Identification of a target protein of Hydra actinoporin-like toxin-1 (HALT-1) using GST affinity purification and SILAC-based quantitative proteomics

Pore-forming moss protein bryoporin is structurally and mechanistically related to actinoporins from evolutionarily distant cnidarians

Pore-forming proteins perforate lipid membranes and consequently affect their integrity and cell fitness. Therefore, it is not surprising that many of these proteins from bacteria, fungi, or certain animals act as toxins. While pore-forming proteins have also been found in plants, there is little information on their molecular structure and mode of action. Bryoporin is a protein from the moss Physcomitrium patens, and its corresponding gene was found to be upregulated by various abiotic stresses, especially dehydration, as well as upon fungal infection. Based on the amino acid sequence, it was suggested that bryoporin was related to the actinoporin family of pore-forming proteins, originally discovered in sea anemones. Here, we provide the first detailed structural and functional analysis of this plant cytolysin. The crystal structure of the monomeric bryoporin is highly similar to those of actinoporins. Our cryo-EM analysis of its pores showed an actinoporin-like octameric structure, thereby revealing a close kinship of proteins from evolutionarily distant organisms. This was further confirmed by our observation of bryoporin's preferential binding to and formation of pores in membranes containing animal sphingolipids, such as sphingomyelin and ceramide phosphoethanolamine; however, its binding affinity was weaker than that of actinoporin equinatoxin II. We determined bryoporin did not bind to major sphingolipids found in fungi or plants, and its membrane-binding and pore-forming activity were enhanced by various sterols. Our results suggest that bryoporin could represent a part of the moss defense arsenal, acting as a pore-forming toxin against membranes of potential animal pathogens, parasites, or predators.

Structures and functions of the membrane-damaging pore-forming proteins

Pore-forming proteins (PFPs) of the diverse life forms have emerged as the potent cell-killing entities owing to their specialized membrane-damaging properties. PFPs have the unique ability to perforate the plasma membranes of their target cells, and they exert this functionality by creating oligomeric pores in the membrane lipid bilayer. Pathogenic bacteria employ PFPs as toxins to execute their virulence mechanisms, whereas in the higher vertebrates PFPs are deployed as the part of the immune system and to generate inflammatory responses. PFPs are the unique dimorphic proteins that are generally synthesized as water-soluble molecules, and transform into membrane-inserted oligomeric pore assemblies upon interacting with the target membranes. In spite of sharing very little sequence similarity, PFPs from diverse organisms display incredible structural similarity. Yet, at the same time, structure-function mechanisms of the PFPs document remarkable versatility. Such notions establish PFPs as the fascinating model system to explore variety of unsolved issues pertaining to the structure-function paradigm of the proteins that interact and act in the membrane environment. In this article, we discuss our current understanding regarding the structural basis of the pore-forming functions of the diverse class of PFPs. We attempt to highlight the similarities and differences in their structures, membrane pore-formation mechanisms, and their implications for the various biological processes, ranging from the bacterial virulence mechanisms to the inflammatory immune response generation in the higher animals.

Expansion of Hydra actinoporin-like toxin (HALT) gene family: Expression divergence and functional convergence evolved through gene duplication

Hydra actinoporin-like toxin 1 (HALT-1) was previously shown to cause cytolysis and haemolysis in a number of human cells and has similar functional properties to the actinoporins equinatoxin and sticholysin. In addition to HALT-1, five other HALTs (HALTs 2, 3, 4, 6 and 7) were also isolated from Hydra magnipapillata and expressed as recombinant proteins in this study. We demonstrated that recombinant HALTs have cytolytic activity on HeLa cells but each exhibited a different range of toxicity. All six recombinant HALTs bound to sulfatide, while rHALT-1 and rHALT-3 bound to two additional sphingolipids, lysophosphatidic acid and sphingosine-1-phosphate as indicated by the protein-lipid overlay assay. When either tryptophan¹³³ or tyrosine¹²⁹ of HALT-1 was mutated, the mutant protein lost binding to sulfatide, lysophosphatidic acid and sphingosine-1-phosphate. As further verification of HALTs' binding to sulfatide, we performed ELISA for each HALT. To determine the cell-type specific gene expression of seven HALTs in Hydra, we searched for individual HALT expression in the single-cell RNA-seq data set of Single Cell Portal. The results showed that HALT-1, 4 and 7 were expressed in differentiating stenoteles. HALT-1 and HALT-6 were expressed in the female germline during oogenesis. HALT-2 was strongly expressed in the gland and mucous cells in the endoderm. Information on HALT-3 and HALT-5 could not be found in the single-cell data set. Our findings show that subfunctionalisation of gene expression following duplication enabled HALTs to become specialized in various cell types of the interstitial cell lineage.

Comparative Analysis of the Soluble Proteome and the Cytolytic Activity of Unbleached and Bleached Millepora complanata ("Fire Coral") from the Mexican Caribbean

Coral bleaching caused by global warming has resulted in massive damage to coral reefs worldwide. Studies addressing the consequences of elevated temperature have focused on organisms of the class Anthozoa, and up to now, there is little information regarding the mechanisms by which reef forming Hydrozoans face thermal stress. In this study, we carried out a comparative analysis of the soluble proteome and the cytolytic activity of unbleached and bleached Millepora complanata ("fire coral") that inhabited reef colonies exposed to the 2015-2016 El Niño-Southern Oscillation in the Mexican Caribbean. A differential proteomic response involving proteins implicated in key cellular processes, such as glycolysis, DNA repair, stress response, calcium homeostasis, exocytosis, and cytoskeleton organization was found in bleached hydrocorals. Four of the proteins, whose levels increased in bleached specimens, displayed sequence similarity to a phospholipase A2, an astacin-like metalloprotease, and two pore forming toxins. However, a protein, which displayed sequence similarity to a calcium-independent phospholipase A2, showed lower levels in bleached cnidarians. Accordingly, the hemolytic effect of the soluble proteome of bleached hydrocorals was significantly higher, whereas the phospholipase A2 activity was significantly reduced. Our results suggest that bleached M. complanata is capable of increasing its toxins production in order to balance the lack of nutrients supplied by its symbionts.

Cytotoxic and apoptosis-inducing effects of wildtype and mutated Hydra actinoporin-like toxin 1 (HALT-1) on various cancer cell lines

Background

Hydra actinoporin like toxin -1 (HALT-1), is a small 18.5 kDa pore forming toxin derived from Hydra magnipapillata which has been shown to elicit strong haemolytic and cytolytic activity when in contact with cell membranes. Due to its cytotoxic potency, HALT-1 was further investigated for its potential as a toxin moiety candidate in immunotoxin developmental efforts, ideally as a form of targeted therapy against cancer.

Methods

In this study, wtHALT-1 (wild type) and its Y110A mutated binding domain counterpart (mHALT-1) were produced and evaluated for their cytotoxic and apoptotic effects on various cancer cell lines. A total of seven different tumour and non-tumour cell lines including HeLa, HepG2, SW-620, MCF-7, CCD841CoN, NHDF and HCT116 were used. Immunofluorescence assays were used to observe membrane binding and localization changes between both HALT-1 recombinant proteins based on 6xHis-tag detection.

Result

Based on MTT data, mHALT-1 demonstrated a significant reduction of 82% ±  12.21% in cytotoxic activity across all cell lines after the membrane recognition domain had been mutated in comparison to the wtHALT-1. Annexin V FITC/PI assay data also indicated that HeLa, HepG2 and MCF-7 demonstrated an apoptosis-mediated cell death after being treated with wtHALT-1. Additionally, a notable difference between wtHALT-1 and mHALT-1 binding affinity was clearly observed where emission of green fluorescence along the cell membrane was observed only in wtHALT-1 treated cells.

Discussion

These results suggest that mHALT-1 (Y110A) can be potentially developed as a toxin-moiety candidate for the development of future immunotoxins against various human cell-based diseases.

Purification and characterization of alginate lyase from Sphingomonas sp. ZH0

Alginate lyases degrade alginate in a beta-elimination reaction to produce oligosaccharides. Thus, alginate lyases are widely used in the food/pharmaceutical industries and are commercially valuable. In this study, four alginate lyase encoding genes were successfully cloned from Sphingomonas sp. ZH0. The expression systems of these alginate lyases were then constructed in Escherichia coli cells. The recombinant ZH0-I, ZH0-II, ZH0-III and ZH0-IV were purified from E. coli cells and were confirmed to be monomeric enzymes with molecular weights of approximately 91, 52, 67, and 113 kDa, respectively. The conditions for enzymes to have the highest specific lyase activities were 53.2 U/mg, 42 °C, pH 7.0 for ZH0-I, 103.9 U/mg, 47 °C, pH 6.5 for ZH0-II, 13.7 U/mg, 52 °C, pH 7.5 for ZH0-III, and 12.3 U/mg, 37 °C, pH 7.0 for ZH0-IV, respectively. These recombinant enzymes were stable over a pH range. Moreover, the enzymes were active in the absence of salt ions, and their activities were substantially reduced by the addition of HgCl₂. ZH0-I, ZH0-II and ZH0-III belong to endotype alginate lyases, while ZH0-IV is an exotype alginate lyase. All types could degrade both poly-β-d-mannuronate and poly-α-l-guluronate blocks, yielding alginate oligosaccharides as the main product. The K_m and V_max values were 0.51 mg/ml and 56.18 U/ml for ZH0-I, 0.47 mg/ml and 27.5 U/ml for ZH0-II, 0.55 mg/ml and 60.24 U/ml for ZH0-III, and 0.41 mg/ml and 5.53 U/ml for ZH0-IV, respectively. These features indicate that these alginate lyases are promising candidates for producing antioxidants from alginates in industrial applications.