Substrate-induced changes in domain interaction of vacuolar H⁺-pyrophosphatase

Exploration of the Catalytic Cycle Dynamics of Vigna Radiata H+-Translocating Pyrophosphatases Through Hydrogen-Deuterium Exchange Mass Spectrometry

Vigna radiata H+-translocating pyrophosphatases (VrH+-PPases, EC 3.6.1.1) are present in various endomembranes of plants, bacteria, archaea, and certain protozoa. They transport H+ into the lumen by hydrolyzing pyrophosphate, which is a by-product of many essential anabolic reactions. Although the crystal structure of H+-PPases has been elucidated, the H+ translocation mechanism of H+-PPases in the solution state remains unclear. In this study, we used hydrogen-deuterium exchange (HDX) coupled with mass spectrometry (MS) to investigate the dynamics of H+-PPases between the previously proposed R state (resting state, Apo form), I state (intermediate state, bound to a substrate analog), and T state (transient state, bound to inorganic phosphate). When hydrogen was replaced by proteins in deuterium oxide solution, the backbone hydrogen atoms, which were exchanged with deuterium, were identified through MS. Accordingly, we used deuterium uptake to examine the structural dynamics and conformational changes of H+-PPases in solution. In the highly conserved substrate binding and proton exit regions, HDX-MS revealed the existence of a compact conformation with deuterium exchange when H+-PPases were bound with a substrate analog and product. Thus, a novel working model was developed to elucidate the in situ catalytic mechanism of pyrophosphate hydrolysis and proton transport. In this model, a proton is released in the I state, and the TM5 inner wall serves as a proton piston.

Crystal structure of Leptospira LSS_01692 reveals a dimeric structure and induces inflammatory responses through Toll-like receptor 2-dependent NF-κB and MAPK signal transduction pathways

Leptospirosis is a commonly overlooked zoonotic disease that occurs in tropical and subtropical regions. Recent studies have divided the Leptospira spp. into three groups based on virulence, including pathogenic, intermediate, and saprophytic species. Pathogenic species express a protein family with leucine-rich repeat (LRR) domains, which are less expressed or absent in nonpathogenic species, highlighting the importance of this protein family in leptospirosis. However, the role of LRR domain proteins in the pathogenesis of leptospirosis is still unknown and requires further investigation. In this study, the 3D structure of LSS_01692 (rLRR38) was obtained using X-ray crystallography at a resolution of 3.2 Å. The results showed that rLRR38 forms a typical horseshoe structure with 11 α-helices and 11 β-sheets and an antiparallel dimeric structure. The interactions of rLRR38 with extracellular matrix and cell surface receptors were evaluated using ELISA and single-molecule atomic force microscopy. The results showed that rLRR38 interacted with fibronectin, collagen IV, and Toll-like receptor 2 (TLR2). Incubating HK2 cells with rLRR38 induced two downstream inflammation responses (IL-6 and MCP-1) in the TLR2 signal transduction pathway. The TLR2-TLR1 complex showed the most significant upregulation effects under rLRR38 treatment. Inhibitors also significantly inhibited nuclear factor κB and mitogen-activated protein kinases signals transduction under rLRR38 stimulation. In conclusion, rLRR38 was determined to be a novel LRR domain protein in 3D structure and demonstrated as a TLR2-binding protein that induces inflammatory responses. These structural and functional studies provide a deeper understanding of the pathogenesis of leptospirosis.

Peptidoglycan mediates Leptospira outer membrane protein Loa22 to toll-like receptor 2 for inflammatory interaction: a novel innate immune recognition

Leptospirosis is an overlooked zoonotic disease caused by pathogenic Leptospira depended on virulence of Leptospira and the host–pathogen interaction. Kidney is the major organ infected by Leptospira which causes tubulointerstitial nephritis. Leptospira outer membrane contains several virulence factors and an outer membrane protein A (OmpA) like protein (Loa22) is essential for virulence. Pull-down assays suggested that Loa22 was a potential Toll-Like Receptor 2 (TLR2) binding candidates from pathogenic Leptospira. Confocal microscopy was employed to observe the co-localization of TLR2 and Loa22-LPGN (Leptospira peptidoglycan) complexes. Atomic force microscopy (AFM), side-directed mutagenesis, and enzyme-linked immunosorbent assay (ELISA) were performed to investigate the affinity between rLoa22, LPGN, and TLR2. Real time PCR was applied to measure the cytokines expression. Downstream signal transduction components were verified by western blot to evaluate the gene regulations. Mutation of two Loa22 key residues (Asp¹²² and Arg¹⁴³) attenuated the affinities for LPGN. rLoa22-LPGN complexes were observed to co-localize with TLR2 and provoked inflammatory responses including CXCL8/IL8, hCCL2/MCP-1, and hTNF-α. Affinity studies suggested that Loa22-LPGN complexes elevated the affinity to TLR2 as compared to Loa22 protein. Downstream signals from TLR2 including p38, ERK, and JNK were regulated under rLoa22-LPGN complexes treatments. This study identified LPGN mediates interactions between Loa22 and TLR2 and induces downstream signals to trigger inflammatory responses. rLoa22-LPGN-TLR2 complexes reveal a novel binding mechanism for the innate immune system.

Crystal structure of Leptospira leucine-rich repeat 20 reveals a novel E-cadherin binding protein to induce NGAL expression in HK2 cells

Leptospirosis is the most common zoonotic disease caused by pathogenic Leptospira, which is classified into three groups according to virulence. Its pathogenic and intermediate species contain leucine-rich repeat (LRR) proteins that are rarely expressed in non-pathogenic strains. In this study, we presented the crystal structure of LSS_11580 (rLRR20) from pathogenic L. santarosai serovar Shermani. X-ray diffraction at a resolution of 1.99 Å revealed a horseshoe-shaped structure containing seven α-helices and five β-sheets. Affinity assays indicated that rLRR20 interacts with E-cadherin on the cell surface. Interestingly, its binds to the extracellular (EC) 1 domain in human epithelial (E)-cadherin, which is responsible for binding to another E-cadherin molecule in neighboring cells. Several charged residues on the concave face of LRR20 were predicted to interact with EC1 domain. In the affinity assays, these charged residues were replaced by alanine, and their affinities to E-cadherin were measured. Three vital residues and mutation variants of LRR20, namely D56A, E59A, and E123A, demonstrated significantly reduced affinity to E-cadherin compared with the control. Besides, we also demonstrated that rLRR20 induced the expression of neutrophil gelatinase-associated lipocalin (NGAL) in HK2 cells. The low ability of the three mutation variants to induce NGAL expression further demonstrates this induction. The present findings indicate that LRR20 from pathogenic Leptospira binds to E-cadherin and interacts with its EC1 domain. In addition, its induction of NGAL expression in HK2 cells is associated with acute kidney injury in human.

Active Components of Leptospira Outer Membrane Protein LipL32 to Toll-Like Receptor 2

Proteins belonging to the toll-like receptor (TLR) family, particularly TLR2, are the major components of innate immunity against Leptospira infection. The ligands for TLR2 harbor several conserved patterns such as lipidation molecules, leucine-rich repeat (LRR) domains, TLR2 binding motifs, and TLR2 binding structure. In Leptospira, LipL32 interacts with TLR2 on human kidney cells concomitantly stimulating inflammatory responses. However, the binding mechanism of LipL32 to TLR2 is unknown. The computational prediction suggests that β1β2, loop-α3-loop, and α4 domains of LipL32 play vital roles in LipL32-TLR2 complex formation. To test these predictions, protein truncation experiments revealed that LipL32ΔNβ1β2 significantly decreased the affinity to TLR2 while LipL32ΔCα4 slightly reduced it. Interestingly, LipL32ΔCenα3 retained affinity to TLR2 in the absence of Ca2+ ions, indicating that Cenα3 play a role preventing the interaction between LipL32 and TLR2. Furthermore, the critical residues of LipL32 involved in interacting with TLR2 suggested that V35S, L36S and L263S variants significantly decreased the affinity to TLR2. The results further confirm that LipL32 interacts with TLR2 through Nβ1β2 and Cα4 domains of LipL32 as well as LipL32-TLR2 complex formation results from hydrophobic interactions. This study provides a detailed mechanism of the interaction between LipL32 and TLR2 and the residues involved in complex formation.