implications of the plant NLR structures that provide insights into distinct mechanisms of action by the different sensor NLRs and discuss plant NLR-mediated innate immune signalling pathways involving the EDS1 family proteins and RNLs

Structural basis of sterol recognition and nonvesicular transport by lipid transfer proteins anchored at membrane contact sites

Membrane contact sites (MCSs) in eukaryotic cells are hotspots for lipid exchange, which is essential for many biological functions, including regulation of membrane properties and protein trafficking. Lipid transfer proteins anchored at membrane contact sites (LAMs) contain sterol-specific lipid transfer domains [StARkin domain (SD)] and multiple targeting modules to specific membrane organelles. Elucidating the structural mechanisms of targeting and ligand recognition by LAMs is important for understanding the interorganelle communication and exchange at MCSs. Here, we determined the crystal structures of the yeast Lam6 pleckstrin homology (PH)-like domain and the SDs of Lam2 and Lam4 in the apo form and in complex with ergosterol. The Lam6 PH-like domain displays a unique PH domain fold with a conserved N-terminal alpha-helix. The Lam6 PH-like domain lacks the basic surface for phosphoinositide binding, but contains hydrophobic patches on its surface, which are critical for targeting to endoplasmic reticulum (ER)-mitochondrial contacts. Structures of the LAM SDs display a helix-grip fold with a hydrophobic cavity and a flexible Omega 1-loop as a lid. Ergosterol is bound to the pocket in a head-down orientation, with its hydrophobic acyl group located in the tunnel entrance. The Omega 1-loop in an open conformation is essential for ergosterol binding by direct hydrophobic interaction. Structural comparison suggested that the sterol binding mode of the Lam2 SD2 is likely conserved among the sterol transfer proteins of the StARkin superfamily. Structural models of full-length Lam2 correlated with the sterol transport function at the membrane contact sites

Structural insights into nonvesicular lipid transport by the oxysterol binding protein homologue family

Sterols such as cholesterol in mammals and ergosterol in fungi are essential membrane components and play a key role in membrane function and in cell signaling. The intracellular distribution and processing of sterols and other phospholipids are in part carried out by oxysterol binding protein-related proteins (ORPs) in eukaryotes. Seven ORPs (Oshl-Osh7 proteins) in yeast have distinct functions in maintaining distribution, metabolism and signaling of intracellular lipids but they share at least one essential function. Significant progress has been made in understanding the ligand specificity and mechanism of non-vesicular lipid transport by ORPs. The unique structural features of Osh proteins explain the diversity and specificity of functions in PI(4)P-coupled lipid transport optimized in membrane contact sites. This review discusses the current advances in structural biology regarding this protein family and its potential functions, introducing them as the key players in the novel pathways of phosphoinositide-coupled directional transport of various lipids. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon

A comparative study of the antioxidant, antimicrobial, cytotoxic and thrombolytic potential of the fruits and leaves of Spondias dulcis

Objective: To investigate the antioxidant, antimicrobial, cytotoxic and thrombolytic property of the fruits and leaves of Spondias dulcis (S. dulcis). Methods: Methanolic extracts of fruits and leaves of S. dulcis were partitioned with chloroform and dichloromethane. The antioxidant potential of the crude extract and partitioned fractions were evaluated in terms of total phenolic content, total flavonoid content, DPPH radical scavenging potential, reducing potential and total antioxidant capacity by specific standard procedures. The antimicrobial activity was evaluated using disc diffusion method. The cytotoxicity was evaluated by using brine shrimp lethality bioassay and compared with vincristine sulfate. The thrombolytic activity was compared with streptokinase. Results: The methanolic fruit extract exhibited the highest phenolic content, flavonoid content and antioxidant capacity, among the other extracts, with the highest DPPH radical scavenging activity at a concentration of 10 μg/mL (IC: 1.91 μg/mL) and maximum reducing power at a concentration of 100 μg/mL (EC: 3.58 μg/mL). Though all extract showed moderate antimicrobial activity against the bacterial strains, weak or no activity against fungus. The range of LC value of all extracts was 1.335-14.057 μg/mL which was far lower than the cut off index for cytotoxicity. All extracts exhibited statistically significant (