Biology subjects

Posbeyikian, A.

Publications and source records attributed to Posbeyikian, A..

4 recordsLinked to original sources

bioRxiv2025

Functional insights into a peculiar tetra-modular LPMO from the human pathogen Enterobacter cloacae

Enterobacter cloacae is a Gram-negative nosocomial human pathogen that inhabits diverse ecological niches. Its genome encodes a conserved set of putative chitin-active enzymes, including a peculiar lytic polysaccharide monooxygenase (LPMO), termed EcLPMO, which we functionally characterized in this study. EcLPMO is a tetra-modular protein consisting of an auxiliary activity family 10 (AA10) catalytic domain, two central domains of unknown function (DUF-A and DUF-B), and a C-terminal carbohydrate-binding module (CBM73). Functional assays using full-length EcLPMO and its truncated variants demonstrated that the AA10 domain oxidatively cleaves chitin at the C1 position. The CBM73 module enhances chitin binding and promotes synergy with endogenous chitinases. Notably, EcLPMO displayed a particularly strong synergistic effect with the unimodular chitinase EcChiA, leading to up to 14-fold and 60-fold increases in GlcNAc release from - and {beta}-chitin, respectively. Deletion of both DUFs reduced EcLPMO activity. While DUF-A alone and the association of DUF-A and DUF-B showed limited chitin binding, DUF-B alone exhibited no binding, suggesting a distinct role. Unexpectedly, using state-of-the-art structural modelling (AlphaFold3), we observed that the DUF-B domain contains two highly conserved histidines that coordinate the AA10-bound copper, forming a previously unreported inter-domain tetra-histidine copper coordination center. These findings highlight the structural and functional complexity of EcLPMO and suggest that its accessory domains, particularly DUF-B, may contribute to enzyme stability and substrate interaction. We speculate that DUF-B may protect the LPMO active site from oxidative damage, a feature that could prove crucial in its ecological and pathogenic contexts.

biochemistry↗

bioRxiv2025

A hierarchical immune receptor network in lettuce reveals contrasting patterns of evolution in sensor and helper NLRs

Nucleotide-binding domain and leucine-rich repeat immune receptors (NLRs) are known for their rapid evolution, even at the intraspecific level, yet the rates of evolution differ significantly across various NLR classes. Within the NRC (NLR Required for Cell Death) network, NLRs operate in complex sensor-helper configurations to confer immunity against a diverse array of pathogens, particularly in Asterids. While helper NLRs are typically conserved and evolve slowly, sensor NLRs tend to evolve more rapidly. However, the functional connections between slow and fast-evolving NLRs remain poorly understood, notably in important crop species. We conducted a comparative analysis of NLRs across 40 Solanales and 29 Asterales genomes to explore NRC network expansion and diversification within the less-studied Asterales order. Our findings reveal that the NRC network has expanded less in Asterales compared to Solanales. We functionally validated a minimal Asterales NRC network with 2 helpers and 9 sensors in common lettuce (Lactuca sativa). Through selection and diversification analysis and structural modeling of NRC helper and sensor subclades in the Lactuca genus, we found varying evolutionary diversification rates between NRC helpers and sensors. We found a correlation between sensor diversification rates and helper dependency, with sensors reliant on a phylogenetically conserved helpers experiencing limited diversification pressure. Our results highlight the lineage- and function-specific evolution of the NRC network, offering insights into the evolutionary pressures shaping plant immune receptor networks.

plant biology↗

bioRxiv2024

A disease resistance protein triggers oligomerization of its NLR helper into a hexameric resistosome to mediate innate immunity

NRCs are essential helper NLR (nucleotide-binding domain and leucine-rich repeat) proteins that execute the immune response triggered by disease resistance proteins, also known as sensor NLRs. The structure of the resting state of NbNRC2 was recently revealed to be a homodimer. However, the sensor-activated state has not yet been elucidated. In this study, we used cryo-EM to determine the structure of sensor-activated NbNRC2, which forms a hexameric inflammasome-like structure known as resistosome. To confirm the functional significance of the hexamer, we mutagenized the interfaces involved in oligomerization and found that mutations in three nucleotide-binding domain interface residues abolish oligomerization and immune signalling. Comparative structural analyses between the resting state NbNRC2 homodimer and the sensor-activated homohexamer revealed significant structural rearrangements before and after activation, providing insights into NLR activation mechanisms. Furthermore, structural comparisons between the NbNRC2 hexamer and previously reported CC-NLR pentameric assemblies revealed features in NbNRC2 that allow for the integration of an additional protomer. We also used the NbNRC2 hexamer structure to assess the recently released AlphaFold 3 for the prediction of activated CC-NLR oligomers. This revealed that AlphaFold 3 allows for high-confidence modelling of the N-terminal 1-helices of NbNRC2 and other CC-NLRs, a region that has proven difficult to fully resolve using structural approaches. Overall, our work sheds light on the structural and biochemical mechanisms underpinning NLR activation and expands our understanding of NLR structural diversity.

plant biology↗

bioRxiv2023

A root-specific NLR network confers resistance to plant parasitic nematodes

Nucleotide-binding domain and leucine-rich repeat immune receptors (NLRs) confer disease resistance to a multitude of foliar and root parasites of plants. However, the extent to which NLR immunity is expressed differentially between plant organs is poorly known. Here, we show that a large cluster of tomato genes, which encodes the cyst and root-knot nematode disease resistance proteins Hero and MeR1 as well as the NLR-helper NRC6, exhibits nearly exclusive expression in the roots. This root-specific gene cluster emerged in Solanum species about 21 million years ago through gene duplication from the ancient NRC network of asterid plants. NLR-sensors in this gene cluster exclusively signal through NRC6 helpers to trigger the hypersensitive cell death immune response. These findings indicate that the NRC6 gene cluster has sub-functionalized from the larger NRC network to specialize for resistance against root pathogens, including cyst and root-knot nematodes. We propose that NLR gene clusters and networks have evolved organ-specific gene expression as an adaptation to particular parasites and to reduce the risk of autoimmunity.

plant biology↗